i

An Assessment of the Extent, Distribution, and Change of Bracken (Pteridium aquilinum) in the Peak District

National Park

by

Julian A.I. McAlpine

August 2014

In submitting this dissertation, I confirm that it is my own work.

A thesis submitted to the Department of Geography, University of Leicester in partial fulfilment of the requirements for the degree of

Global Environmental Change

Master of Science

ii

Abstract

An Assessment of the Extent, Distribution, and Change of Bracken (Pteridium aquilinum) in the Peak District National Park

Bracken (Pteridium aquilinum) has a global distribution (apart from Antarctica). By origin a

component of open woodland habitats, due to anthropogenic activities its range has

broadened, and on open ground it displays competitive and invasive characteristics. Bracken

is thus viewed as a pest - in the UK it encroaches into habitats of greater conservation and

economic value, notably in the uplands where control is difficult. The application of asulam,

the most effective narrow-margin herbicide, was made illegal in 2013, however Emergency

Authorisations have allowed spraying to continue in the short-term. An absolute ban will have

wide-ranging implications for the uplands; without asulam spraying bracken extent would

possibly be 50% greater.

In ArcGIS this study mapped 1960s and 2005 imagery for the Peak District National Park. A

comparison of the study periods observed an increase in bracken extent of nearly 1% per

annum, which is not exceptional. However, the data suggest that bracken may have covered

an area of 62 km2 in the Peak District National Park 2005, a 1.8% p.a. increase on 1990

estimations. Despite steady, persistent control since 1990, bracken’s rate of spread may have

accelerated. Land management practices, such as burning, and potentially the influence of

climate change, may be assisting the bracken’s spread. This study observed an increase in

bracken cover at higher elevations and on north- and east-facing slopes, but the diversity of

change, by virtue of anthropogenic activities, masked any evidence of climate change as a

factor in upland bracken distribution. In the future, if the viability of upland economies

becomes more tenuous, the threats from invasive species worsen, and asulam (or herbicide

spraying in general) is prohibited, a decision on the successional direction of upland

environments will be unavoidable. Should plagio-climax communities be maintained, or

should succession towards woodland habitats be promoted?

iii

Acknowledgements

Firstly, I would like to thank Dr Ben Clutterbuck – I am hugely grateful for all his help, advice,

and encouragement, which was always afforded with good humour. He is a good friend and an

inspiration. This study is dedicated to him.

I would also like to thank Professor Sue Page, her enthusiasm has been a real tonic, and her

support, advice and feedback are greatly appreciated. The literature review is much improved

due to her recommendations.

From the Peak District National Park Authority I would like to thank Angela Johnson, Philippa

Davey and Rhodri Thomas. Angela was extremely helpful, patient, and welcoming when I was

searching for imagery. Philippa, too, has been most helpful, and Rhodri’s knowledge of the

Peak District is second to none.

Huge thanks also to Professor Rob Marrs and Penny Anderson, whose replies to my queries

were invaluable, and their inputs have been integral to this study.

I sincerely thank all the above, and all those referenced throughout this study - you have

inspired and informed this study, however all errors and oversights are my own.

And a huge thank you to all my family and friends for their continued support and

encouragement, especially my wife Jennifer, children Sienna and Thomas, and my parents –

I’ll see a lot more of you now (whether you like it or not!).

iv

Table of Contents

Front page i

Abstract ii

Acknowledgements iii

Table of contents iv

Table of figures vii

Table of tables

Title page

ix

xi

1. Introduction 1

1.1 Is the extent of bracken increasing? 2

1.2 The natural history and ecology of bracken 3

1.3 The competitive ability of bracken 5

1.3.1 Factors that have influenced bracken spread in the UK 5

1.3.2 The competitive characteristics of bracken 6

1.3.3 The dynamics of bracken spread 7

1.4 The Uplands 10

1.4.1 Upland characteristics 10

1.4.2 The Peak District National Park (PDNP) 13

1.5 Bracken: extent and distribution, management and control 14

1.5.1 Bracken extent and distribution 14

1.5.2 Bracken management 16

1.5.3 Bracken control 18

1.6 Study rationale 20

1.6.1 Bracken spread in the PDNP 20

1.6.2 Climate change 21

1.6.2.1 Climate change and the uplands 21

1.6.2.2 Bracken and climate change 23

1.6.3 Burn regimes 24

1.7 Summary of study intentions 25

2. Method 26

2.1 Introduction 26

2.2 The study area – the Peak District National Park 26

2.2.1 Dark Peak landscape characteristics 27

2.2.2 Climate 29

2.3 Site selection 29

2.4 Aerial imagery 32

v

2.4.1 Orthocorrection 32

2.4.2 Georeferencing 34

2.5 Geographic Information Systems (GIS) 34

2.6 Mapping bracken cover in ArcGIS 35

2.6.1 Identification of bracken 35

2.6.2 Polygon mapping 36

2.6.3 Image uncertainty 36

2.6.4 Altitude, aspect, and slope data 37

2.7 Statistical analysis 38

2.7.1 Assessment of mapping consistency 38

2.7.2 Bracken coverage in the PDNP in 2005 38

2.7.3 Bracken data for the 2005 sample and sub-sample 38

2.7.3.1 The Pearson correlation test 38

2.7.3.2 The Kolmogorov-Smirnov test 38

2.7.3.3 Sub-sample analysis 39

2.7.3.4 Changes in bracken cover at slopes related to altitude 1960s-2005

39

2.8 Historic climate data 39

3. Results 40

3.1 Assessment of mapping consistency 40

3.2 Bracken coverage in the PDNP in 2005 40

3.3 Bracken extent by combined aspect and slope at altitudinal zones 44

3.4 Representation of bracken in sub-sample data 45

3.5 Change in bracken cover 1960s-2005 48

3.5.1 Changes in bracken cover at altitudinal zones 1960s-2005 49

3.5.2 Changes in bracken cover at aspect 1960s-2005 51

3.5.3 Changes in bracken cover at aspect related to altitude 1960s-2005

51

3.5.4 Changes in bracken cover on slopes 1960s-2005 54

3.5.5 Changes in bracken cover on slopes related to altitude 1960s-2005

56

3.6 Ranking changes in bracken cover (%) 1960s-2005 59

3.7 Results for historic climate data 60

4. Discussion 63

4.1 Bracken in the PDNP in 2005 63

4.2 Comparison of bracken in the 1960s and 2005 66

4.2.1 Comparison of aspect 68

4.2.2 Comparison of aspect 69

vi

4.3 Climate change 70

4.3.1 Climate change and future bracken distribution 70

4.4 The relationship between bracken and heather 71

4.4.1 The effects of climate change on the bracken-heather relationship

73

4.4.2 Heather: state and distribution 73

4.5 The role of anthropogenic activities on bracken distribution 74

4.5.1 Wildfires and burn regimes 74

4.5.2 Bracken control 77

4.6 Future conservation strategies 78

4.6.1 Contradictions of future strategies 80

4.6.2 The succession paradox 80

4.6.3 Confronting misdeeds 81

4.7 Study limitations 82

4.7.1 1960s imagery versus 2005 imagery 82

4.7.2 Problems associated with identifying bracken 83

4.7.2.1 Growing and senesced fronds 83

4.7.2.2 Shadows 84

4.7.3 Climate data 85

4.8 Recommendations for future research 85

4.9 Concluding summary 86

5. Annex 88

5.1 Introduction 88

5.2 The evolution of the study 88

5.2.1 Site selection 88

5.2.2 Variables and the elevation boundary 89

5.2.3 Study period selection 91

5.3 Mapping 91

5.3.1 Mapped data 91

5.3.2 ArcGIS 93

5.4 The results 93

5.5 Bracken: further discussion 93

5.5.1 The value of bracken 94

5.5.2 Albedo and succession 95

5.5.3 Bracken toxicity and hydrology 95

6. References 97

7. Appendix 115

vii

Table of figures:

Figure 1.1 The major morphology of Pteridium spp.: 4

Figure 1.2 The regeneration cycle of bracken and the associated changes in litter and shoots

8

Figure 1.3 The change in height and cover continuity, and depth of origin of fronds between the margin and hinterland in a bracken community

8

Figure 1.4 The distribution of bracken in the British Isles 10

Figure 1.5 Extent and location of the British uplands and marginal uplands recorded by the Countryside Survey 2000

11

Figure 1.6 Location of the Peak District National Park 13

Figure 1.7 Bracken in the Peak District, 2005 15

Figure 1.8 The effects of control on the mean position (m) of the bracken front relative to study start in 1993 at Levisham Moor, North York Moors

19

Figure 1.9 A model of the inter-relationships between climate change, visitors, the environment, and wildfires in UK uplands

23

Figure 1.10 Visible bracken encroachment into burn scars on the North Yorkshire Moors 2002-2009

24

Figure 2.1 The PDNP, with the Dark Peak 26

Figure 2.2 Landscape character types in the PDNP 28

Figure 2.3 An illustration of the problem of identifying bracken with confidence in the 1960s imagery

30

Figure 2.4 The 2005 sample 31

Figure 2.5 The sub-sample of 13 1 km2 study grids in PDNP 31

Figure 2.6 Orthocorrection 33

Figure 2.7 ArcMap screen grab in which a georeferencenced 1960s image is compared with orthocorrected 2005 imagery from which its GCPs were taken

34

Figure 2.8 Bracken identification 35

Figure 2.9 ArcGIS mapping 36

Figure 2.10 ArcMap screen-grab showing the attribute table for ‘2005 all bracken aspect’

37

Figure 3.1 Proportion of bracken in altitudinal (50 m zones) in PDNP in 2005 41

Figure 3.2 Areal extent (ha) of bracken by aspect in PDNP in 2005 41

Figure 3.3 Aspects in which bracken cover (ha) was observed at altitudinal zones (50 m) in PDNP, 2005

42

Figure 3.4 Areal extent (ha) of bracken by slope in PDNP in 2005 43

Figure 3.5 Slope zones in which areal extent (ha) of bracken was observed at altitudinal zones (50 m) in PDNP in

44

Figure 3.6 The 2005 sample and sub-sample of bracken in altitudinal zones (m) in PDNP

46

viii

Figure 3.7 Correlation between the full 2005 sample and sub-sample in altitudinal zones (m) in PDNP, illustrating a linear association

47

Figure 3.8 Correlation between the full 2005 sample and sub-sample for aspect 47

Figure 3.9 Correlation between the full 2005 sample and sub-sample for slope 48

Figure 3.10 Change in area (ha) of bracken at altitudinal zones (50 m) from the 1960s to 2005 in PDNP

50

Figure 3.11 Annual change (%) of bracken at altitudinal zones (50 m) from the 1960s to 2005 in PDNP

50

Figure 3.12 Change (% p.a.) of bracken at a north aspect from the 1960s to 2005 in PDNP

52

Figure 3.13 Annual change in area (ha) of bracken at an east aspect from the 1960s to 2005 in PDNP

53

Figure 3.14 Change of bracken cover for aspect at altitudinal zones (50 m) from the 1960s to 2005 in PDNP

54

Figure 3.15 Change in area (ha) of bracken at slope zones from the 1960s to 2005 in PDNP

55

Figure 3.16 Annual change (%) of bracken at slope zones from the 1960s to 2005 in PDNP

56

Figure 3.17 Change in area (ha) of bracken at consolidated slope zones from the 1960s to 2005 in PDNP

58

Figure 3.18 Change (%) of bracken at consolidated slope zones from the 1960s to 2005 in PDNP

58

Figure 3.19 Changes in distribution (%) of bracken in the slope zones for the 1960s and 2005 in PDNP

59

Figure 3.20 Mean summer maximum and minimum temperatures 61

Figure 3.21 Mean winter maximum and minimum temperatures 62

Figure 4.1 1 km2 study grid showing bracken mapped for 2005, illustrating clipping out

64

Figure 4.2 The observed altitudinal limit, 514 m, of bracken in the study grids and bracken at 536 m, Kinder Scout, PDNP

65

Figure 4.3 An example of land-cover change next to the Upper Derwent Reservoir 67

Figure 4.4 The lifecycle of heather is comparable with that of bracken 72

Figure 4.5 Moorland burns at a local scale in the PDNP 75

Figure 4.6 Moorland burns at a large scale in the PDNP 76

Figure 4.7 Areas sprayed by asulam 77

Figure 4.8 The extent and potential extent of bracken in Great Britain with and without sprayed asulam (from Pakeman, 2012)

78

Figure 4.9 The directional options for upland landscapes 80

Figure 4.10 Possible breaches of regulations in the PDNP from 2005 imagery 81

Figure 4.11 Bracken is easily identifiable 83

Figure 4.12 Bracken is not identifiable in the 1960s, but easy to identify in 2005 83

ix

Figure 4.13 The images illustrate how shadows, woodland, and other features such as walls and lone trees were dealt with when mapping bracken in this study

84

Sheep amongst bracken, the PDNP, July 2013 (by author) 87

Figure 5.1 The differences between two- and three-dimensional images 90

Figure 5.2 A 1km2 study grid in which is shown mapped bracken from 2005 overlaid with mapped bracken from the 1960s

91

Figure 5.3 The mapped extents for the 1960s and 2005, and the losses and gains in

bracken extent from the 1960s to 2005 in a 1km2 study grid

91

Table of tables:

Table 1.1 Bracken’s ecological and amenity provisions 2

Table 1.2 Main upland habitats, recorded by the Countryside Survey 2000 13

Table 1.3 Examples of ecosystem services provided by PDNP 14

Table 1.4 Bracken broad habitat in Great Britain 1984-2007 and the UK 1998-2007

15

Table 1.5 Management options for bracken control 17

Table 1.6 The processes driven by climate and their associated upland impacts 22

Table 2.1 Date, source, format and resolution of aerial imagery 32

Table 2.2 When and why orthocorrection is necessary 33

Table 2.3 Typical questions GIS can answer 35

Table 2.4 Month photograph taken and bracken growth-stage 36

Table 3.1 Three bracken stands mapping consistency assessment results 40

Table 3.2 Proportion (%) of bracken by altitudinal zones (50 m) in PDNP in 2005 40

Table 3.3 Proportion (%) of bracken by aspect in PDNP in 2005 41

Table 3.4 Distribution of bracken cover by aspect at altitudinal zones (50 m) in PDNP, 2005

42

Table 3.5 Proportion (%) of bracken by slope in PDNP in 2005 43

Table 3.6 Distribution of bracken cover by slope at altitudinal zones (50 m) in PDNP, 2005

43

Table 3.7 Bracken extent (ha) by aspect and slope in relation to altitude in the PDNP, 2005

45

Table 3.8 2005 sample and sub-sample datasets for bracken in 50 m zones in PDNP

46

Table 3.9 Change in bracken extent (ha) by aspect and slope in relation to altitude in the PDNP, 1960s-2005

49

Table 3.10 Bracken cover and change at altitudinal zones (50 m), 1960s-2005 in PDNP

49

x

Table 3.11 Bracken cover and change at four aspects, 1960s-2005 in PDNP 51

Table 3.12 Bracken cover and change at north aspect, 1960s-2005 in PDNP 51

Table 3.13 East aspect for bracken in the 1960s and 2005 in PDNP 52

Table 3.14 South aspect for bracken in the 1960s and 2005 in PDNP 53

Table 3.15 West aspect for bracken in the 1960s and 2005 in PDNP 53

Table 3.16 Bracken in the 1960s and 2005 in 8 slope zones in PDNP 55

Table 3.17 Low slope for bracken in the 1960s and 2005 in PDNP 56

Table 3.18 low-mid slope for bracken in the 1960s and 2005 in PDNP 57

Table 3.19 Steep-mid slope for bracken in the 1960s and 2005 in PDNP 57

Table 3.20 Steep slope for mapped bracken in the 1960s and 2005 in PDNP 57

Table 3.21 Bracken in the 1960s and 2005 in the consolidated slope zones in PDNP

58

Table 3.22 Distribution (%) of bracken in the slope zones for the 1960s and 2005 in PDNP

59

Table 3.23 Ranking areas (n) and proportion (%) of bracken by percentage change from the 1960s to 2005 in PDNP

60

Table 3.24 Summer period decadal comparison of climatic variables during the study period

60

Table 3.25 Winter period decadal comparison of climatic variables during the study period

61

Table 4.1 Examples of potential causes and effects of climate change in PDNP that might proliferate the spread of bracken

70

Table 4.2 Projected temperature (°C) and precipitation (%) changes for PDNP in 2020s, 2050s and 2080s

71

Table 4.3 The Twelve Ecosystem Approach Principles 79

Table 4.4 A vision for the uplands in 2060 compared to 2009 79

xi

An Assessment of the Extent, Distribution, and Change of Bracken (Pteridium aquilinum) in the Peak District

National Park

by

Julian A.I. McAlpine

August 2014

1

1. Introduction

Bracken (Pteridium aquilinum) is an invasive plant of worldwide significance (Maren et

al., 2008; Da Silva and Matos, 2006; Marrs and Watt, 2006; Schneider, 2004; Potter and

Baird, 2000; Blackman and Pitman, 1999). It is distributed globally apart from the

coldest and hottest desert environments (Hatcher and Batty, 2011; Marrs and Watt,

2006; Page, 1976), the most widely of all pteridophytes (ferns and fern allies) (Marrs

and Watt, 2006), and is one of the five most common plants in the world (Werkman and

Callaghan, 1999; Heads and Lawton, 1984). In temperate regions it is a particularly

successful competitor where it colonises all habitats except calcareous grassland and

mires (Elkington et al., 2001; Marrs et al., 2000).

Farmers, foresters, land managers and conservationists in the UK consider bracken a

weed, and the problems it causes are copious and diverse; invasive, competitive and

toxic, it encroaches into habitats of higher economic and conservation value (Alday and

Marrs, 2014; Alday et al., 2013; Deluca et al., 2013; Hatcher and Batty, 2011; Marrs and

Watt, 2006; Pakeman et al., 1994; Anderson and Radford, 1988; Taylor, 1985; Page,

1982). Bracken can out-compete other vegetation and endure non-polluting herbicides,

repeated burning, and all but the worst droughts due to its physiological and

morphological properties, including an extensive, deep rhizomal system, high

productivity, efficient water-use, dense frond-litter, and a tolerance of a broad range of

soils (Marrs and Watt, 2006; McGlone et al., 2005; Marrs et al., 2000).

Distributed mainly in upland areas and on marginal land, bracken colonises

conservation-priority communities such as heather (Calluna vulgaris) moorland and acid

grassland (Le Duc et al., 2007; Pakeman et al., 2000; Pakeman and Marrs, 1992),

fragmenting or completely replacing these habitats which diminishes biodiversity,

ecosystem functioning and ecosystem services (Anderson et al., 2009; Tong et al., 2006;

Pakeman and Marrs, 1992). Additionally this affects the viability of important upland

economies including farming, forestry and grouse (Lagopus lagopus scotica) shooting

(Alday and Marrs, 2014; Anderson et al., 2009; Pakeman et al., 1997; Varvarigos and

Lawton, 1991).

The control of bracken is viewed as essential to protect unique habitat assemblages and

vulnerable upland economies (Stewart et al., 2007). Control of invasive plants must be

2

cost-effective and ecologically sound (Wiles, 2004), however a ban on the herbicide

asulam, a potent and narrow-spectrum control, may give rise to upland range expansion

(Pakeman, 2012). However, where bracken is not dominant and suppressive, it provides

ecological and amenity value as a component of the landscape mosaic (Marrs and Watt,

2006; Anderson and Radford, 1988, Table 1.1).

Table 1.1 Bracken’s ecological and amenity provisions (references: Robinson, 2007; Marrs and Watt, 2006; Hartig and Beck, 2003; Pakeman and Marrs, 1992; Anderson and Radford, 1988)

Beneficiaries Provision

Fauna Shelter and food for mammals, reptiles, birds and invertebrates. In Nardus-dominant pasture bracken offers only significant vegetational structure for breeding whinchat (Saxicola rubetra), wrens (Troglodytidae spp.) and ring ouzel (Turdus torquatus).

Flora Shelter and protection from herbivory, frost and wind for rare woodland relicts, and a nursery for saplings, bilberry (Vaccinium myrtillus), wavy hair-grass (Deschampsia flexuosa) or bent (Agrostis spp.), but not for Nardus.

Ecosystem services Prevention of erosion, notably on steeper slopes, carbon sequestration, attractive seasonally-changing colour and form

1.1 Is the extent of bracken increasing?

Extensive land clearance and habitat change was attended by an expansion in bracken’s

distribution in many global regions (e.g. Taylor, 1986; Thomson et al., 1986; Rymer,

1976). In the twentieth century this was exacerbated by a reduction in the use of

bracken as upland farming and economies changed (Cox 2007).

It is estimated that bracken spreads by 1% to a maximum of 3% per annum (p.a.) (Miller

et al., 1990; Hopkins et al., 1988; Lawton, 1988; Taylor, 1986), although present bracken

cover is viewed as “not exceptional,” and pollen records demonstrate greater former

abundance (Pakeman et al., 2000). Moreover, the Countryside Survey 2007 found a

decline in bracken’s areal extent (Carey et al., 2008). However data on distribution and

abundance are unsatisfactory, lacking in precision and consistency (Holland and Aplin,

2013; Ustin and Gamon, 2010; Laba et al., 2008; Dare, 2005; Giles, 2001; Birnie et al.,

2000; Pakeman et al., 2000). Therefore the true extent of bracken cover is unknown, and

may be substantially greater than estimations. Furthermore, it is suggested that

bracken’s range may be extending in latitude and elevation due to climate change effects

(House et al., 2010; Pakeman et al., 1994) and/or alterations in management practices.

Unpublished data (McAlpine, 2013) found discrete bracken stands increased in area

significantly above 330 m in the Peak District National Park (PDNP) between 1989 and

3

2005. Anderson (pers. comm., 2014) regards bracken spread in some areas of PDNP as

“significant”, notably in the region of the Howden/Derwent National Trust estate, whilst

other areas show no appreciable spread.

Effective control is difficult - bracken is exceptionally resilient to non-polluting

herbicides (Cox et al., 2008; Stewart et al., 2008; Marrs and Watt, 2006), and cutting and

ploughing are labour-intensive and confined to accessible areas (Burge and Kirkwood,

1992). In addition, anthropogenic activities can stimulate or accelerate invasion

(Schneider and Fernando, 2010; Mack, 2005), for example by grazing (Holden, 2009),

burning (Alday et al., 2013; Roos et al., 2011; Hartig and Beck, 2003), drainage, and

afforestation (Haigh, 2006).

1.2 The natural history and ecology of bracken

Vascular plants (tracheophytes) are the most species rich and abundant flora, the most

conspicuous of all organisms, and the most influential terrestrial primary producers

(Pryer et al., 2004). After angiosperms (the flowering plants, ~257,000 extant species),

leptosporangiate ferns, the grouping of modern ferns, are the most diverse vascular

plants with over 9,000 extant species in 267 genera (Hatcher and Batty, 211; Sharpe et

al., 2010; Schuettpelz and Pryer, 2007), including the family to which bracken belongs -

Dennstaedtiaceae (rhizomal and mostly creeping) (ITIS, 2011; Lehtonen, 2011; Smith et

al., 2006, appendix i). Bracken is recorded in fossil records dating back to the Tertiary

(~55 mybp) before the continents fully separated, which helps explain its widespread

contemporary distribution (Moran, 2004). Ferns also produce millions of spores, usually

falling very locally but which can disperse over thousands of kilometres (Kessler, 2010;

Schneller and Liebst, 2007), rare occurrences that are however disproportionately

important in their distribution (Renner, 2005). It is thought that unsuitable climatic and

edaphic conditions in the UK prevent successful widespread germination by this method

(Pakeman et al., 2002).

Here, bracken spreads as a travelling geophyte (having the capacity for underground

storage), advancing horizontally through the soil by means of a perennating, expansive,

starch-rich, branching rhizome system (Marrs and Watt, 2006; Parks and Werth, 1993;

Watt, 1940). As the rhizome advances segments die back behind it, and a single,

interconnected plant develops into a myriad of discrete clones. Large stands can be the

product of just one or a few plants, clones of great size and age - one clone’s stand in the

Appalachian mountains covered over 1km2 and was estimated to be 1,200 years old

4

(Hatcher and Batty, 2011). They produce solitary fronds on stems that can reach

between (15)30–180(440) cm in length, procuring sunlight and carbon dioxide (CO2)

that sustain rhizomal branching (Marrs and Watt, 2006).

Bracken’s physiology is key to its success as an invader. After producing around ten

fronds the sporophyte shoots bifurcate (branch into two) and go beneath the soil,

branching more extensively and producing primordia that arise at the surface as croziers

which unfurl into fronds (Hatcher and Batty, 2011; Marrs and Watt, 2006 ). The

morphology is illustrated in Figure 1.1.

Figure 1.1 The major morphology of Pteridium spp.: (a) frond lamina (blade); (b) rachis; (c) pinna; (d) stipe; (e) nectary; (f) crozier with hairs; (g) leaf primordium on short-shoot; (h) apex of shoot; (i) lateral line; (j) petiolar roots (in P. esculentum); (k) roots; (l) rhizome; (m to p) pinna; with (m) pinnule; (n) midrib of pinna; (o) pinnulet; and (p) midrib of pinnule; (q) lower surface of pinnule; (r to w) section of the margin of pinnulet; with (r) upper surface; (s) lower surface; (t) mature sporangium; (u) indusium; (v) sporangium after discharge of spores; (w) false indusium; (y to z) shows (x) frond primordium; (y) abaxial bud and (z) adaxial bud. Redrawn by author from Marrs and Watt (2006)

5

1.3 The competitive ability of bracken

“Bracken is one of the most accommodating of plants as regards it requirements.”

C.E. Moss (1913)

Bracken is well adapted to the abiotic conditions where grows because it’s range is

natural (Page, 1982). A “non-aggressive understory species of open-canopied

woodland” (Atkinson, 1989), suppressed by limiting factors of light and water

availability (Berget, 2012; Harmer et al., 2005), bracken often signifies locations of

historic ancient woodland (Rodwell, 1998), and the altitudinal limits of bracken and

trees appear correlated (Moss, 1913). However, the beginning of agriculture by Neolithic

man 6,000 years ago initiated large-scale deforestation and habitat disturbance, due to

which bracken was unshackled from beneath the woodland canopy, and quickly

achieved its modern extensive distribution (Yerkes et al., 2012; Hatcher and Batty, 2011;

Page, 1976).

Contemporary UK bracken distribution - to its ecological limits, excluding permanently

cultivated areas (Marrs and Watt, 2006) - has been encouraged by its dwindling use,

especially in marginal and upland areas (Hatcher and Batty, 2011). Historically bracken

was viewed not as a pest but as a valuable resource as food for humans and livestock,

fuel, fertilizer and mulch, for thatching, bedding, medicine, soap and insect repellent.

Folklore mentions fern seed as an aphrodisiac, with the powers also to bestow invisibility

and invoke rains (Marrs and Watt, 2006; Rymer, 1976).

1.3.1 Factors that have influenced bracken spread in the UK

Modern changes in the activities that preserve open uplands character, predominantly

grazing and burning, have aided bracken’s spread (Holden et al., 2007; Evans et al.,

2006), exacerbated by modern anthropogenic-induced increases in nitrogen deposition

and other atmospheric pollution (Edmondson et al., 2013; Worrall and Evans 2009;

Barker et al., 2004).

These changes include:

Land-use change (e.g. arable to grazing) or abandonment, for example due to the

enforced Highland clearances of the eighteenth and nineteenth centuries (Dodgshon

and Olsson, 2006; Ader, 1988), or, more recently, because farming and shooting often

exist on the edge of economic viability (Arblaster et al., 2009; Holden et al., 2007;

Pakeman et al., 2003).

6

Changes away from mixed-livestock to sheep-dominated grazing practices. Sheep are

less effective than cattle and horses in trampling pioneer bracken, and their grazing

habits (of close cropping) allow bracken invasion into grassland and heather

moorland (Gardner et al., 2009; Evans et al., 2006; Birch, 2002; Pakeman et al., 2002).

Common Agricultural Policy (CAP) subsidies for less favourable areas (LFAs)

resulted in an intensification of sheep farming in the UK uplands, and have

contributed to local soil erosion, a decline of dwarf shrubs (notably Calluna vulgaris),

the spread of unpalatable, competitive species (including bracken), and a decline in

upland biodiversity (Gardner et al., 2009; Fraser et al., 2009; Fuller and Gough,

1999).

There has been an increase in peatland exploitation (Connolly and Holden, 2013;

Holden et al., 2007), and over half of UK peatland has been drained (1.5 of 2.9 million

ha) to lower the water table and make available lands for grazing or forestry (Worrall

and Evans, 2009), and more susceptible to bracken invasion (Marrs and Watt, 2006).

Upland burning of heather moorland for grouse has become more widespread and

extensive (Clutterbuck and Yallop, 2010; Yallop et al., 2009; Evans et al., 2006), and

provide opportunities for bracken invasion (McMorrow et al., 2009; Anderson,

1986).

1.3.2 The competitive characteristics of bracken

The characteristics of bracken are typically those of a plant ‘competitor’ (Grime, 2001),

defined by Trinder et al. (2013) as “the capture of essential resources from a common,

finite pool by neighbouring individuals.” Competition is dynamic, driven by the

mechanisms of leaf, stem and root maintenance and functioning. The resources for

which plants compete are light, water, C02, oxygen and minerals (especially nitrogen and

phosphorus) (Niklas and Hammond, 2013; Trinder et al., 2013).

High productivity and a rhizome system of multitudinous buds that, early in the season,

grow rapidly and form wide-ranging frond canopies, help make bracken a successful

competitor (Hatcher and Batty, 2011; Marrs and Watt, 2006; McGlone et al., 2005; Marrs

et al., 2000). Beneath bracken fronds light penetration is minimal, and in temperate

regions, when fronds senesce in autumn, substantial leaf litter accumulations combine to

prevent the establishment of other species (Marrs and Watt, 2006; Le Duc et al., 2003;

Lawton, 1988). However, Watt (1976) found opportunities for other species to

(re)colonise where the rate of litter accumulation was greater than that of removal.

7

Bracken is especially successful in temperate regions where it maximises growth in mid-

summer (Hatcher and Batty, 2011; Marrs and Watt, 2006), and prospers in a wide range

of non-woodland habitats, thriving most of all in full sunlight, where its maximum

photosynthetic rate is much greater than other ferns (Hatcher and Batty, 2011; Marrs

and Watt, 2006; Dring, 1965). Light levels influence the abundance and vigour of

bracken, which alters its frond morphology and stand structure is response to the

amount of light received (Marrs and Watt, 2006; Dring, 1965), an adaptation of

developmental plasticity which contributes to its success (Hatcher and Batty, 2011;

Alonso-Amelot et al., 2001). Such traits have caused it to be regarded as a “native thug” –

a weed possibly worse than alien invasive species (Marrs et al., 2010; Marrs, 2005;

Pearman, 2004).

Furthermore, bracken is unpalatable and almost completely unaffected by grazing,

invertebrate herbivory, or pathogens (Marrs and Watt, 2006; Lee et al., 1986). In

addition bracken toxins forestall litter decay, reduce soil pH, and, it is suggested, further

prevent re-invasion by allelopathy (Marrs and Watt, 2006; Rasmussen et al., 2005;

Hartig and Beck, 2003; Gliessman and Muller, 1978), whilst stray seeds from potential

competitors can conceivably be consumed by small mammals residing within the litter

(Marrs and Watt, 2006; den Ouden, 2000; Gliessman and Muller, 1978).

On open ground bracken is recognised as a successional species; replacing existing species

and establishing itself throughout a habitat, preventing most re-invasion, and persisting

in relative stability (Rodwell, 1998; Marrs et al., 2000). Hence there is a requirement for

effective, meticulous long-term control, for which it is important to recognize the

dynamics that promote or limit bracken spread (Page, 1982).

1.3.3 The dynamics of bracken spread

When covering large areas bracken is often patchily distributed due to the plant’s cycle

of regeneration - with stands experiencing cyclical change and a degeneration phase -

and topographic conditions (Marrs and Watt, 2006; Watt, 1976). Typically degeneration

occurs in the ‘hinterland’ in which all five cyclical phases (grass-heath, pioneer, building,

mature, degenerate) exist together in a mosaic, located behind an “advancing margin,” a

zone of uniformly dominant bracken (Marrs and Watt, 2006; Watt, 1976; 1947; 1940,

Figure 1.2).

8

a)

b)

Figure 1.2 a) The regeneration cycle of bracken and b) the associated changes in litter and shoots (black = living, white = dead). Redrawn from Marrs and Watt (2006)

In the hinterland bracken becomes patchy, often in concentric rings, allowing re-

invasion by other species (Marrs and Watt, 2006; Watt, 1940, Figure 1.3). “The effects of

one phase become part cause of the next” explains Watt (1947). For example, bracken’s

litter, a component of its competitive success, can exacerbate degeneration in the

hinterland when the rhizome grows into deep litter and be exposed to the effects of frost

and drought. However, a return to bracken dominance is likely where grazing occurs

because species of greater palatability are preferentially selected (e.g. sedges, grasses,

tree shoots) (Hatcher and Batty, 2011; Watt, 1947).

Figure 1.3 The change in height and cover continuity, and depth of origin of fronds between the margin and hinterland in a bracken community. Adapted and redrawn from Watt (1947)

9

Topography determines where bracken grows (Marrs and Watt, 2006), and in general

bracken flourishes most on a southerly aspect (Atkinson, 1986). It is discernibly affected

by air and ground frosts, which therefore limit its range both in latitude and elevation

(Marrs and Watt, 2006; Watt, 1954). Early frosts can restrict the growing season and

hasten senescence, and growth ceases following the first lethal frost (Pakeman et al.,

1994). Hard frosts during winter are extremely damaging, particularly if there is

insignificant insulation provided by litter or snow, and especially if rhizome apices are

close to the surface (Pakeman et al., 1994; Pakeman and Marrs 1992; Watt, 1950).

Conversely, substantial accumulations of litter can prevent the emergence of fronds and

offer protection from late frosts assuming the rhizome has not grown into the litter

(Atkinson, 1986).

Bracken growth is also affected by wind exposure, and its distribution excludes very

exposed sites (Marrs and Watt, 2006; Paterson et al., 1997; McVean and Ratcliffe, 1962),

and by water-logging, a constraint due to a lack of aeration and nutrients rather than

saturation (Marrs and Watt, 2006; Poel; 1961; Summerhayes et al., 1924). Nowhere in

the UK is it impeded due to excessive rainfall or humidity (Marrs and Watt, 2006).

Although bracken can become water-stressed, notably in spring as it starts to grow, it is

exceptionally drought tolerant, especially if the rhizome is buried deep (Anderson, pers.

comm., 2014; Marrs and Watt, 2006).

Moss (1913) claimed that bracken’s altitudinal limit in the Peak District was about 457-

472 m, above which “it is local and rare,” although he added that Woodhead (1906) had

noted bracken reached 518 m (on Moss Moor in the Pennines), however “in England it is

quite exceptional to meet with the plant at such an altitude.” The recorded limit in the

UK (in the Scottish Highlands) is about 600 m. Globally, as latitudes decrease, bracken

grows at higher altitudes, reaching 3,000 m in parts of the Caribbean and South and

Central America (Marrs and Watt, 2006).

The northern fringes of the British Isles are above bracken’s latitudinal limit, and it is not

found in some East Anglian frost pockets, otherwise it has an almost ubiquitous

distribution up to its altitudinal limit (Figure 1.4). Temperature is regarded as an

important factor in distribution, and climate change warming may assist in extending

bracken’s range in the UK uplands (McMorrow et al., 2009; Marrs and Watt, 2006).

10

Figure 1.4 The distribution of bracken in the British Isles. Each dot represents at least one recording in a 10km square of the British National Grid. The black dots are recordings from 1950 onwards, and the white circled dots (e.g. western Ireland) are pre-1950 recordings. Note East Anglian frost pockets. From Marrs and Watt (2006)

1.4 The Uplands

“The uplands hold a special appeal… perhaps it is the scenery, the great character of the land... these places are a living expression of nature and our culture… with roots as much in the great geological epochs as in the tenacious grip of man.”

Des Thompson, Scottish Natural Heritage, 1995

Burt et al. (2002)

1.4.1 Uplands characteristics

Uplands boundaries differ according to local climate, topography and plant communities,

and absolute demarcations can be constraining (Bonn et al., 2009). However, globally,

elevations of over 200 m above sea level (asl), or land above the alluvial plain or stream

terrace, are commonly recognised as upland areas, and represent one-quarter of all land

(Orr et al., 2010; Clutterbuck, 2009; Stuki et al., 2004; Holmes and Duff, 1998). Uplands

cover 28% of UK land (~6.5 million ha), where they are semi-natural environments,

whose landscapes humans have played an important role in creating and maintaining,

above enclosed agricultural land, roughly 300 m, but with geographical variety (e.g. sea-

11

level in areas of northern Scotland), up to the montane zone, the ecological limit of trees,

approximately 600-750 m (Evans, 2009; JNCC, 2009).

However, there is a powerful connection between enclosure and less intensively farmed

uplands, and as a boundary the term ‘Less Favoured Areas’ (LFA) is used, introduced by

the European Union (EU) as a socio-economic designation for land naturally hindered

by, for example, harsh conditions, low soil fertility, abbreviated growing seasons,

inaccessibility, and remoteness, and as such LFAs correspond usefully with upland

ranges (Defra, 2011; Bonn et al., 2009). Agriculture, determined by climatic and abiotic

factors that affect plant growth, and grouse shooting, reliant upon renewing bird stocks,

function on the margins of economic viability (Dougill et al., 2006).

Figure 1.5 illustrates the extent of British uplands and upland marginal areas as

recorded in the Countryside Survey 2000 (Haines-Young et al., 2000).

Figure 1.5 Extent and location of the British uplands and marginal uplands recorded by the Countryside Survey 2000. From Haines-Young et al. (2000)

12

The JNCC (2009) list following habitats as present in UK uplands:

Dwarf shrub heath, dry to wet

Grassland, dry to wet

Moss and lichen heath

Flush and valley bogs

Blanket bogs

Crags, screes and boulder-fields

Springs, rills and flushes

Rivers

Lakes and tarns

The UK uplands contend with prevailing winds from the Atlantic ocean which impose an

oceanic temperate-humid climate characterised by cold conditions, wind and frost

exposure, excessive precipitation, humidity, persistent cloud cover with a corresponding

paucity in sunshine hours, and long-standing snow cover (Clutterbuck, 2009; Holden,

2009; Ratcliffe and Thompson, 1988). This has influenced the formation of rare and

unique habitats and floral assemblages, heritage landscapes, and ecosystem services of

national and international importance (Fontana et al., 2014; Clutterbuck, 2009; Reed et

al., 2009; Thompson et al., 1995). The services provided include the provision of food,

fuel (including renewable energies), timber, clean water supplies, flood mitigation,

carbon sequestration and climate mitigation, and tourism and recreation (Bonn et al.,

2009; Natural England, 2009a; Reed et al., 2009; Yallop and Clutterbuck, 2009, e.g. Table

1.3).

The present-day uplands are characterised primarily by heather/dwarf-shrub heath,

bogs, alpine communities and rough grassland, and, in England and Wales, a recent move

towards improved grassland dominance (Alday et al., 2013; Clutterbuck, 2009; JNCC,

2009; Carey et al., 2008; Haines-Young et al., 2000, Table 1.2). Sheep grazing, controlled

burning and forestry are the most widespread activities and highlight the persistent

effect of humans on the upland landscape (Gardner et al., 2009; Bonn et al., 2009; Carey

et al., 2008; Evans et al., 2006), although biodiversity is of naturally occurring (excepting

livestock) rather than cultivated species (Gimingham, 1975).

13

Table 1.2 Main upland habitats, recorded by the Countryside Survey 2000. From Haines-Young et al. (2000)

Upland habitat % GB upland % England and Wales upland

% Scotland upland

Bog 20.6 6.0 32.4 Dwarf shrub heath 19.1 16.1 21.6 Acid grassland 16.7 17.9 15.8 Improved grassland 14.2 28.1 2.9 Conifer woodland 10.7 5.8 14.7

1.4.2 The Peak District National Park (PDNP)

The PDNP (Figure 1.6), established in 1951, was the first national park in Britain

(PDNPA, 2008), and is one of the most visited protected areas (PAs) in the world with

more than 22 million annual visitors (Dougill et al., 2006).

Figure 1.6 Location of the Peak District National Park (circled) (PDNPA, 2014)

The PDNP uplands provide valuable ecosystem services typical of most upland

environments (Table 1.3), yet its central location within four governmental regions (East

Midlands, West Midlands, Yorkshire and Humber, and North West), with 16 million

people (~25% of UK population) within one hours travel, and with 38,000 people

resident, accentuate the PDNP’s significance (Bonn et al., 2009; Dougill et al., 2006).

14

Table 1.3 Examples of ecosystem services provided by PDNP. Adapted from Bonn et al. (2009 and 2009b)

Ecosystem Service PDNP contributions

Food and fibre 79% farmed for livestock, of which 80% sheep. Forestry mainly around reservoirs.

Water provision 55 reservoirs of more than 2 ha supplying local cities and towns with 450 ml per day.

Climate regulation, carbon storage and sequestration

30-40 mt carbon stored in moorlands, with the capacity to sequester up to 41,000 t of carbon annually.

Regulation of air quality Upland areas contribute to cooling the air, and comparatively high levels of precipitation deposit pollutants in upland regions, a process which cleans the air

Recreational and cultural value 600 km of public rights of way, 500 km2 of open-access land and more than 10 m visitors annually. 65% of upland moorlands managed for the grouse shooting industry. Unique habitat and species assemblages of international conservation value

Biodiversity Biodiversity is not only valued (conservation organisations) but is integral for healthily functioning ecosystem. Unique habitat and species assemblages of international conservation value

“A land of many uses” (Reed et al., 2013) - the natural landscape and biodiversity of the

PDNP is shaped, managed, and influenced by humankind, arguably to a greater degree

than any other UK upland environment (PDNP, 2012; McMorrow et al., 2009; Finney et

al., 2005). It is representative of the uplands and marginal montane regions of the UK

and Europe, facing social and environmental pressures that include land management

reform and transformation, habitat degradation, biodiversity loss, and the impacts of

climate change (Reed et al., 2013).

1.5 Bracken: extent and distribution, management and control

1.5.1 Bracken extent and distribution

Bracken is a major driver of land-use change in the uplands (Anderson and Radford,

1988), and with its widespread range, environmental tolerances, and competitive

nature, it is perhaps surprising that bracken distribution has not been more extensively

surveyed. There are no categorically accepted figures for bracken cover and spread in

the UK, and Marrs (pers. comm., 2014) regards data as poor and inconsistent. Previous

studies have used different methodologies which make comparisons problematic and

may explain the variation in estimations of bracken extent, which range between 1.2%

to 2.7% of the land surface (Cherrill and Lane, 1994). Taylor (1986) projected bracken to

cover 2.6% (6,338 km2), Brown (1986) estimated encroachment into moorland in the

North York Moors to be at least 120 ha per annum, and Varvarigos and Lawton (1991)

estimated bracken to cover 14% of LFAs in Northumberland and Durham (a likely over-

estimation). Yet these data (and other) are extrapolated from samples, and it is

15

suggested uplands-derived estimations of spread may not be applicable elsewhere in the

UK. For example, Marrs and Hicks (1986) chronicled minimal increases (0.2% p.a.) of

cover at Lakenheath Warren, Norfolk over a 60 year period despite an absence of

management. An illustration of upland bracken distribution from the PDNP in 2005 is

shown overleaf (Figure 1.7).

Figure 1.7 Bracken, its fronds evident as a green-blue hue, in the Peak District, 2005 (image courtesy of NE)

The Countryside Survey 2007 records a decline of 17.3% in ‘bracken broad habitat’

(containing 95% or more bracken) in the UK to 2,630 km2 (1% of land surface) between

1998 and 2007 (Carey et al., 2008, Table 1.4), and observe that bracken is primarily

being usurped by broadleaved, mixed, yew, and coniferous woodland, dwarf-shrub

communities, and acid grassland (Carey et al., 2008).

16

Table 1.4 Bracken broad habitat: estimated area (‘000s ha) in Great Britain 1984-2007 and the UK 1998-2007. Arrows denote significant change 1998-2007 (p <0.05), na = no available data. Re-tabulated from the Countryside Survey 2007 (Carey et al., 2008)

1984

1990

1998

2007

Change 1998-2007

GB 439 272 315 260 ↓ England na 93 109 91 Scotland na 107 121 132

Wales na 71 84 37 ↓ N. Ireland na na 3 3

UK na na 318 263

However, The Countryside Survey definition of ‘bracken broad habitat’ fails to take into

account bracken present at densities below 95%. It can be found from lone plants to

homogenous, extensive stands occupying large areas (Marrs and Watt, 2006). The

Survey of 2007 recorded 57,000 ha of bracken broad habitat in the English uplands, yet

none is shown for the PDNP in the Landcover Map (NERC, 2008).

Even sparsely distributed bracken has the potential to colonise or reinvade in upland

and lowland areas, but distribution is influenced by a diversity factors and there can be

considerable variation in the patterns of bracken spread over relatively small areas, for

example as observed in the Howden/Derwent National Trust estate (Anderson,pers.

comm., 2014). Marrs and Hicks (1986) postulate phases of dominance in the lowlands

are more overtly cyclical, in which the “massive degeneration” of bracken has significant

implications for bracken research and management. Data and observation indicate that

bracken management seemingly requires regional or even site specific solutions.

1.5.2 Bracken management

Bracken occupies a mid-successional stage; it spreads into early-successional

communities (e.g. grass, heath, and moor), and can prevent the establishment late-

successional woodland communities (Tong et al., 2006; Marrs et al., 2000). Therefore

two management pathways are available:

1. Reversal of succession in favour of priority habitats such as moorland communities

(Tong et al., 2006).

2. Forward towards woodland, more significant carbon sinks which in theory mitigate

for climate change and help the UK meet emission and stabilizations targets (e.g. EC,

2014b; UNFCCC, 2013).

17

Loss of upland habitats reduces plant diversity and decreases their capacity to provide

ecosystem services (Fontana et al., 2014). Restoration of moorland (through BAPs and

latterly Aichi Biodiversity Targets) is an International, European, and UK priority (JNCC,

2012), supported nationally by, for example, agri-environment and environmental

stewardship schemes (Natural England, 2009b), and restoration projects such as Moors

for the Future (MFP, 2008), the Moorland Regeneration Project (Moorland Association,

2002), and the Yorkshire Peat Partnership (NYMNPA, 2013). A policy option to maintain

and enhance moorland landscapes and conservation species was integral within 1987’s

Environmentally Sensitive Area (ESA) scheme (Natural England, 2014), and with ESA

funding bracken control increased hugely (Thomas, pers. comm., 2014).

The notional assumption is bracken control increases biodiversity, yet this depends

upon successful re-establishment of priority communities, like heather moorland, rather

than, for example, species-poor grassland (Maren et al., 2008; Pakeman et al., 1997;

Pakeman and Marrs, 1992). Effective restoration requires a programme for replacement

vegetation (Pakeman et al., 2008; Pienkowski et al., 1998), and an understanding of

bracken’s place in community succession (Marrs et al., 2000).

Bracken management requires the consideration of ecological factors. For example

reversal to moorland is difficult in soils enriched by bracken control (e.g. cutting,

burning), in which competitive or late-successional species will have an advantage

(Mitchell et al., 2000; Marrs, 1993). To suit communities such as heather that persist in

nutrient-poor podzols restoration needs to reduce soil nutrient content (Mitchell et al.,

2000; Hetherington and Anderson, 1998; Mitchell et al., 1997). However approaches

such as stripping topsoil and litter, adding or burning straw, and sowing nutrient-

extracting crops have often been unsuccessful, sometimes even increasing nutrient

levels (Marrs, 1985).

Bracken is a native species, with a rightful place and important role in the ecosystems in

which it inhabits, which at appropriate levels adds ecological and amenity value

(Robinson, 2007). Control should be undertaken at appropriate levels to achieve desired

aims and not degrade the environment (Natural England, 2008a, Table 1.5).

18

Table 1.5 Management options for bracken control (Natural England, 2008)

Management options

Situation

No control On steep slopes, heavily grazed areas, where regeneration of other habitats would be difficult or where wildlife considerations important

Conservation management

Where it is desirable to maintain low intensity or patchy cover for species of conservation value – limited and selective control

Limited control Where the aim is to reduce bracken cover or severely limit vigour and spread, but not eliminate

Eradication Where bracken is to be replaced with other vegetation

1.5.3 Bracken control

“There is no quick fix!”

Dr R.C. Robinson, IATG (2007)

Bracken’s morphology and life-traits make complete eradication extremely difficult,

which have resulted in extensive studies on its control (e.g. Alday and Marrs, 2014; Le

Duc et al., 2007; Pakeman et al., 2007; Robinson, 2007; Pienkowski et al., 1998; Lawton,

1988, and 1986; Williams and Foley, 1975). In essence there are two management

options; that of no control or a commitment to long-term, wide-ranging control

(Pakeman and Marrs, 1992). The first option risks widespread invasion, biodiversity loss

and diminished ecosystem heterogeneity, the second option is expensive, time-

consuming, dependent upon access, requires assiduous and thorough implementation,

yet still is not guaranteed to be successful (Natural England, 2008; Le Duc et al., 2007;

Pakeman and Marrs, 1992). Control involves long-term strategies, usually mechanical

and/or chemical, to achieve management aims which generally fall into 4 categories:

1. Low-level equilibrium grazing: cattle and horses ensure bracken kept in-check by

trampling, a sustainable, long-term option (note that in most upland areas cattle have

been replaced by sheep that are ineffective).

2. Containment: treatment of peripheries and boundaries to halt encroachment into

areas free of bracken.

3. Releasing: a forestry term denoting temporary bracken control during which other

flora can be established, such as saplings.

4. Clearance: eradication of bracken to free-up land for an alternative preferred

vegetation (Robinson, 2007).

The most permanent mode of eradication is by ploughing, however this is unsuitable and

inappropriate for most upland areas (Robinson, 2007). Here the best single method of

19

control by spraying asulam, a narrow-spectrum herbicide, onto bracken fronds

(Pakeman, 2012; Maren et al., 2008; Pakeman et al., 2007; Robinson et al., 2000, Figure

1.8). The combination of spraying and cutting, where access permits, is an effective

control (Marrs et al., 2000; Pakeman et al., 1999; Lowday, 1986).

Figure 1.8 The effects of control on the mean position (m) of the bracken front relative to study start in 1993 at Levisham Moor, North York Moors. Redrawn from Pakeman et al. (2007)

The spraying of asulam in the UK has remained relatively constant since 1990 at about

70km2 per year, and without this bracken areal extent has been projected to be 50%

higher (Pakeman, 2012). However, a European Union-wide ban came into force in 2013

making possession of asulam illegal, although Emergency Authorisations were granted

in the UK for the 2013 and 2014 seasons (Bracken Control, 2014).

Pakeman (2012) claims that without asulam spraying “the long-term consequences for

farming and conservation in the UK uplands are serious as control by aerial spraying is

the only option in many areas.” Asulam is “the only herbicide that can be used for

selective bracken control” until a superior replacement can be created Marrs (pers.

comm., 2013; 2014). Alternative herbicides such as broader-spectrum glyphosates are

viewed by Anderson (pers. comm., 2014) as poor alternatives. Their usual use is as

precise ‘follow-up’ sprays and they cannot (or should not) be used in the ‘gardening

phase’ to target small fronds amongst and beneath heather due to their sweeping

toxicity (Robinson, 2001). Wide-scale application is potentially much more harmful to

biodiversity.

20

1.6 Study rationale

“It is increasingly clear that a business as usual approach will be less and less able to address the challenges of the future and that we urgently need to consider how best to sustain the value of the uplands.”

Poul Christensen, Chairman of Natural England Natural England (2009a)

Originally part of open woodland communities, bracken has taken advantage of land-

cover changes to expand its range (Cox, 2007; Marrs et al., 2000; Watt, 1976), with

ecological, economic and cultural costs (e.g. Robinson, 2007; Pakeman and Marrs, 1992).

However, bracken extent is uncertain and rates and patterns of spread are unpredictable

(Pakeman et al., 1995), which, when aligned with changing agricultural and management

practices, and the potential impacts of climate change, intimate that a study of temporal

bracken distribution is timely. This study assesses change among variables of altitude,

aspect, and slope to determine whether bracken has encroached into habitats of greater

conservation and economic value during the study period. The study also attempts to

appraise the influences of climate change on past and future bracken distribution,

following on from unpublished findings (McAlpine, 2013).

1.6.1 Bracken spread in the PDNP

In the PDNP bracken is considered “a serious problem” (Tong et al., 2006). Its historical

distribution in the region was on the steeper slopes and sheltered depressions in the

more lowland areas, however by 1907 bracken control was necessary for grouse

management (Anderson and Radford, 1988), and Moss (1913) recorded spread into

upland heather moorland. By the 1980s it was estimated 6% (~30 km2) of moorland was

bracken-dominant (Anderson and Radford, 1988), a figure that has remained relatively

constant but with considerable variation in the patterns of spread and retreat

determined by climatic and edaphic conditions (Anderson, pers. comm., 2014). In 1990 it

was estimated bracken covered 2.5% of PDNP (35 km2) (Pakeman et al., 1996).

21

1.6.2 Climate change

“Cumulative emissions of CO2 largely determine global mean surface warming by the late 21st century and beyond. Most aspects of climate change will persist for many centuries even if emissions of CO2 are stopped. This represents a substantial multi-century climate

change commitment created by past, present and future emissions of CO2.”

Intergovernmental Panel on Climate Change (IPCC, 2013)

Hotter summers and warmer winters are predicted for the UK, with altered precipitation

patterns, extended summer droughts, increased winter precipitation, and more frequent

and intense extreme weather events (IPCC, 2013; Albert et al., 2012; McMorrow et al.,

2009; Jentsch et al., 2007). A warming climate presents opportunities for invasive

species, pests and diseases to extend their ranges which jeopardize more specialised

species (IPCC, 2013; Bishop, 2012; Stafford-Smith et al., 2012). This can result in

homogenised ecosystems of less conservation value, decreased long-term resilience, and

which provision fewer ecosystem services (Albert et al., 2012; Barnosky et al., 2012;

Ross et al., 2012; Strayer and Dudgeon, 2010).

1.6.2.1 Climate change and the uplands

Future climate projections for the uplands are very difficult due to highly diverse and

localised climatic conditions (Orr et al., 2008). Nonetheless temperatures are recorded

to be rising at a faster rate than lowlands (House et al., 2010; Burt and Holden, 2010; Orr

et al., 2008), notably winter minimum temperatures (Burt and Holden, 2010). By the

2050s ‘heatwave’ summers like 1990 and 1995 are expected once in every three years,

and summer rainfall will 37% less than present (Hulme et al., 2002). The PDNP

currently receives around 400 mm of summer rainfall, and decreases of 10% by 2020,

and 23-45% by 2080 are projected. In winter, during which there is usually more

precipitation, precipitation is projected to increase by 12-23% by 2080, with 50-90%

less as snowfall (McMorrow et al., 2009).

Clark et al. (2010) suggest rainfall patterns and growing season length are the limiting

climatic variables which determine the distribution of upland habitats and soils. Climate

change is likely to transform upland habitats into something more characteristic of the

lowlands, resulting in the loss of stored carbon, notably in peat bogs (Billett et al., 2010),

and other ecosystem services such as water purification (Cornell, 2010). Eastern English

uplands like the PDNP are considered particularly vulnerable in all but the highest places

(Clark et al., 2010).

22

Upland species adapted to harsh environments are more threatened by climate change

impacts (Callaghan et al., 1992). These unique and important ecosystems face increasing

isolation and threats from lowland ecosystems (IPCC, 2013; Albert et al., 2012; House et

al., 2010; Hickling et al., 2006). Environmentally sensitive, specialist species constrained

to higher elevations, in shrinking refuges, are suggested to act as sentinels to impacts

which may materialise at lower altitudes (Lenoir et al., 2008; Nogues-Bravo et al, 2007;

Hickling et al., 2006; Beniston, 2003). Therefore it is important to understand the drivers

and impacts of climate change (such as the effects on bracken distribution) so that

adaptation and mitigation responses can be implemented for both upland and lowland

areas (Orr et al., 2008, Table 1.6).

Table 1.6 The processes driven by climate and their associated upland impacts (re-tabulated from Orr et al., 2008)

Drivers Impacts

Physical component Changes to: duration, magnitude, frequency, suddeness

Changes to: resilience, vulnerability, flexibility, scale

Climate Temperature, precipitation, wind, radiation

Snow melt, frost-free period, flooding, drought, soil

Hydrology Total runoff and seasonal variation, flood, drought

Water quality and quantity, increased flood exposure, water supply reliability, wetland sustainability

Geomorphology Weathering, erosion, landslides, channel activity, sediment mobilisation

Sediment supply, transport, source channel connectivity, pollutant mobilisation

Soils Fertility, nutrient cycling, biophysical processes, erosion

Increased water pollution, loss of soil function, loss of carbon

Ecology Species migration, invasive species, extinction, ecological processes, primary productivity

Habitat loss and change, loss of ecosystem services (e.g. buffering, filtering, carbon sequestration)

With longer growing seasons, hotter, drier summers (PDNPA, 2011), and enhanced

levels of atmospheric CO2 projected in the uplands, plant biomass production and ET will

increase, and soil moisture content (SMC) will diminish (Keenan et al., 2013; Damgaard

et al., 2009; McMorrow et al., 2009). Consequently, plants and leaf litter will become

increasingly flammable, potentially leading to frequent and intense fires (McMorrow et

al., 2009; Werkman and Callaghan, 1999; Thomson et al., 1986). Additionally it is

expected there will be rising visitor numbers to national parks increasing the threat of

fire ignition (McMorrow et al., 2009, Figure 1.9). Resultant short- and long-term changes

in the properties of soil (e.g. increases in nutrient levels, pH, hydrophobicity and erosion;

decreases in porosity and soil organic matter) could make burned areas unsuitable for

23

the persistence of heather moorland and peat bog species (NWCG, 2001; Thomson et al.,

1986).

Figure 1.9 A model of the inter-relationships between climate change, visitors, the environment, and wildfires in UK uplands (adapted from McMorrow et al., 2009)

1.6.2.2 Bracken and climate change

Climate change presents bracken with opportunities for latitudinal and altitudinal

expansion. Enhanced levels of atmospheric greenhouse gases that cause global warming

also affect vegetation dynamics and interactions (Albert et al., 2011; Caporn and Emmet,

2009; Gordon et al., 1999; Werkman and Callaghan, 1999). Photosynthetic carbon

uptake is controlled directly by the factors that are predicted to change in the future,

atmospheric CO2 concentration, temperature and water availability (Morison & Lawlor

1999; Sage & Kubien 2007; Lawlor & Tezara 2009), and will therefore almost certainly

be affected by climate change.

Terrestrial plants fix CO2 into organic compounds through photosynthesis, of which the

carbon flux is gross primary production (GPP) (Gough, 2012; Beer et al., 2010). About

half GPP returns to the atmosphere as CO2 due to autotrophic respiration, mainly from

roots and mycorrhizal fungi, and what remains is net primary production (NPP), mostly

manifested as plant growth (Heimann and Recihstein, 2008; Heimann et al, 1998).

Higher levels of atmospheric CO2 increase the water efficiency of plants, as stomatal

conductance lessens and thus transpiration is suppressed (Zhu et al., 2012; Costa and

24

Foley, 2000), therefore increasing NPP. Increases in CO2 will potentially aid bracken

spread, but are considered less influential than rising temperatures (Pakeman and

Marrs, 1996), and water and nutrient availability (Albert et al., 2011, Reich et al., 2006;

Caporn et al., 1999), although there exists considerable uncertainty as ecosystem

responses are extremely complex (Albert et al., 2011).

1.6.3 Burn regimes

Controlled burn regimes are of greater consequence than wildfires in the PDNP and most

UK uplands, and burn scars are conspicuous (Yallop et al., 2009; Carr and Middleton,

2004, Figure 4.6). Burning assists bracken growth and spread as a) bare ground present

a colonising opportunity (Schneider and Fernando, 2010), and b) fronds exhibit a burst

of growth following fire, and their lifespan is usually longer unburned bracken (Roos et

al., 2010). Many upland areas are being burned more extensively than ever before, and it

is therefore timely to assess post-burn bracken invasion (e.g. Clutterbuck, pers. comm.,

2014; Yallop et al., 2009, Figure 1.10).

Figure 1.10 Visible bracken encroachment into burn scars on the North Yorkshire Moors 2002-2009. From Google Earth (2014): 54° 26’13.31 N 1° 01’50.14 W, elevation 389 m, view from 950 m.

25

1.7 Summary of study intentions

Historic bracken cover is mapped from available aerial imagery from the 1960s and

2005 in the PDNP to assess bracken extent and distribution with the intent to contribute

knowledge, especially the gaps regarding distributional data, and to answer the

following questions:

Has bracken cover changed in the PDNP?

If yes, how and where has it changed?

Is it possible to determine if climate has had an impact on any change identified?

26

2. Method

2.1 Introduction

In this study bracken extent was mapped for the years 1966, 1968 and 1971 (henceforth

the 1960s), contingent upon the availability of aerial imagery, and the ability to map

bracken cover with confidence, and the year 2005 (the most recent available aerial

imagery). The study assessed and compared bracken distribution during the periods in

order to distinguish patterns of change, and evaluate the importance of elevation, slope,

and aspect, with considerations for the impacts of climate change, historically and in the

future (historical climate data for PDNP spanning the study period were analysed), and

land management practices (see Discussion). Furthermore an estimate was made of

modern bracken extent and patterns of distribution in the PDNP based on the 2005

imagery.

This chapter explains the methods used and the following are described:

Peak District landscape characteristics and climate

Site selection

Aerial imagery

Orthocorrection in ERDAS Imagine 2013 and georeferencing in ArcGIS 10.1

Geographic Information Systems (GIS)

Mapping bracken in ArcGIS 10.0 and 10.1

Statistical analysis

2.2 The study area – the Peak District National Park

The PDNP covers 1,438 km2 and contains 509 km2 of upland moorland (PDNP, 2008).

The most extensive stands of bracken are found above enclosed farming in the Dark

Peak upland areas (Thomas, pers. comm., 2012; Anderson and Radford, 1988, Figure

2.1).

27

Figure 2.1 The PDNP, with the Dark Peak shaded in grey (from Reed et al., 2013)

2.2.1 Dark Peak landscape characteristics

The Dark Peak is characterised by a widespread upland moorland landscape, sparsely

populated and in which sheep farming (about 0.5 animals ha-1) and management for

grouse are the main human activities (Natural England, 2012; Bonn et al., 2009;

Sotherton et al., 2009). The habitat comprises relatively flat elevated plateaux of blanket

bog and dry heaths of dwarf shrubs (heather, bilberry Vaccinium spp., and crowberry

Empetrum nigrum), dissected by drainage channels (groughs) through the peat. These

contrast with steep, sheltered valleys (cloughs) and streams that support a diversity of

vegetation, including semi-natural broadleaved woodland and Sphagnum species

(Natural England, 2012b; PDNPA, 2009; Dougill et al., 2006).

Soils are acid podzols and gleys (Tong et al., 2006), overlaying Millstone Grit, hard

gritstone beds and shales that are exposed on steep slopes and tors (Natural England,

2012b). There are wide flooded valleys that form large reservoirs, around which

coniferous plantation dominates (PDNPA, 2009). The Dark Peak’s blanket bog and dry

heath habitats are of international conservation importance, and the region contains

40,530 ha of Sites of Special Scientific Interest (SSSIs), 39,730 ha of Special Areas of

Conservation (SACs) and 39,725 ha of Specially Protected Areas (SPAs) (PDNPA, 2014).

28

The region includes an agricultural characteristic of dry gritstone walls, and

commonplace at lower altitudes are enclosed fields, hedgerows, and gritstone farm

buildings (Natural England, 2012b). Bracken is found both in large stands and as a

component of a habitat mosaic that includes dwarf shrubs, grasses and woodland (Tong

et al., 2006). Figure 2.2 illustrates the landscape character types found within the PDNP.

Figure 2.2 Landscape character types in the PDNP (PDNPA, 2008)

29

2.2.2 Climate

The mean annual precipitation in the study area is 1050mm (Tong et al., 2006), and the

mean annual temperature from 1961-1990 was 8.20C, which from 1991-2003 rose to

8.90C (PDNPA, 2004). The Peak District has more rain, colder temperatures and fewer

hours of sunshine than the average for England and Wales, although these disparities

have recently become smaller (Caporn and Emmett, 2009; PDNPA, 2004, appendix ii).

However, within the Park exist local variations and micro-climates, and the northern

upland areas tend to be colder and wetter than the south and east (PDNPA, 2004).

Albertson et al. (2011) report no significant recent climatic trends in the PDNP, and

claim “the weather in the Peak District [has been] stationary” since 1976, apparently

contradicting findings in other studies (e.g. PDNPA, 2011; Caporn and Emmett, 2009;

Clutterbuck, 2009).

2.3 Site Selection

The selection of sites in PDNP was random in order to avoid bias (Ennos, 2012).

Originally this was achieved by delineating the study area using 1 km2 ordnance survey

(O/S) grids which prevents manipulation and is an accepted method (e.g. Birnie et al.,

2000; Pakeman et al., 1996). 75 1 km2 O/S grids - equating to approximately 5% of the

national park and thus enabling worthwhile estimations of bracken extent (Loveland et

al., 2002) - were randomly generated by R 3.1.1, “a system for statistical computation

and graphics” (Hornik, 2014).

Using the 2005 imagery, with the criterion that mappable 1 km2 O/S grids must contain

bracken, 31 study grids were identified from the original 75 1 km2 O/S grids in which

bracken would be mapped. However, due to 1) the unavailability of imagery, or 2) the

lack of confidence in bracken identification (Figure 2.3), comparison between the study

periods was limited to 13 grids (the sub-sample), an quantity from which useful data

could still arise.

30

Figure 2.3 An illustration of the problem of identifying bracken with confidence in the 1960s imagery: a) 1960s image with mapping for 2005 overlaid, b) the 1960s image in which bracken cannot be confidently identified, and c) 2005 imagery in which bracken was mapped with confidence

31

The 2005 sample and sub-sample grids are shown in Figures 2.4 and 2.5.

Figure 2.4 The 2005 sample: 31 randomly selected 1 km2 study grids in PDNP. The yellow box delineates the sub-sample area

Figure 2.5 The sub-sample of 13 1 km2 study grids in PDNP from which 1960s was mapped and compared with 2005 imagery

32

2.4 Aerial imagery

In environmental monitoring observing change is fundamental (Verbesselt et al., 2010;

Aplin, 2005; Coppin et al., 2004), and the interpretation of aerial imagery using GIS is the

most reliable method with which to map landcover and its temporal change (Rocchini

and Di Rita, 2005; Rocchini, 2004). GIS is the most practical and accurate method for

large-scale bracken monitoring (Holland and Aplin, 2013; Blackburn and Pitman, 1999)

and a fine-scale resolution (e.g. 25cm per pixel) allows the mapping of actual extent

rather than approximated, thematic landcover classifications (Jensen, 2007; Aplin,

2004).

The multi-temporal advantages of satellite imagery are tempered presently by limited

resolution quality, and on-the-ground field surveys, which whilst useful, are

substantially error-strewn (Holland and Aplin, 2013 Pakeman et al., 1996). The aerial

imagery spans 39 years from 1966 to 2005 (Table 2.1).

Table 2.1 Date, source, format and resolution of aerial imagery. B/W = black and white, RGB = red green blue (colour)

Date Source Format Resolution (cms/pixel)

21/07/1966 Ordnance Survey/PDNPA B/W 25 26/04/1968 Ordnance Survey/PDNPA B/W 25

25/08/1971 Ordnance Survey/PDNPA B/W 25 08/2005 PDNPA/NE RGB 25

2.4.1 Orthocorrection

The curvature of the Earth, the camera lens or sensor, and the angle from which a

photograph is taken, can all distort a raw image (ERDAS, 1999). Hence, geometrical

correction – of topography, curvature, camera distortion and tilt, altitudinal variations -

is necessary for accuracy (Obade and Lal, 2013; Clutterbuck, 2009; Jensen, 2007; Novak,

1992, Table 2.2). The increased accuracy of an object’s location, dimension and outline

allows a collage of images to be meshed together and coincided with precision (Boccardo

et al., 2004; ERDAS, 1999), also taking into account relief diversity (Clutterbuck, 2009;

Bailey and Pearson, 2007).

33

Table 2.2 When and why orthocorrection is necessary, adapted from ERDAS (1999)

Orthocorrection is needed to:

Compare pixels in scene-to-scene applications

Develope GIS data for GIS models

Overlay an image with vector data (e.g. real world features using point, line or polygon)

Compare images originally at different scales

Extract accurate measurements of distance and area

Collate a mosaic of images

Create accurate scaled photomaps

Raw aerial photographs displace objects from their true location which orthocorrection

corrects to their proper location (Jensen, 2007; Novak, 1992). The 2005 imagery was

supplied corrected, and from these six 1960s images were orthocorrected. Raw images

were corrected using aero-triangulation (Jensen, 2007). X and Y location and Z altitude

information is available for every pixel and these data form a three-dimensional system

called the map projection (Clutterbuck, 2009). Landmarks plainly visible in both the raw

and 2005 imagery were identified as Ground Control Points (GCPs). Multiple GCPs

spread evenly throughout the image were collected per raw image (9 minimum). The

precision and accuracy with which these are obtained is decisive in attaining a properly

corrected image (Wangfei et al., 2009). Mapping consistency was over 99%, or less than

one pixel (<250mm). A comparison between raw and corrected imagery is shown in

Figure 2.6.

Figure 2.6 Orthocorrection: a) a raw aerial photograph and b) orthocorrected.

34

2.4.2 Georeferencing

Imagery covering the remaining study grids was georeferenced. Raster data from raw

aerial imagery are assigned spatial information so that GIS can display the data in their

correct location on the Earth, however the Earth’s curvature, the camera lens, and the

photograph angle are not taken into account by georeferencing, affecting accuracy,

notably in areas of varied topography. This to some extent can be countered by

accurately positioning numerous GCPs. Moreover, the imagery remains accurately

corrected to within a few pixels (50-75cm), and sufficient for this study in which data are

studied according to specified zones rather than for specific points (Clutterbuck, pers.

comm., 2014, Figure 2.7).

Figure 2.7 ArcMap screen grab in which a georeferencenced 1960s image is compared with orthocorrected 2005 imagery from which its GCPs were taken. The images appear to fit well. Red box delineates a 1 km2 study grid. (ArcMap is the main ArcGIS application in which imagery is explored, mapping is undertaken, datasets are created etc.)

2.5 Geographic Information Systems (GIS)

GIS is “a system for capturing, storing, checking, integrating, manipulating, analysing

and displaying data which are spatially referenced to the earth” (O’Riordan, 2000). It is a

tool for integration with which different information can be related (IGIC, 2014), such as

population trends, habitat loss, crime rates, or temporal and spatial bracken distribution.

GIS can be used to answer questions such as those posed in Table 2.3:

35

Table 2.3 Typical questions GIS can answer, adapted from O’Riordan (2000)

Type of Question Example

Identification What is at a specific location? Location Where does a specific feature occur?

Trend What features have changed over time? Routes Which way to travel between points? Pattern Is there an association? What if? What will happen if a particular change occurs?

2.6 Mapping bracken cover in ArcGIS

2.6.1 Identification of bracken

Bracken displays considerable seasonal variation (Holland and Aplin, 2013; Birnie et al.,

2000; Pakeman et al., 1996). Between July and August frond growth is at its peak, and

bracken is identifiable due to its blue-green hue and rippled texture. Following the first

frosts (usually mid-October) bracken turns brown, as it senesces, until late March or

early April in the uplands (Birnie et al., 2000; Pakeman et al., 1996). This provides a vivid

contrast to other upland vegetation which can aid identification in colour or black and

white imagery (Figure 2.8). Imagery of poor quality and resolution, and shadows make

identification more problematic.

Figure 2.8 Bracken identification: a) shows senesced bracken amongst heather in 1968, evident even in b/w, grasses at the bottom of the image appear more grey and wispy, b) shows the same area in 2005, bracken’s blue-green hue standing out. Note the darker green grass in the bottom right of the image.

Imagery in which bracken was growing (1966, 1971, and 2005) and was senesced

(1968) was mapped in this study (Table 2.4).

36

Table 2.4 Month photograph taken and bracken growth-stage

Year Month photograph taken Bracken growth stage

1966 July Growing 1968 April Senesced 1971 August Growing 2005 August Growing

2.6.2 Polygon mapping

A polygon is a closed line or set of lines that define boundaries, area, and location, and

that represent an entity in terms of its connected X and Y coordinates (ArcGIS, 2011;

Burrough and McDonnell, 1998). Label points link a polygon to its attributes, and hold

information about its boundary and area (NDIIPP, 2011; Jensen, 2007). Points were

entered around bracken using a mouse, and the areal extent, the sum, and mean of each

polygon (vector data) were displayed in the attribute table of a shapefile (an Esri vector

data storage format for storing the location, shape, and attributes of geographic features)

being used (e.g. Figure 2.9 from 2005 bracken cover file). Vector data spatially describe

features like polygons or lines, whereas raster data are based on pixels that are each

assigned a specific value from e.g. aerial and satellite imagery, digital maps and pictures

(Arc GIS, 2008).

Figure 2.9 ArcGIS mapping: a) polygon mapping of bracken, b) mapped outline, c) mapped area representing cover in summer, 2005. Scale 1:400.

2.6.3 Image uncertainty

In the substantial majority of the imagery identification of bracken was straightforward,

however there were areas of uncertainty. Predominantly this occurred where bracken

existed in a mosaic with moor grasses, bilberry, and where in the past it had been

controlled. This uncertainty could, for the most part, be removed by:

cross-checking with recent historical imagery (e.g. Bing Maps and Google Earth)

manipulating the colour spectrum to alter the definition

37

The 1960s black-and-white imagery presented the most uncertainty, and hence resulted

in a smaller dataset (the sub-sample). Areas of uncertainty within this dataset were

cross-checked with other historic imagery and the previously-mapped 2005 imagery.

2.6.4 Altitude, aspect and slope data

The study aimed to assess bracken change between the study periods, and breakdown

any change by the categories of elevation, aspect and slope. It was necessary to limit the

variables to a workable scale:

altitude was measured zones of 50 metres (from 100-550 m)

aspect was measured in four zones (north, east, south and west)

slope was measured in zones of 5 degrees (from 0-40°)

This was achieved in ArcGIS by merging the mapped areal data, and intersecting them

with created shapefiles of the three categories (geoprocessing → intersect). Once

updated, the areal data for each category was found in the attribute table of the newly

combined shapefile of category and mapped study period (e.g. “ 2005 all bracken

aspect”, Figure 2.10).

Figure 2.10 ArcMap screen-grab showing the attribute table for “2005 all bracken aspect” shapefile

38

2.7 Statistical Analysis

2.7.1 Assessment of mapping consistency

To assess mapping consistency three discrete bracken stands (outside the study grids)

from 2005 imagery were subjected to four repeat mappings at the scale 1:400 on three

separate occasions: before the commencement of mapping the study sample, at mid-

point, and upon completion.

2.7.2 Bracken coverage in the PDNP in 2005

The 2005 sample is analysed in order to estimate bracken extent and distribution by the

categories of altitude, aspect and slope within the PDNP.

The three categories are organised in predetermined zones:

Altitude range (m) in 9 zones increasing in increments of 50 m from 100-550 m

Aspect in 4 zones: north, east, south and west

Slope in 8 zones increasing in increments of 5° from 0-40°

2.7.3 Bracken data for the 2005 sample and sub-sample

The sub-sample data cover all the aspect and slopes zones of the 2005 data, however in 3

altitudinal zones no bracken was recorded (100-150 m, 150-200 m and 500-550 m), and

therefore are excluded from the sub-sample datasets, and comparisons between the

2005 sample.

2.7.3.1 The Pearson correlation test

The Pearson correlation, in which normally distributed data are tested for their linear

association (where 1 equals total correlation and zero represents no correlation), was

performed in MINITAB 17, statistical software for data analyses, to test the correlation of

the two data (Ennos, 2012).

2.7.3.2 The Kolmogorov-Smirnov test

The Kolgomorov-Smirnov (K-S) test was performed in MINITAB 17 to assess whether

the distribution of the 2005 sample data were significantly different from a normal

distribution (p = >0.05) (Ennos, 2012).

2.7.3.3 Sub-sample analysis

The sub-sample data were analysed by category, and aspect and slope were also

considered by elevation, because a) climate change predicts species ranges will

39

experience altitudinous expansion (IPCC, 2013), and b) the 2005 sample suggests

altitude is an important factor in the distribution of bracken.

2.7.3.4 Changes in bracken cover at slopes related to altitude1960s-2005

Due to the small areal coverage of bracken at individual slope zones in the sub-sample

data change was analysed by consolidating two neighbouring zones:

0-5° and 5-10°, henceforth the low slope

10-15° and 15-20°, henceforth the low-mid slope

20-25° and 25-30°, henceforth the steep-mid slope

30-35° and 35-40°, henceforth the steep slope

2.8 Historic climate data

Historic climate data was analysed by looking at summer (April-September) and winter

(October-March) decadal means for maximum and minimum temperature, frost days,

precipitation and sunlight hours. The data were created from Met Office Sheffield station

historic climate data (location 4339E 3872N, 131 m). It is acknowledged that this data is

from just outside the PDNP boundary, at a lower altitude that most study grids, however

it was considered satisfactory to highlight general local climatic trends.

40

3. Results

3.1 Assessment of mapping consistency

For the three stands of bracken that were each mapped individually at the start,

approximately halfway through and upon completion of the project, the overall

concordance of area of stand mapped was 99.6% (Table 3.1). This suggests that any

modification or improvement in image interpretation skill during the study did not have

significant influence on the results.

Table 3.1 Three bracken stands mapping consistency assessment results

Bracken stand

First assessment (ha)

Second assessment (ha)

Third assessment (ha)

Range (ha)

Concordance (%)

1 1.171 1.184 1.177 0.013 99.5 2 2.259 2.268 2.276 0.016 99.6 3 0.427 0.424 0.426 0.003 99.6 Total 3.86 3.88 3.88 0.02 99.6

3.2 Bracken coverage in the PDNP in 2005

From the 5% sample of the PDNP mapped in this study, a total of 309.5 ha of bracken

were identified (Table 8.2). Assuming that the random sample is representative of

bracken distribution across the remainder of the PDNP, these data suggest that bracken

may have covered an area of 6190 ha (62 km2) in 2005.

When examined in relation to altitude, nearly 90% of bracken was found between 250-

450 m, with more than 55% between 300 and 400 m. Interestingly these data are

normally distributed (K-S test 0.25; p = 0.09), and suggest that bracken in the PDNP

favours these altitudes (Table 3.2; Figure 3.1, the graphs for all test results are found in

appendix iii).

Table 3.2 Proportion (%) of bracken by altitudinal zones (50 m) in PDNP in 2005

Altitudinal range (m) 2005 ha

Proportion in zone (%)

100-150 2.9 1.0

150-200 3.7 1.2

200-250 15.8 5.1

250-300 59.4 19.2

300-350 88.2 28.5

350-400 82.3 26.6

400-450 46.6 15.1

450-500 10.2 3.3

500-550 0.4 0.1

41

0

10

20

30

40

50

60

70

80

90

100

100-150 150-200 200-250 250-300 300-350 350-400 400-450 450-500 500-550

Altitudinal range (m)

Are

a (h

a)

Figure 3.1 Proportion of bracken in altitudinal (50 m zones) in PDNP in 2005

When this sample is examined in relation to aspect, there appears to be less significant

variation than that noted with altitude. Although the largest amount of bracken was

found on south-facing aspects (32%), comparable amounts of bracken were found on

other aspects (19-25%; Table 3.3; Figure 3.2). These data are also normally distributed

(K-S test 0.21; p = >0.15).

Table 3.3 Proportion (%) of bracken by aspect in PDNP in 2005

Aspect 2005 ha Proportion in

zone (%)

North 69.2 22.3

East 61.2 19.7

South 100.4 32.4

West 79.2 25.6

0

20

40

60

80

100

120

North East South West

Aspect

Are

a (h

a)

Figure 3.2 Areal extent (ha) of bracken by aspect in PDNP in 2005

42

When bracken extent by aspect was broken down into altitudinal zones it confirmed that

90% bracken cover was found between 250-450 m. Interestingly the west-facing aspect

is the only aspect in which bracken was observed at all altitudinal zones, and which

showed an increase in areal extent from 350-400 m to 400-450 m (Table 3.4; Figure 3.3).

Table 3.4 Distribution of bracken cover by aspect at altitudinal zones (50 m) in PDNP, 2005

Altitudinal range (m) North East South West

100-150 0.1 0.0 2.2 0.6

150-200 0.5 0.0 2.7 0.5

200-250 2.1 8.7 3.4 1.6

250-300 13.4 13.5 20.3 12.2

300-350 20.4 15.6 28.8 23.4

350-400 21.7 12.9 29.5 18.2

400-450 8.8 6.6 12.8 18.5

450-500 2.2 4.0 0.2 3.8

500-550 0.0 0.0 0.0 0.4

0

5

10

15

20

25

30

35

100-150 150-200 200-250 250-300 300-350 350-400 400-450 450-500 500-550

Altitudinal range (m)

Are

a (h

a)

North

East

South

West

Figure 3.3 Aspects in which bracken cover (ha) was observed at altitudinal zones (50 m) in PDNP, 2005

In relation to slope, 88% of bracken in the sample was found on slopes ranging from 5-

25°, with 59% on slopes from 5-15°. Only 3.5% of bracken was on slopes steeper than

25°. These data are normally distributed (K-S test 0.16, p = >0.15), but are skewed

towards lower angle slopes suggesting that in the PDNP bracken favours slopes of 5-25°

(Table 3.5; Figure 3.4).

43

Table 3.5 Proportion (%) of bracken by slope in PDNP in 2005

Slope (°) 2005 ha Proportion in

zone (%)

0-5 27.4 8.9

5-10 82.0 26.5

10-15 99.9 32.2

15-20 51.5 16.6

20-25 38.4 12.4

25-30 8.9 2.9

30-35 1.8 0.6

35-40 0.05 0.02

0

20

40

60

80

100

120

0-5 5-10. 10-15. 15-20 20-25 25-30 30-35 35-40

Slope

Are

a (h

a)

Figure 3.4 Areal extent (ha) of bracken by slope in PDNP in 2005

When bracken cover by aspect was analysed by altitudinal zones 78% was found on less

steep slopes 5-25° in the altitudinal range of 250-450 m, and 65% of bracken was found

on slopes from 5-15° between 300-450 m. The 250-300 m zone was more evenly

distributed, yet slopes from 5-15° still accounted for 49% of bracken found (Table 3.6;

Figure 3.5).

Table 3.6 Distribution of bracken cover by slope at altitudinal zones (50 m) in PDNP, 2005

Altitudinal range (m) 0-5° 5-10° 10-15° 15-20° 20-25° 25-30° 30-35° 35-40°

100-150 1 1.2 0.7 0.0 0.0 0.0 0.0 0.0

150-200 1.6 1.3 0.8 0.0 0.0 0.0 0.0 0.0

200-250 1 6.9 4.4 1.7 1.6 0.1 0.0 0.0

250-300 9.4 14.7 14.3 7.8 11.6 1.5 0.02 0.05

300-350 2.7 23.4 26 16.2 9.7 1.4 0.2 0.0

350-400 0.4 20.9 31.2 12.3 10.3 3.7 1.1 0.0

400-450 0.05 11.8 20.6 7.8 3.5 2.1 0.4 0.0

450-500 0.0 1.7 1.8 5.3 1.3 0.001 0.0 0.0

500-550 0.0 0.0 0.0 0.3 0.1 0.0 0.0 0.0

44

0

5

10

15

20

25

30

35

100-150 150-200 200-250 250-300 300-350 350-400 400-450 450-500 500-550

Altitudinal range (m)

Are

a (h

a)0-5°

5-10°

10-15°

15-20°

20-25°

25-30°

30-35°

35-40°

Figure 3.5 Slope zones in which areal extent (ha) of bracken was observed at altitudinal zones (50 m) in PDNP in 2005

3.3 Bracken extent by combined aspect and slope at altitudinal zones

The importance of altitude as a factor has been mentioned, and thus, when aspect and

slope were analysed by altitude, trends appeared to be confirmed and some interesting

results were observed (Table 3.7). Bracken on north- and east-facing aspect/slope (a/s)

areas were generally restricted to above 200 m. This was most notable between 250-400

m on slopes of 5-20°; where 9 of 38 north-facing a/s areas accounted for 62% of bracken

cover. Between 200-400 m on slopes of 5-20° 12 of 31 a/s areas accounted for 66% on

an east-facing aspect.

The south- and west-facing a/s areas were better represented throughout all altitude

zones, however bracken distribution also favoured higher elevations, from 250-450 m.

Of the largest a/s areas, 5 were observed in the south-facing zone, with a largest area of

17.6 ha.

There was noticeable relationship between the presence of bracken in a/s areas and

altitudes between 200-450 m, most clearly between 5-25°.

45

Table 3.7 Bracken extent (ha) by aspect and slope in relation to altitude in the PDNP, 2005 (ten top ranked in bold, to 1 decimal point)

Altitudinal range (m)

Aspect Slope(°) 100-150

150-200

200-250

250-300

300-350

350-400

400-450

450-500

500-550

0-5 0.5 0.1 0.1 1.2 1.3 0.2

5-10 0.1 0.1 0.6 3.5 9.3 3.5 0.9

10-15 0.8 6.4 6.2 5.0 2.3 0.4

North 15-20 0.3 2.3 6.0 3.1 0.9 0.7

20-25 0.7 2.7 3.0 0.8 0.7 0.2

25-30 0.1 1.2 0.4 1.3 0.7

30-35 0.8 0.4

35-40 0.1

0-5 0.4 1.1 3.8 0.1 0.1

5-10 6.3 7.1 3.7 3.1 1.1 0.6

10-15 1.6 2.5 3.5 3.1 0.2

East 15-20 0.4 3.7 3.4 3.1 2.6

20-25 0.1 0.6 0.6 2.3 0.4 0.6

25-30 0.3 0.6 0.1

30-35 0.1

35-40

0-5 0.6 0.7 0.5 6.4 4.1 1.0 0.1

5-10 0.9 1.2 0.3 4.0 10.7 4.9 1.2

10-15 0.7 0.8 1.1 2.2 6.4 17.6 8.5

South 15-20 0.8 1.5 3.2 2.0 2.0 0.1

20-25 0.7 5.9 4.3 2.8 0.6

25-30 0.3 0.1 1.1 0.4

30-35 0.2 0.2 1.2

35-40

0-5 0.4 0.5 1.7 2.2 0.2

5-10 0.3 0.1 0.2 3.1 5.5 3.7 5.9 0.2

10-15 0.9 3.1 9.8 5.5 8.1 1.3

West 15-20 0.3 1.8 3.3 3.8 1.7 1.8 0.3

20-25 0.2 2.3 1.8 4.4 1.8 0.5 0.1

25-30 0.6 0.6 0.9

30-35 0.2

35-40

3.4 Representation of bracken in sub-sample data

Bracken mapped in 2005 in the sub-sample area (13 sites) occurred on all aspects and

the full range of slopes observed in the full sample. However, bracken was not found at 3

altitudinal zones (100-150, 150-200 and 500-500 m) (Table 3.8; Figure 3.6).

46

Table 3.8 2005 sample and sub-sample datasets for bracken in 50 m zones in PDNP (nc = not covered)

Altitudinal range (m)

2005 full dataset (ha)

2005 sub-dataset (ha)

2005 full dataset proportion in

zone (%)

2005 full dataset proportion in

zone (%)

100-150 2.9 nc 1.0 nc

150-200 3.7 nc 1.2 nc

200-250 15.8 12.1 5.1 10.3

250-300 59.4 24.2 19.2 20.6

300-350 88.2 32.1 28.5 27.3

350-400 82.3 30.5 26.6 22.6

400-450 46.6 13.3 15.1 11.3

450-500 10.2 5.5 3.3 4.6

500-550 0.4 nc 0.1 nc

Total 309.5 117.6

0

10

20

30

40

50

60

70

80

90

100

100-150 150-200 200-250 250-300 300-350 350-400 400-450 450-500 500-550

Altitudinal range (m)

Are

a (h

a)

2005sample

2005 sub-sample

Figure 3.6 The 2005 sample and sub-sample of bracken in altitudinal zones (m) in PDNP

The area of bracken in each altitudinal zone mapped in both the 2005 sub-sample and

the complete 2005 sample are very strongly correlated (R2 = 0.93, Pearson correlation =

0.96, p = 0.002; Figure 3.7). This indicates that although the sub-sample does not allow

assessment of change below 200 m or above 500 m, the data provide valid

representation for the 200-500 m zone.

47

3530252015105

90

80

70

60

50

40

30

20

10

0

2005 sub-dataset

20

05

sam

ple

Scatterplot of 2005 sample vs 2005 sub-dataset

Figure 3.7 Correlation between the full 2005 sample and sub-sample in altitudinal zones (m) in PDNP, illustrating a linear association

Correlations were less strong between the sample and sub-sample for aspect and slope,

although slope was strongly correlated, but may have slightly under-represented

bracken on slopes of 10-15° (Figures 3.8 and 3.9).

4035302520

100

90

80

70

60

2005 sub-sample

20

05

sam

ple

Scatterplot of 2005 sample vs 2005 sub-sample

Figure 3.8 Correlation between the full 2005 sample and sub-sample for aspect (R2 = 0.60, Pearson = -0.775, p=0.225)

48

403020100

100

80

60

40

20

0

2005 sub-sample

20

05

sam

ple

Scatterplot of 2005 sample vs 2005 sub-sample

Figure 3.9 Correlation between the full 2005 sample and sub-sample for slope (R2 = 0.74, Pearson = 0.86, p=0.006)

The 2005 sub-sample (K-S test 0.18, p = >0.15) and corresponding 1960s data (K-S test

0.20, p = >0.15) showed normal distributions.

3.5 Change in bracken cover 1960s-2005

A total of 87.5 ha of bracken were mapped from the 1960s imagery compared to 117.6

ha for the same area 2005, an increase of about 0.9% p.a. over the study period (c. 35

years).

Table 3.9 shows change in bracken areal extent between the study periods. Bracken

spread by only 1 ha when the 5-25° zones were discounted, of which 88% was observed

between 250-400 m, and 77% between 300-400 m. The greatest increase was observed

on the north-facing aspect, between 5-25° and 300-400 m, which accounted for 53% of

the sub-sample increase, and where 5 of the top ten ranked areal increases were found,

with 3 more found on the east facing aspect between 5-15° and 300-400 m.

The west-facing aspect showed the largest single decrease of 2.69 ha between 15-20°

and 200-250 m, however a 2.79 ha increase was recorded between 5-25° and 300-350

m. The south-facing aspect was the only aspect in which there was almost parity in a/s

area which gained and lost bracken extent during the study period, and in which

patterns of change were concealed.

49

Table 3.9 Change in bracken extent (ha) by aspect and slope in relation to altitude in the PDNP, 1960s-2005 (in bold are the top ten ranked changes between study periods, boxed are the areas of greatest extent in 2005 (to 2 decimal points)

Altitudinal range (m)

Aspect Slope(°) 200-250 250-300 300-350 350-400 400-450 450-500

0-5 -0.27 0.18 0.51 0.15 -0.09

5-10 0.22 0.68 1.58 0.83

10-15 -0.02 0.89 1.21 2.91 0.93 0.04

North 15-20 -0.38 0.6 1.95 1.69 0.20 0.23

20-25 0.02 0.31 1.76 0.29 0.12 0.20

25-30 0.04 0.24 0.82 0.59

30-35 0.61 0.25

35-40 0.05

0-5 -0.02 0.44 0.24 0.04

5-10 0.47 0.66 1.73 0.80 0.20 0.15

10-15 -0.25 0.92 1.46 1.34 0.31

East 15-20 0.00 0.23 0.43 0.45 0.42 1.3

20-25 -0.01 0.02 0.35

25-30 0.26 0.22 0.01

30-35 0.04

35-40

0-5 0.02 -0.01 -0.02 0.08

5-10 0.23 0.06 0.00 0.09

10-15 0.11 0.04 0.81 0.23 -0.13

South 15-20 0.62 -0.90 -1.45 -0.22 0.03

20-25 0.19 -1.17 -0.09 0.02

25-30 -0.40 0.06

30-35 -0.23 -0.18

35-40

0-5 0.03 0.1 0.02 0.01

5-10 0.07 0.06 0.66 0.05 -0.23

10-15 0.44 0.04 0.60 0.06 -0.28

West 15-20 -2.69 -0.02 0.98 -0.29 -0.07

20-25 -0.21 0.55 0.10 0.43

25-30 -0.97 0.01 0.10 0.03

30-35 -0.01 0.18

35-40

3.5.1 Changes in bracken cover at altitudinal zones 1960s-2005

For all altitudinal zones assessed, except the 200-250 m zone, increases were observed

during the study period (Table 3.10; Figure 3.10).

Table 3.10 Bracken cover and change at altitudinal zones (50 m), 1960s-2005 in PDNP

Altitudinal range (m)

1960s (ha)

2005 (ha)

Change (ha)

% change

% annual change (c.35 yrs)

200-250 14.4 12.1 -2.4 -16.6 -0.5

250-300 22.3 24.2 1.9 8.3 0.2

300-350 19.5 32.1 12.6 64.5 1.8

350-400 19.0 30.5 11.5 60.6 1.7

400-450 9.4 13.3 3.9 42.2 1.2

450-500 2.9 5.5 2.6 87.9 2.5

Total 87.5 117.7 30.1 25.6 0.7

50

The greatest areal change was observed between 300 and 400 m (24 ha) equating to

61% change (1.7% p.a.) and corresponded with the area of most extensive bracken

mapped in the 2005 sample. The decrease in cover observed in the 200-250 m zone was

by less than 0.5% (Figures 3.10).

-4.0

-2.0

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

200-250 250-300 300-350 350-400 400-450 450-500

Altitudinal range (m)

Are

a (h

a)

Figure 3.10 Change in area (ha) of bracken at altitudinal zones (50 m) from the 1960s to 2005 in PDNP

Assessed as proportional change, however, the largest increase was observed at 450-500

m (2.5% p.a.; Figure 3.11).

-1

-0.5

0

0.5

1

1.5

2

2.5

3

200-250 250-300 300-350 350-400 400-450 450-500

Altitudinal range (m)

Ch

ange

(%

p.a

.)

Figure 3.11 Annual change (%) of bracken at altitudinal zones (50 m) from the 1960s to 2005 in PDNP

51

3.5.2 Changes in bracken cover at aspect 1960s-2005

Bracken from the 1960s imagery was observed to be relatively evenly distributed in the

aspect zones, with a range of 5.2 ha. The range from 2005 imagery increased to 22 ha.

The greatest areal and percentage change was observed for the north-facing aspect (20.4

ha, 5.9% p.a.). The east-facing aspect also recorded a large increase (11.6 ha, 4.2% p.a.).

The south- and west-facing aspects recorded small decreases. A paired T-Test found the

difference between the 1960s and 2005 bracken distribution was not significant (T = -

1.44, P = 0.24). However, the 2005 sample and sub-sample were not closely correlated

(Pearson correlation = -0.78, p = 0.22) (Table 3.11).

Table 3.11 Bracken cover and change at four aspects, 1960s-2005 in PDNP

Aspect 1960s (ha)

2005 (ha)

change (ha)

% change

% annual change (c. 35 yrs)

North 18.9 39.3 20.4 207.6 5.9

East 24.1 35.7 11.6 148.0 4.2

South 23.4 21.8 -1.6 -7.4 -0.2

West 21.1 20.8 -0.3 -1.3 -0.04

Total 87.5 117.6 30.1 34.4 1.0

3.5.3 Changes in bracken cover at aspect related to altitude 1960s-2005

The largest areal and percentage change for the north-facing aspect was observed above

250 m, most notably at 250-350 m where bracken cover nearly doubled during the study

period (>5% p.a.). The largest areal increase (8.4 ha) was observed in the 350-400 m

zone. Above 400 m the areal increases were smaller than at lower altitudes, however

increases of over 2% p.a. were observed (Table 3.12; Figure 3.12).

Table 3.12 Bracken cover and change at north aspect, 1960s-2005 in PDNP

Altitudinal range (m) Aspect

1960s (ha)

2005 (ha)

Change (ha)

% change

% annual change (c. 35 yrs)

200-250 North 0.2 0.2 0.00 0.0 0.0

250-300 North 1.0 3.1 2.1 218.9 6.3

300-350 North 3.6 9.6 6.0 169.5 4.8

350-400 North 9.4 17.8 8.4 89.9 2.6

400-450 North 3.4 6.5 3.1 89.2 2.6

450-500 North 1.4 2.2 0.8 53.1 1.5

52

0

1

2

3

4

5

6

7

200-250 250-300 300-350 350-400 400-450 450-500

Altitudinal range

Ch

ange

(%

p.a

.)

Figure 3.12 Change (% p.a.) of bracken at a north aspect from the 1960s to 2005 in PDNP

The largest areal and percentage change for the east-facing aspect was observed in the

300-350 m zone (4.3 ha, 5% p.a.). Also, large increases were observed in the 350-400 m

and 450-500 m zones (3.1 ha, 2.6% p.a. and 1.8 ha, 3.5% p.a. respectively) (Table 3.13

and Figure 3.13).

Table 3.13 East aspect for bracken in the 1960s and 2005 in PDNP

Altitudinal range (m) Aspect

1960s (ha)

2005 (ha)

Change (ha)

% change

% annual change (c. 35 yrs)

200-250 East 8.6 8.2 -0.4 -5.1 -0.1

250-300 East 5.0 6.8 1.8 35.7 1.0

300-350 East 2.5 6.8 4.3 174.9 5.0

350-400 East 3.4 6.5 3.1 90.4 2.6

400-450 East 3.1 4.1 1.0 31.8 0.9

450-500 East 1.5 3.3 1.8 121.2 3.5

53

-1

0

1

2

3

4

5

6

200-250 250-300 300-350 350-400 400-450 450-500

Altitudinal zone

Are

a (h

a)

Figure 3.13 Annual change in area (ha) of bracken at an east aspect from the 1960s to 2005 in PDNP

Compared to north- and east-facing aspects, the only large increase for the south was

observed in the 200-250 m zone (1.2 ha, 2.2% p.a.), and decreases were observed

between 250 and 350 m (2.9 ha, 0.4% p.a.; Table 3.14).

Table 3.14 South aspect for bracken in the 1960s and 2005 in PDNP

Altitudinal range (m) Aspect

1960s (ha)

2005 (ha)

Change (ha)

% change

% annual change (c. 35 yrs)

200-250 South 1.6 2.7 1.2 75.5 2.2

250-300 South 11.7 9.7 -2.0 -17.1 -0.5

300-350 South 8.8 7.9 -0.9 -9.9 -0.3

350-400 South 1.4 1.4 0.04 2.9 0.1

400-450 South 0.0 0.04 0.04 nc nc

Decreases were observed for the west-facing aspect at all altitudinal zones apart from

the 300-350 m zone, which increased by 3.1 ha (1.9% p.a.) during the study period. The

largest areal and percentage decrease was in the 200-250 m zone (3.1ha, 12.7% p.a.),

whilst decreases in the remaining altitudinal zones were small (Table 3.15).

Table 3.15 West aspect for bracken in the 1960s and 2005 in PDNP

Altitudinal range (m) Aspect

1960s (ha)

2005 (ha)

Change (ha)

% change

% annual change (c. 35 yrs)

200-250 West 4.1 1.0 -3.1 -426.0 -12.2

250-300 West 4.6 4.6 -0.03 -0.7 -0.02

300-350 West 4.7 7.8 3.1 65.9 1.9

350-400 West 4.9 4.8 -0.04 -0.8 -0.02

400-450 West 2.8 2.7 -0.1 -5.3 -0.2

54

In the sub-dataset north- and east-facing aspects recorded large changes in bracken

cover during the study period, with increases of 208% and 148% respectively. The

largest areal increase was recorded at the north-facing aspect in the 350-400 m zone

(8.4 ha). Similarly, the largest percentage increase for any aspect was the north-facing

aspect in the 250-300 m zone (219%). For the east-facing aspect the largest areal and

percentage increases were in the altitudinal zones above 300 m, less conspicuoisly so in

the 400–450 m zone.

The south- and west-facing aspects recorded small changes overall. The largest areal and

percentage decrease of any aspect was found at the east-facing aspect in the 200-250 m

zone (3.1 ha, 12.2% p.a.). A small combined increase was observed for the south-facing

aspect (1.6 ha, 0.05% p.a.; Figure 3.14).

-4

-2

0

2

4

6

8

10

150-200 200-250 250-300 300-350 350-400 400-450 450-500

Altitudinal range

Are

a (h

a)

North

East

South

West

Figure 3.14 Change of bracken cover for aspect at altitudinal zones (50 m) from the 1960s to 2005 in PDNP

The pattern of bracken distribution changed over the study period, although not

significantly (Kruskall-Wallis test, p = 0.392). Observed increases in bracken distribution

in the north- and east-facing aspects (15%) apparently corresponded closely to

decreases in the south- and west-facing aspects (13%).

3.5.4 Changes in bracken cover on slopes 1960s-2005

The largest percentage change was observed on the steepest slopes, with increases of

more than 200% (>6% p.a.) between 25-35°, whilst the largest areal change was

observed between 0-15° (21.8 ha, 1.8 p.a.). The mid-level slopes (15-25°) recorded

comparatively small increases.

55

In the 1960s 94% (82.6 ha) of bracken was observed between 5-25°. In 2005 the same

slope range accounted for 92% (108.7 ha) of bracken, suggesting that bracken favours

these regions. Large percentage increases on steeper slopes are tempered by relatively

small areal increases (Table 3.16; Figures 3.15 and 3.16).

Table 3.16 Bracken in the 1960s and 2005 in 8 slope zones in PDNP

Slope (°) 1960s (ha)

2005 (ha)

Change (ha)

% change

% annual change (c. 35 yrs)

0-5 2.5 4.0 1.5 60.7 1.7

5-10 28.7 37.4 8.7 30.2 0.9

10-15 11.8 23.5 11.7 98.8 2.8

15-20 21.7 25.5 3.7 17.1 0.5

20-25 20.4 22.3 1.9 9.4 0.3

25-30 1.9 3.9 2.0 203.7 5.8

30-35 0.5 1.2 0.7 236.7 6.8

35-40 0.0 0.05 0.05 nc nc

Total 87.5 117.6 30.3 25.7 0.7

0

2

4

6

8

10

12

14

0-5° 5-10° 10-15° 15-20° 20-25° 25-30° 30-35° 35-40°

Slope

Are

a (h

a)

Figure 3.15 Change in area (ha) of bracken at slope zones from the 1960s to 2005 in PDNP

56

0

1

2

3

4

5

6

7

8

0-5° 5-10° 10-15° 15-20° 20-25° 25-30° 30-35° 35-40°

Slope

Ch

ange

(%

p.a

.)

Figure 3.16 Annual change (%) of bracken at slope zones from the 1960s to 2005 in PDNP

3.5.5 Changes in bracken cover at slopes related to altitude1960s-2005

Due to the small areal coverage of bracken at individual slope zones change was

assessed by consolidating two neighbouring zones (see Method). The greatest change in

the low slope zone was observed above 300 m (7.9 ha, 1.3% p.a.), with smaller increases

observed below 300 m (1.6 ha, 0.4% p.a.). The only decrease was observed in the

altitudinal zone of 450-500 m (Table 3.17).

Table 3.17 Low slope for bracken in the 1960s and 2005 in PDNP

Altitudinal range (m)

Slope (°)

1960s (ha)

2005 (ha)

change (ha)

% change

% annual change (c. 35 yrs)

200-250 0-10 6.3 6.8 0.5 7.9 0.2

250-300 0-10 5.5 6.6 1.1 20 0.6

300-350 0-10 5.1 8.8 3.7 72.5 2.1

350-400 0-10 10.3 13.6 3.3 32.0 0.9

400-450 0-10 2.5 3.5 1 40.0 1.1

450-500 0-10 0.1 0.03 -0.07 -70.0 -2.0

In the low-mid slope zone increases were observed above 250 m, of which the greatest

areal and percentage increases were at 300-400 m (12.1 ha, 2.6% p.a.). Of note was an

increase of 1.5 ha (2% p.a.) in the 450-500 m altitudinal zone (Table 3.18).

57

Table 3.18 low-mid slope for bracken in the 1960s and 2005 in PDNP

Altitudinal range (m)

Slope (°)

1960s (ha)

2005 (ha)

Change (ha)

% change

% annual change (c. 35 yrs)

200-250 10-20 6.5 4.3 -2.2 -33.8 -1.0

250-300 10-20 6.5 9.5 3.0 46.2 1.3

300-350 10-20 8.4 14.4 6.0 71.4 2.0

350-400 10-20 4.8 10.9 6.1 127.8 3.6

400-450 10-20 5.8 7.3 1.5 25.9 0.7

450-500 10-20 2.1 3.6 1.5 71.4 2.0

In the steep-mid slope zone change was observed at 300 m, with bracken increasing

above and decreasing below this altitude. The largest areal increases were between 300-

400 m (4.4 ha, 1.5% p.a.), whilst the largest percentage increases were above 400 m

(3.3% p.a., 1.6 ha). Decreases below 300 m were 2.3 ha (0.5% p.a.) for two combined

zones (Table 3.19).

Table 3.19 Steep-mid slope for bracken in the 1960s and 2005 in PDNP

Altitudinal range (m)

Slope (°)

1960s (ha)

2005 (ha)

Change (ha)

% change

% annual change (c. 35 yrs)

200-250 20-30 1.7 0.9 -0.8 -88.9 -2.5

250-300 20-30 10.7 9.2 -1.5 -16.3 - 0.5

300-350 20-30 5.7 8.6 2.9 50.9 1.5

350-400 20-30 2.9 4.4 1.5 51.7 1.5

400-450 20-30 1.1 2.2 1.1 100.0 2.9

450-500 20-30 0.3 0.8 0.5 166.7 4.8

The increases and decreases observed in the steep zone concerned small areal extent

and therefore confidence in these results were low however there was a very large

percentage increase above 350 m that was of interest, as it appeared bracken was

colonising an altitude in which it was barely present in the 1960s (Table 3.20).

Table 3.20 Steep slope for mapped bracken in the 1960s and 2005 in PDNP

Altitudinal range (m)

Slope (°)

1960s (ha)

2005 (ha)

Change (ha)

% change

% annual change (c. 35 yrs)

250-300 30-40 0.2 0.00 -0.2 n/a n/a

300-350 30-40 0.2 0.2 0.0 0.0 0.0

350-400 30-40 0.1 0.8 0.7 700 20

400-450 30-40 0.001 0.3 0.3 30000 857.1

The consolidated slope zones confirmed the established patterns of change observed in

the study - the greatest areal increase was for the mid-low slope zone, and the greatest

percentage increase was for the steep zone (Table 3.21; Figures 3.17 and 3.18).

58

Table 3.21 Bracken in the 1960s and 2005 in the consolidated slope zones in PDNP

Slope 1960s (ha)

2005 (ha)

Change (ha)

% change

% annual change (c. 35 yrs)

low (0-10) 29.8 39.3 9.5 32.0 0.9

mid low (10-20) 34.1 50.0 15.9 46.6 1.3

mid steep (20-30) 22.4 26.1 3.7 16.5 0.5

Steep (30-40) 0.5 1.3 0.8 160.0 4.6

0

2

4

6

8

10

12

14

16

18

low (0-10°) mid low (10-20°) mid steep (20-30°) steep (30-40°)

Slope

Ch

ange

(h

a)

Figure 3.17 Change in area (ha) of bracken at consolidated slope zones from the 1960s to 2005 in PDNP

0

20

40

60

80

100

120

140

160

180

low (0-10°) mid low (10-20°) mid s teep (20-30°) s teep (30-40°)

Slope

Ch

ange

(%

)

Figure 3.18 Change (%) of bracken at consolidated slope zones from the 1960s to 2005 in PDNP

59

In the 1960s and 2005 imagery bracken was distributed principally on slopes ranging

between 5-25°, however within this range increases and decreases in bracken areal

extent were observed. Bracken was less abundant at 0-5° and 25-40°, but increases in

cover were noted during the study period (Table 3.22 and Figure 3.19).

Table 3.22 Distribution (%) of bracken in the slope zones for the 1960s and 2005 in PDNP

Slope (°) 1960s

distribution % 2005

distribution %

Change

0-5 2.8 3.3 0.5

5-10° 32.8 31.8 -1.0

10-15° 13.5 20.0 6.5

15-20 24.8 21.7 -3.1

20-25 23.3 18.9 -4.4

25-30 2.2 3.3 1.1

30-35 0.6 1.0 0.4

35-40 0.0 0.04 0.04

-6

-4

-2

0

2

4

6

8

0-5 5-10. 10-15. 15-20 20-25 25-30 30-35 35-40

Slope zone

Dis

trib

uti

on

(%

)

Figure 3.19 Changes in distribution (%) of bracken in the slope zones for the 1960s and 2005 in PDNP

3.6 Ranking changes in bracken cover (%) 1960s-2005

40 areas (30% of sub-sample) in which the largest change (>100%) was observed were

analysed (appendix iii). 75% of change in bracken extent in the PDNP between 1960s

and 2005 imagery occurred between 250-400 m, with largest change on the north- and

east-facing aspects. Slope zones appear to be distributed more uniformly and the largest

changes were noted between 5-30°. The 3 categories were normally distributed:

altitudinal range (K-S test 0.26), aspect (K-S test 0.29), and slope (K-S test 0.24) (Table

3.23).

60

Table 3.23 Ranking areas (n) and proportion (%) of bracken by percentage change from the 1960s to 2005 in PDNP

Altitudinal range

Ranking areas

% of Ranking

areas Aspect Ranking

areas

% of Ranking

areas Slope Ranking

areas

% of Ranking

areas

200-250 4 10 North 18 45 0-5 1 2.5

250-300 8 20 East 12 30 5-10 5 12.5

300-350 11 27.5 South 5 12.5 10-15 7 17.5

350-400 11 27.5 West 5 12.5 15-20 5 12.5

400-450 3 7.5 20-25 4 10

450-500 3 7.5 25-30 5 12.5

30-35 2 5

The data suggest that in 2005 the greatest likelihood of encountering bracken in the

PDNP would have been on a north-facing aspect, at 10-15°, between 300 m and 400 m

elevation.

3.7 Results for historic climate data

Historic climate data were analysed by looking at summer (April-September) and winter

(October-March) decadal means for maximum and minimum temperature, frost days,

precipitation and sunlight hours.

The summer period comparison of the 1960s and 2000s found that maximum and

minimum temperatures increased by more than 1 degree, frost days decreased, and

sunlight hours increased by almost 20%. After 3 drier decades (1970s-1990s)

precipitation narrowly increased (3%) (Table 3.24; Figure 3.20).

Table 3.24 Summer period decadal comparison of climatic variables during the study period (derived from Met Office Sheffield station historic data)

Decade Temp max (°C) Temp min (°C) Frost days Precipitation

(mm) Sunlight hours

1960s 16.8 9.1 1.9 400.2 843.5

1970s 17.1 9.2 2.2 348.3 949.4

1980s 17.4 9.2 2 376.8 992.18

1990s 17.8 9.6 1.6 344.3 1062.4

2000s 18.2 10.2 0.9 414.4 1042.5 Change 1960s-2000s 1.4 1.1 -1 14.2 199

% Change 7.6 10.8 47.4 3.4 19.1

61

0

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p (

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Figure 3.20 Mean summer maximum (light grey) and minimum (dark grey) temperatures, with trendline added (Met Office Sheffield station historic data)

The winter period comparison of the 1960s and 2000s found that maximum and

minimum temperatures increased by over 1 degree, of which the mean winter minimum

temperature increase of 28% was most marked. Frost days decreased by 35%, sunlight

hours increased by 25%, and, as in summer, little change in precipitation was recorded

(Table 3.25; Figure 3.21, appendix iv).

Table 3.25 Winter period decadal comparison of climatic variables during the study period (compiled from Met Office Sheffield station historic data)

Decade Temp max (°C) Temp min (°C) Frost days Precip (mm) Sunlight hours

1960s 8.03 2.84 42.7 445.1 361.3

1970s 8.43 3.28 29.8 432.0 395.4

1980s 8.48 3.14 35.4 476.8 349.3

1990s 9.06 3.64 29.7 463.5 388.6

2000s 9.39 3.95 27.7 441.2 482.1 Change 1960s-2000s 1.4 1.1 -15 -3.9 120.8

% Change 14.5 28.1 -35.1 -0.88 25.1

62

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Figure 3.21 Mean winter maximum (light grey) and minimum (dark grey) temperatures, with trendline added (Met Office Sheffield station historic data)

63

4. Discussion

4.1 Bracken in the PDNP in 2005

The rate of bracken spread of around 1-3% p.a. has been identified in previous studies

(e.g. Miller et al., 1990; Hopkins et al., 1988), with annual advances of up to 87 cm

recorded in heather moorland (Cox, 2007). However there is uncertainty of bracken’s

true coverage in the UK (Birnie et al., 2000; Pakeman et al., 2000). The 1990 Countryside

Survey estimated there were 930 km2 of bracken broad habitat in England, by 1998

increasing to 1090 km2, and by 2007 declining to 910 km2 (Carey et al., 2008). However,

accurate identification of vegetation classes is problematic (Clutterbuck, pers. comm.,

2014), and standard errors are large (Pakeman et al., 1996).

In the PDNP previous surveys (up to 1990) have broadly consented on bracken’s

distribution and extent – about 30 km2 in the moorlands (Anderson and Radford, 1988),

and 35 km2 in the whole park (Pakeman et al., 1996). This study suggests that bracken

may have covered 62 km2 of PDNP in 2005. If the 2005 sample is representative of the

PDNP, between 1990 and 2005 bracken cover increased by 27 km2, a 1.8% p.a. gain

comparable with previously identified rates of spread, but which is twice that observed

in the sub-sample (0.9% p.a.) comparison between the 1960s and 2005, suggesting both

more rapid recent range expansion, and possibly more effective bracken control

between the 1960s and 1990.

However bracken control remained prevalent in the PDNP after 1990, and management

practices and climate were not exceptional compared to the rest of the country. It is

suggested that if the methods employed in this study - only identified bracken was

mapped, and, as far as possible, within bracken stands areas of grassland, trees, heather,

and bare ground were excluded (or clipped out, Figure 4.1) - were applied to the earlier

surveys the bracken extent may have been larger. For example in the 1990 Countryside

Survey (from which Pakeman et al., 1996 derived bracken’s extent) “estimates were

based on assessments of the cover of bracken as a proportion of each square” (Pakeman

et al., 1996), and may have underestimated bracken’s true extent. Thus, rather than

bracken’s expansion rate doubling after 1990, perhaps gradual and continuous spread

occurred that had not been recognised.

Birnie et al. (2000) acknowledge data deficiencies on actual bracken distribution. There

is poor understanding of the dynamics and drivers of bracken spread and retreat

because it persists over such a broad spectrum of environmental conditions, and it’s

64

distribution is influenced by locally distinctive environmental factors such as exposure,

soil depth, SMC, nutrient levels, and micro-climates (Marrs and Watt, 2006; Pakeman

and Marrs, 1996), as well as anthropogenic factors that shape the UK uplands (Bonn et

al., 2009; Holden et al., 2007).

Figure 4.1 1 km2 study grid showing bracken mapped for 2005, illustrating clipping out (one example of many in this image is circled)

Bracken’s altitudinal limit in the PDNP was estimated by Moss (1913) to be 457-472 m,

and an encounter a 518 m was considered to be “quite exceptional.” By 2005 bracken

was observed, albeit infrequently, in study grids above 472 m. It remains unusual to find

bracken over 500 m in the PDNP – it was found in just one study grid (NE of Howden

Reservoir, 53°27’29.53” N 1°43’19.33” W) at 514 m, although it appeared evident that

bracken control occurred nearby above 520 m.

Satellite imagery from 2009 (Google Earth, 2014) shows bracken at 533 m in the same

area, and the 2005 imagery reveals stands of bracken (~75-100 m2) above 500 m,

observed to a maximum of 536 m at Kinder Scout (53°27’46.09” N 1°50’28.37” W, Figure

4.2). It is suggested that upland warming (Caporn and Emmett, 2009; Holden and

Adamson, 2002) has allowed bracken to extend its altitudinal range in the PDNP by

approximately 18 m in 92 years (1913-2005).

65

Figure 4.2 a) the observed altitudinal limit, 514 m, of bracken in the study grids, b) bracken at 536 m, Kinder Scout, PDNP (Google Earth, 2009)

The study data suggest that bracken favours altitudes between 250-400 m in the PDNP,

which accounts for 90% of the bracken distribution. The greatest extent was on the

south-facing aspect, in agreement with Atkinson’s (1986) findings. In the 2005 sample

31% of bracken was found on a south-facing aspect, at 10-15°, between 300-350 m.

Deviating from observed trends there was also a small increase in bracken cover

between 350-450 m, whereas the other aspects recorded a decrease of 43% between

66

these altitudes. The west-facing slope was the only aspect in which bracken was present

at all altitudes, however this may be due to a lack of representativeness of the sample

below 150 m and above 500 m, sample error, or possibly a greater resilience to change

and/or climatic extremes?

4.2 Comparison of bracken in the 1960s and 2005

In the sub-sample 87.5 ha of bracken were mapped in the 1960s and 117.6 ha in 2005,

an increase of 30 ha (0.9% p.a.). Extrapolation suggests bracken may have covered

46km2 of the PDNP in the 1960s however there is little confidence in this estimation as

the sub-sample represented just 1% of PDNP area, compared to 5% of the 2005 sample.

Bracken’s areal change does not appear to simply correlate with its earlier extent and

distribution. These findings seem to concur with Anderson’s (pers. comm., 2014)

observations that bracken rates of spread differ considerably in the PDNP, conceivably

expanding its range at rates beyond 3% p.a. where able, whilst being stymied, repelled,

or controlled elsewhere. The sub-sample 0.9% p.a. rate of spread is at the lower end of

previous estimations, and may be indicative of factors were an impediment to rapid

bracken spread during the study period. However, the sub-sample observes a trend of

larger increases in bracken above 300 m countered by losses (or very small increases) at

lower elevations.

This is likely due to concerted efforts to eradicate bracken at the upper limits of enclosed

land and the lower limits of the open uplands, encouraged by CAP aid to farmers in LFA’s

(EC, 2009), BAPs (JNCC, 2012), ESA and upland ELS schemes (Natural England, 2014),

other grants (Robinson, 2001), and financial incentives in restoring/extending heather

moorland for grouse (Carr and Middleton, 2004). An example of change during the

study period is shown in Figure 4.3.

Comparisons between the periods underline the importance of altitude in bracken

distribution in the PDNP. At 200-300 m bracken cover decreased or annual spread was

minimal, however above 300 m bracken spread by 1.2-2.5% p.a., of which the largest

increase was observed at 450-500 m. Despite the smallest extent of bracken being

mapped in this zone an increase in area from 2.9 ha to 5.5 ha remains notable. The sub-

sample concurs with the 2005 sample: in the PDNP bracken favours altitudes between

250-400 m.

67

Figure 4.3 An example of land-cover change next to the Upper Derwent Reservoir. In the 1960s the enclosed field in the centre of the images was mostly bracken (pink outline delineates mapped bracken), which has been eradicated by 2005 (yellow outline). The plantations along the reservoir fringe and at the bottom of the images have both extended. Heather is being encouraged: in the centre-right of the 1960s image bracken was observed in the north of the wall, which, by 2005 has changed to heather and grassland, apparently the result of mowing and possibly ploughing.

4.2.1 Comparison of aspect

The comparison of change for aspect presents arguably the most interesting results. In

the 1960s bracken was distributed mostly on south- and west-facing aspects, but by

2005 there was an apparent reversal with the most bracken found on north- and east-

aspects, increasing respectively by a considerable 6% and 4% p.a. Interestingly, the

north-facing aspect showed a normal distribution (K-S test 0.23, p = >0.15) in which

68

there was no change below 250 m, the greatest at 250-300 m (6.3% p.a.), and an

incremental decrease in change through the other altitudinal zones (from 4.8-1.5% p.a.),

which suggests bracken to favour altitudes around 250-350 m. A more haphazard

distribution was observed for the east-facing aspect. There were small decreases in

extent in the south- and west-facing aspects, presenting a correlation in which two

aspects increased and two decreased. However, the distributions between aspects were

relatively analogous, suggesting previous barriers to bracken spread in north- and east-

facing slopes had diminished.

The reasons for this change can only be hypothesised. Marrs (pers. comm., 2014)

suggests bracken may be more rapidly expanding into areas that were previously on the

periphery of its range (i.e. northern and eastern areas), whereas in southern and

western areas at corresponding altitudes bracken was already more established. It might

be presupposed that under climate change bracken spread at all aspects would be

similar, but possibly such contrasts may be explained by the diversity of upland

microclimates.

Anderson (pers. comm., 2014) suggested that spring droughts, a more recent

phenomenon, have stymied bracken spread on slopes more susceptible to drying from

the sun and prevailing winds (i.e. south and west facing). Her study of dunes in Jersey

found bracken in dune grassland disappeared during very dry summers. Moreover

heather is less affected by drought unless close to their flowering period in late summer,

and despite drought-tolerance, bracken is sensitive in its early growing phase. One-off

droughts are believed to have more impact on plant phenology than gradual climate

change over the short- and mid-term (Nagy et al., 2013). Such events are likely to

diminish the suitability of a locale for one species, and thus help increase its suitability

for another species (Evans and Pearce-Higgins, 2013).

The data differ from the 2005 sample, in which the most bracken was observed on

south-facing aspects, and it is possible a larger sample for the 1960s would have found

similar results. However, the sub-sample may be an accurate indication of changes above

200 m, local changes, and/or specific, local environmental conditions that both

encourage north- and east-facing bracken spread and discourage south- and west-facing

spread. Or – unlikely as it is - the sub-sample may be more representative and highlight a

trend of bracken distribution

69

The mean location of the sub-sample was more northerly than the 2005 sample, and 11

(85%) of the sample grids covered open heather moorland compared to 21 (60%) of the

2005 sample. This prompts a question of whether this change is representative of

upland moorland change in the Dark Peak, or whether the change is site specific, one

that may not be observable in a larger sample.

In the 2005 sample south-facing aspects remained the dominant aspect in which to find

bracken between 250-400 m, whilst the most bracken at 400-450 m was observed on

west-facing aspects, therefore the findings may be local, even site specific, however they

should not be dismissed. Anderson (pers. comm., 2014) noted variation in bracken

spread, from “significant” change, especially the Howden/Derwent National Trust

region, to areas in which bracken distribution appeared stable.

4.2.2 Comparison of slope

The largest areal extent of bracken was observed on slopes from 5-25° (92%), however

the greatest change occurred on the steepest slopes (>25°). This trend was commonly

observed at all altitudinal ranges, although the mid-slope (20-30°) increased less than

the other zones (0.5% p.a. compared to 0.9-1.6% p.a.). Observations suggest that

bracken is less able to advance on steep slopes due to local physical and environmental

conditions (e.g. shallow soils, leaching, exposure, microclimates). The presence of

bracken on steeper slopes is viewed as beneficial, stabilising the habitat and preventing

erosion (Natural England, 2008).

4.3 Climate change

Anthropogenic climate change is unequivocal and unprecedented change can be

expected (IPCC, 2013). The distributions of species, their capacity to reproduce,

community interactions, and ecosystem functions are under threat (Barnosky et al.,

2012; Hulme, 2005; Berry et al., 2003). Environments of climatic extremes are especially

vulnerable (Callaghan et al., 1992), including uplands that provide important ecosystem

services and are refuges for many rare species adapted to harsh conditions (PDNPA,

2011; Grime et al., 2008, Table 4.1). The ability to adapt or migrate will determine the

future of many upland species, as broad-niche generalists, like bracken, prosper at the

expense of many specialists (Higgins and Richardson, 2014; Albert et al., 2012; Trivedi et

al., 2008).

70

Table 4.1 Examples of potential causes and effects of climate change in PDNP that might proliferate the spread of bracken. Key: red shade = summer impacts; blue shade = winter impacts; DOC: dissolved organic carbon; SMC: soil moisture content; ET: evapotranspiration (adapted from a table in PDNPA, 2011)

Climate change cause

Potential climate change effect

Consequences

Water table lowered

Drying of blanket bogs Peat dries, desiccates, and erodes; loss of sphagnum spp. etc, loss of unique and climate change-

mitigating habitat, release of CO2 into atmosphere and DOC into watercourses, invasion by bracken

Drier moorlands Invertebrate decline, wildfires

Upland breeding birds under threat, including grouse, which threatens shooting economy, change

in vegetation structure,

Increased visitors Trampling, fires, increased pollution

habitat damage and loss, nutrient loading (C, N) enriching poor upland soils which suits generalists

like bracken

Increased surface water

temperatures

Increase in biological respiration, Less oxygen

Low flows, dry river beds, eutrophication, anoxia, less standing water, increase in invasive species

Reduction in SMC Loss of grazing pastures, wildfires, reduced NPP

Grazing not viable, abandoned pasture encroached by bracken, difficulty in managing burn regimes, risk

of peatland fires, reduced growth and carbon storage, increased ET and downwind rainfall

Flooding, extreme weather events,

and water logging

Extended growing season, change in chemical

make-up of SMC, loss of grazing pastures

Conditions benefitting invasive species, including bracken, poaching and trampling, increased erosion,

inadequate drainage. Also a threat to local businesses

Fewer frost days Increased rates of disease and pests

Effects most upland species, although bracken remarkably tolerant

4.3.1 Climate change and future bracken distribution

Climate change and land management decisions will determine the future extent and

distribution of bracken (Pakeman et al., 1996). Its spread will potentially be assisted by

future increases in water demand, declines of grouse, and greater visitor numbers that

may alter the landscape character and biodiversity of the PDNP (PDNPA, 2009).

However, the impacts and effects are complex and poorly understood, and projections

are replete with uncertainties (Orr et al., 2008; IPCC, 2007) – all climate models are a

‘best guess’ (WMO, 2013), but upland forecasts are more problematic due to their very

diverse, often very localised climatic conditions (Orr et al., 2008).

Yet the uplands are warming, and this is predicted to continue (possibly by 5 °C by 2100

from 1990 levels). The PDNP is forecast to experience hotter, driers summers and

warmer, wetter winters with more extreme weather events (PDNPA, 2011; Caporn and

Emmett, 2009, Table 4.2). The historic data 1960s-2000s from the Met Office (2014)

Sheffield station show that minimum and maximum summer and winter temperatures

have risen by more than one degree, snow cover and frost days have declined and

sunlight hours increased. Therefore a proposal that climatic trends will aid bracken

71

spread into areas previously beyond its ecological limits seems logical. The most

influential factor according to Marrs (pers. comm., 2014) is the increase in minimum

temperatures. Climate change effects have the potential of intensifying bracken spread in

the uplands.

Table 4.2 Projected temperature (°C) and precipitation (%) changes for PDNP in 2020s, 2050s and 2080s (PDNP, 2011)

Climate projection

Projected temperature and precipitation changes

2020s 2050s 2080s

Hotter, drier summers

+1.4 °C +2.4 °C +3.4 °C -7% -18% -22%

Warmer, wetter winters

+1.3 °C +2.2 °C +3.0 °C +4% +10% +13%

However, despite recognising apparent trends in climate warming and observing

increases in bracken’s altitudinal distribution in the PDNP over the study periods, there

is insufficient supporting evidence in this study. Bracken would be expected to show

increases in extent at higher altitudes on all aspects in the sub-sample dataset, yet this

was not observed. Bracken on south- and west-facing aspects was found to decline

slightly in area over the study period, which would not be an expected effect of climate

change. Any potential effects of climate change upon bracken distribution in the PDNP

were masked by pronounced anthropogenic activities, whose influences are visible

everywhere (e.g. burning, farming, quarrying, forestry), possibly assisted by bracken’s

complex relationship with heather.

4.4 The relationship between bracken and heather

Bracken and heather in no way resemble one another, heather is smaller, evergreen,

procumbent, and propagates by seed (Rogers, 1996; Gimingham, 1975). It endures

frosts, a competitive advantage the plant enjoys over bracken, although this is countered

by bracken’s greater resilience to grazing, fire and drought (Gordon et al., 1999; Marrs,

1993). The environments in which bracken and heather are found overlap appreciably,

they persist in broadly similar climatic and edaphic conditions, and their lifecycles – with

pioneering to degenerate phases - are comparable (Marrs and Watt, 2006; Watt 1955,

Figure 4.4). The species exist in an association of fluctuating fortunes, conditional on

many environmental and ecological factors (Marrs and Watt, 2006).

72

Figure 4.4 The lifecycle of heather is comparable with that of bracken, Watt (1955) describing it thus: “mixed to give an uneven cover, young dense Calluna or old straggly bushes occur, each kind being repeated again and again.” Redrawn from Watt (1955)

Heather moorland is retained and managed in the PDNP primarily because of the

economic gains provided by grouse shooting. Without it the landscape might be very

different, possibly one in which bracken might not be seen as a pest. A correlation

between the discontinuation of shooting and heather loss has been identified (Sotherton

et al., 2009; Usher and Thompson, 1993). Compared to areas in which land management

has ceased, those under controlled burning regimes experience 70% less heather loss,

and curtail woodland and bracken encroachment (Robertson, 2001).

At Lakenheath Warren, a managed heath in Norfolk, Watt (1954; 1955; 1976) found

bracken and heather persisting in an inexact and ill-defined equilibrium, in which both

would encroach into the degenerate or pioneer regions of the other, and be retarded by

building and mature plants (Marrs and Watt, 2006; Marrs and Le Duc, 2000). Once

management ceased this equilibrium stopped and bracken became dominant. Over time,

when it degenerated, exposed patches were inhabited by grasses, and more latterly pine

(Pinus sylvestris). Natural succession towards woodland was evident even within dense

bracken stands (Marrs and Hicks, 1986). Although a lowland study the findings are

relevant in an upland scenario because a) there is a commensurate bracken-heather

relationship in uplands (Whitehead, 1993), b) land management methods, succession,

and ‘re-wilding’ are similarly important upland policy options (Reed et al., 2009), and c)

lowland and upland climates are converging (Burt and Holden, 2010; House et al., 2010).

4.4.1 The effects of climate change on the bracken-heather relationship

Studies suggest bracken and heather do not increase productivity in conditions of

enhanced CO2 (Eatough-Jones et al, 2011; Caporn et al., 1999). However nutrient inputs

are thought to benefit bracken, which creates an N-rich inorganic soil less suited to

heather (Deluca et al., 2013). Currey et al. (2011) propose this may be less due to soil

conditions, but rather N-inputs being retained within the bryophyte layer, which are

73

therefore unavailable to heather. However, heather naturally persists in nutrient-poor

soils and Clutterbuck (pers. comm., 2014) questions the validity of this hypothesis.

Pakeman et al. (2002) found bracken, despite asulam spraying, spread by a mean 87cm

p.a. in upland heather moorland, twice the rate of spread recorded by Watt (1954) in

lowland heath. This implies that bracken spread is more influenced by the communities

into which it encroaches than by climate. For the true effects of climate and climate

change on bracken distribution it would be necessary to eliminate the variables of

species community, edaphic conditions, and micro-climates, allowing for a study in

which the only variable is altitude.

4.4.2 Heather: state and distribution

Heather is distributed throughout the UK, home to three-quarters of the global stock,

and the most archetypal communities are found in moorlands (Thompson et al., 1995).

Here, in a landscape mostly free of trees, they are characteristically found on peats soils,

supporting grasses, sedges, dwarf shrubs, ferns, mosses and lichens beneath their

canopy, forming an indiscernible frontier with peat bogs (Holden et al., 2007; Bardgett et

al., 1995; Gimingham, 1975). Heather moorland is widely protected, including under the

Habitats Directive (92/43/EEC), The Wildlife and Countryside Act (1981), the Heather

and Grass etc Burning Regulations for England (2007) and Wales (2008) (EC, 2014a;

RSPB, 2010; Littlewood et al., 2006; English Nature, 2001), and the Control of Pesticides

Regulations 1986 (Defra, 2008).

However just 46% of UK upland heather moorland is in favourable condition (AMEC,

2013), and coverage has declined by an estimated 90% in the last two-hundred years

(Stevenson and Thompson, 1993). Historically this has been due to grazing and

afforestation, and recently perturbations such as burning and trampling (from increased

visitor numbers) have aided encroachment by competitive, invasive species (Anderson

et al., 2009), most notably bracken (Alday and Marrs, 2014), and purple moor grass

(Molina caerulea), which in damper soils can exist with bracken to the exclusion of other

plants (Marrs and Watt, 2006; Milligan et al., 2004; Chambers et al., 1999).

In the PDNP 36% (6,500 ha) of heather moorland was lost from 1913-1991 (PDNPA,

2001), yet recently, with local exceptions, the moorlands have recovered due to

increased control of bracken and restoration initiatives enabled by ESA and agri-

environment schemes, and the Moors for the Future project (Natural England, 2014;

74

Anderson and Ross, 2011; Moors for the Future, 2008). However, bracken encroachment

into heather moorland is stimulated by enduring factors like overgrazing, deforestation,

abandonment of hill pastures, and fire (Anderson and Radford, 1988).

4.5 The role of anthropogenic activities on bracken distribution

Anthropogenic activities can both prevent and aid bracken spread. In the imagery used

for this study human intervention is unmistakable, including forestry, farming,

quarrying, construction and infra-structure changes, peat drainage and gulley blocking,

burning, and vegetation control. The anthropogenic activities that appear to have the

most influence on bracken distribution are those most evident and proximate - burning

and bracken control.

4.5.1 Wildfires and burn regimes

Fire is a natural regenerator, and in the uplands it is important in creating new shoots

for grouse (Yallop et al., 2009; Carr and Middleton, 2004), conserving biodiversity and

fulfilling upland management polices (Davies et al., 2008). However the frontiers of

newly burnt areas, in which there is exposed ground, increased nutrient availability, and

diminished competition, are vulnerable to bracken invasion (Anderson and Radford,

1988; Smith, 1977). Furthermore the consequences of poorly controlled or excessive

burning causes losses of heather, moss and lichen (Sotherton et al., 2009; Tucker, 2003),

associated biodiversity loss (Ramchunder et al., 2009), erosion and degradation of peat

(Hooijer et al., 2014; Worrall and Evans, 2009), and releases CO2 into the atmosphere

and dissolved organic carbon (DOC) into local watercourses (Palmer et al, 2013; Yallop

et al., 2012).

With hotter, drier summers more dry, flammable biomass will be generated (PDNPA,

2011), and pressures - from increased visitor numbers (McMorrow et al., 2009) and to

extend moorland and increase grouse stocks (Harvey-Miller, pers. comm., 2014) - are

expected to lead to an increase in the frequency and intensity of accidental wildfires and

controlled burns (Milin-Chalabi et al., 2014: McMorrow et al., 2009; Davies et al., 2008,

Figures 4.5 and 4.6).

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Figure 4.5 Moorland burns at a local scale in the PDNP: in the 1960s only a few burn scars can be identified, by 2005 many more are evident. Image 1:5,000

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Figure 4.6 Moorland burns at a large scale in the PDNP: in a ten year period (1999-2009) observable burn scars have increased in the High Peak region. Image altitude 12 km (Google Earth, 2014)

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4.5.2 Bracken control

Asulam is the most effective means of controlling bracken (Stewart et al., 2008), and

without it bracken extent might be 50% greater (Thorp, 2012). Thus, based on this

study’s extrapolations, bracken may have covered an area of 93 km2, not 62 km2, of the

PDNP in 2005. Other methods such as cutting and mowing are limited by accessibility,

safety, practicality, and cost (Pakeman, 2012), moreover their effects are limited, and

more suited to follow-up control after asulam application (Marrs and Le Duc, 2000).

Control is pointless without restoration targets, and Alday et al. (2013) found than the

most effective methods in restoring heather moorland communities was by asulam

treatment followed by grazing. This study observed large extents of apparently treated

areas at various stages of recovery, and unpublished data (McAlpine, 2013) found in 1

km2 grids that up to 48% of bracken was treated by asulam (Figure 4.7).

Figure 4.7 Areas sprayed by asulam are evident in the image on the left; the mapped image (right) shows the comparable extents of bracken (green, 4.7 ha) and sprayed areas (yellow, 4.6 ha) in a 1 km2 study grid in PDNP, 2005 (image on right from McAlpine, 2013)

Asulam was introduced in 1978, and cumulatively 1,643 ha have been sprayed, reducing

bracken spread into habitats such as heather moorland and acid grassland (Pakeman,

2012, Figure 4.9). The EU ban on asulam has huge implications for future bracken

control. An Emergency Authorisation was granted for the 2014 season (Bracken

Control, 2014), but future authorisations may not be, and long-term grazing, shooting

and conservation in the uplands are threatened as aerial spraying is the only recourse in

many places (Pakeman, 2012). However, Thorp (nd) asks “how long will we be able to

justify spraying chemicals onto open hill ground?” For example concerns have been

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raised regarding asulam’s contamination of dairy, honey and meat (Maren et al., 2008).

Alternative methods of bracken control and/or management and policy directions are

required. Although it has been difficult to associate bracken spread with climate change,

perhaps the prescience of the climate change debate can help hasten research,

management and policy actions.

Figure 4.9 The extent and potential extent of bracken in Great Britain with and without sprayed asulam (from Pakeman, 2012)

4.6 Future conservation strategies

“There needs to be a fundamental shift in the way we look at the uplands. Rather than seeing them as areas of severe disadvantage – which in turn influences policy and its delivery – they should be considered as areas of significant environmental, cultural and social value and opportunity.”

CRC (2010)

The uplands depend upon human stewardship to maintain their landscape character

(Bonn et al., 2009). Assorted different bodies and individuals have contrasting

perceptions of what the uplands represent: wildernesses; playgrounds; important,

priority habitats that harbour rare, threatened species; provisiders of important

ecosystem services; homes and workplaces. Future conservation strategies and policies

must account for this diversity of standpoints, and recognise the future requirements for

climate change adaptation and mitigation (Fagundez, 2013; House et al., 2010; Orr et al.,

2008). An ecosystems approach has been proposed (e.g. JNCC, 2014; Natural England,

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2009, Table 4.3), but to achieve their aims the endorsement of all stakeholders is

fundamental.

Table 4.3 The Twelve Ecosystem Approach Principles that aim “to promote conservation and sustainable use of natural resources in an equitable way through the integrated management of land, water and living resources” (from JNCC, 2014)

Ecosystem approach principle

1. Recognise objectives as society’s choice 2. Aim for decentralised management 3. Consider the extended impacts, or externalities 4. Understand the economic context and aim to reduce market distortion 5. Prioritise ecosystem services 6. Recognise and respect ecosystem limits 7. Operate at an appropriate scale, spatially and temporally 8. Manage for the long-term, considering lagged effects 9. Accept change as inherent and inevitable 10. Balance use and preservation 11. Bring all knowledge to bear 12. Involve all relevant stakeholders

The Vital Uplands Report sets out a vision for the uplands of 2060 (Table 4.4), which aim

to reduce environmental degradation whilst also providing:

Sustainable production of food, timber and other raw materials

Resilient uplands that adapt to, and mitigate for climate change

Clean water

flooding and wildfire reduction

Vital, healthy, happy and prosperous upland communities (Natural England, 2009)

Table 4.4 A vision for the uplands in 2060 compared to 2009 (adapted from Natural England, 2009)

Uplands in 2009 Vision for uplands in 2060

Areas of artificially drained peat bogs with exposed peat and eroding gullies

Restored blanket bog community, biodiversity increased, carbon losses prevented and downstream water quality improved

Burning regimes for the benefit of grouse, but can have negative environmental effects

Burning continues, but away from blanket bog

Footpath and track erosion worsened by increase extreme rainfall events

Woodland and scrub encourage to grow on bracken covered slopes, which mitigate for climate change, filter water, help prevent erosion, and provide a diverse habitat for species

Erosion resulting in increased sediment loads and DOC in streams, further downstream nitrates washed into stream can cause eutrophication

Abandoned pastures reclaimed from bracken infestation, and unimproved hay meadows restored

Bracken spread Farms using renewable energy technologies, possibly include camp sites to boost revenue from increased visitor numbers

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4.6.1 Contradictions of future strategies

The ecosystem approach gives rise to several policy inconsistencies and contradictions

(Marrs et al., 2000). Habitat and biodiversity conservation targets for the UK, for

example the Habitats Directive (92/43/EEC), the Birds Directive (2009/147/EC), the

Convention on Biological Diversity, and Aichi Targets, have objectives that focus on

halting natural succession. “The contemporary landscape is inherited as an integration of

past impacts” – e.g. moorland burning and grazing prevent the establishment of

woodland (Evans, 2009, Figure 4.9). Landscapes revert to a natural, climax state if

management is halted (Evans, 2009; Pakeman et al., 2008, Figure 4.9). However

‘rewilding’, advocated by for example Monbiot (2013), has been criticised by farmers

(Driver, 2014), and it is speculated that visitors would not want to see expanses

woodland, rather than a traditional, open upland panorama (Page, pers. comm., 2014).

Figure 4.9 The directional options for upland landscapes (from Evans, 2009)

4.6.2 The succession paradox

The management of heather moorland releases carbon, nutrients and biomass, and

maintains infertile soils (Marrs et al., 2007; Haigh, 2006). There exists a paradox: the

Kyoto Protocol (UNFCCC, 2013), the 2008 Climate Change Act (DECC, 2014), and other

UK policies target reducing greenhouse gas emissions, of which sequestration in soils

and woody biomass is an important component. Moreover, upland burning degrades

peatland (an important carbon sink), and contributes to watercourse pollution

(Clutterbuck and Yallop, 2010; Haigh, 2006), which is in conflict with achieving Water

Framework Directive (2000/60/EC) targets of ‘good ecological status’ (EC, 2014c).

Upland management has two options: 1) maintain plagio-climax communities, or 2)

allow succession, and Marrs et al. (2000) suggest taking “the path of least resistance”

towards an alternative priority habitat such as upland oak wood, which is the easiest and

cheapest recourse, and should address the bracken problem (Anderson, pers. comm.,

2014). Woodlands sequester much more carbon (Ostle et al., 2009), accumulate more

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litter and nutrients (Odum, 1985), and intercept pollutants (Helliwell et al., 2013). There

is a policy to increase oak wood cover in bracken invaded slopes in the north and east of

the PDNP (PDNPA, 2014), and Anderson (pers. comm., 2014) inquires whether, if grouse

shooting becomes less viable due to climate change, this policy should be extended to

heather moorland?

4.6.3 Confronting misdeeds

In the future the remote upland areas, much under private ownership, require robust

protection and good governance. This study observed burns and spraying right up to

watercourses in breach of regulations (Defra, 2008; 2007, Figure 4.10).

Figure 4.10 Possible breaches of regulations in the PDNP from 2005 imagery, the arrows show watercourses up to which a) asulam appears to have been sprayed, and b) there are apparently burn scars

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4.7 Study limitations

The most effective and accurate way to assess the dynamics of vegetation distribution is

to map in real time, allowing for in situ confirmation and analyses of on-the-ground

conditions. This study, on the other hand, maps snapshots of bracken cover at two

temporal periods. Comparing imagery from different periods in GIS to assess land-cover

change, such as habitat loss and fragmentation, deforestation, or ice retreat, is a common

application used to collect data and reveal trends, and one frequently pivotal in

answering numerous, diverse geographical and environmental questions (Kuzera and

Pontius, 2008; Foody, 2007; Visser and de Nils, 2006). Remote sensing is “the only

realistic, cost-effective means of acquiring data over large areas,” (Aplin, 2005).

However, although the concept appears straightforward, in practice it is complicated and

problematic (e.g. Pontius et al., 2008; Foody, 2007; Boots and Csillag, 2006; Hargrove et

al., 2006; Pontius and Lippitt, 2006). Comparisons are only as good as the imagery used

(Yu et al., 2006), and the facets and quality of the imagery being compared had an

influence on the methodological approach required to assess and answer the study’s

questions. The discrepancies of imagery between the study periods – of scale, resolution,

quality, and palette – makes absolute, unconditional comparison unattainable, however

this is normal with any comparison of historic imagery, and does not invalidate the

method or study (Tzotsos et al., 2011).

4.7.1 1960s imagery versus 2005 imagery

Bracken identification was the primary focus, and although the 1960s imagery was

black-and-white (b/w) and of lesser resolution than the 2005 imagery, bracken was

identifiable in the sub-sample grids. Identification of bracken in b/w imagery was most

clear against the backdrop of purple heather, and thus the sub-sample is located

generally in moorland in the north of the PDNP. However, these grids offered a good

representation of the 2005 sample above 200 masl (R2 = 0.93). The original, random 75

1 km2 grids were spread throughout the PDNP. Stipulating bracken presence eliminated

44 grids, many of which were in more southerly, lowland regions of the PDNP, a

landscape dominated by arable farmland and pasture, affording the 2005 sample a

northern bias.

The value in this study was a comparison over a relatively long time period (39 to 33

years), allowing for a noteworthy assessment of the change in bracken distribution.

However this necessitated a compromise; the sub-sample compared 13 rather than 31

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grids due to a) no available imagery, or b) lack of confidence in bracken identification.

Therefore there is greater confidence in a smaller sub-sample of 13 than there would

have been for an extended sample that included the grids covered by all the available

b/w imagery (Figures 4.11 and 4.12)

Figure 4.11 Bracken is easily identifiable in both images (of the identical area and scale) amongst heather, although grass is more easily identifiable in 2005 being darker and more brown-green. In b/w imagery it is identifiable amongst bracken by appearing darker and wispy

Figure 4.12 Bracken is not identifiable in the 1960s, but easy to identify in 2005. Imagery of the identical area and scale

4.7.2 Problems associated with identifying bracken

4.7.2.1 Growing and senesced fronds

When comparing mapped bracken ideally all the images should be from the same season

as there are small differences in extent at stand boundaries. Bracken mapped in the

growing season (1966, 1971, 2005) compared to in its senesced state (1968) can

overstate ground-level extent by up to 50 cm (Marrs, pers. comm., 2014). However,

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these small differences are unlikely to have affected the data, especially as only 1968 (7

grids) differed seasonally.

4.7.2.2 Shadows

Shadows can result in partial or total loss of image identification (Tolt et al., 2011; Care,

2005). Shadows by reason of topography, trees, rocks, and walls were all observed in

the study imagery, however these were minimised fortuitously by the times of day and

seasons (late April and summer) the imagery was taken, so long shadows were not cast.

Mapping continued through shadows where bracken’s presence seemed obvious (Figure

4.13). The overall extent of bracken mapped in shadow is small and it is considered did

not skew the data.

Figure 4.13 The images illustrate how shadows, woodland, and other features such as in a) walls and in b) lone trees were dealt with when mapping bracken in this study. It seems obvious that shadows cast from lone trees in large stands of bracken (in b) fall on bracken.

Trees under which bracken might have been present were not mapped or were clipped

out (Figure 4.13), therefore it is possible that bracken’s extent in the PDNP was greater

than recorded. However it is not regarded as a pest beneath woodland (Harmer et al.,

2005; Watt, 1976), although it is adept at colonising nearby open land.

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4.7.3 Climate data

The temperature data were obtained from a meteorological station (Sheffield) situated

at a lower altitude (131 masl) than the study sites, and about 15 km from the PDNP.

Additionally, temperatures acquired near towns can be higher on account of heat island

effects (Barry and Chorley, 2010). However, it was apparent that establishing a link

between climate and the altitudinal spread of bracken would be problematic due to

masking by anthropogenic upland activities. Therefore, establishing regional climate

trends was considered appropriate for an appraisal of their potential influences.

4.8 Recommendations for future research

“What I decided I could not continue doing was making decisions about intervening when I had no idea whether I was doing more harm than good”

Dr Archibald L. Cochrane (1909-1988)

From Pullin et al. (2013)

This study mapped 310 ha of bracken from 2005 imagery, and from this sample it was

extrapolated that bracken covered 62 km2 in the PDNP. Bracken extent and distribution

from the 1960s and 2005 was also compared using altitude, aspect, and slope to isolate

and analyse data (Chapter 3). However, it would be of great benefit to ascertain

conditions on-the-ground (edaphic, ecological, micro-climatic, and land management) to

understand better the dynamics of bracken spread, both driving and limiting it. Thus,

combined with remotely sensed data, areas vulnerable to colonisation can be identified.

Bracken distribution in other UK upland areas may be different than in the PDNP, and

therefore similar mapping and ground surveys are recommended, ideally to compile a

complete up-to-date UK upland database.

Climate change threatens to alter the uplands (e.g. House et al., 2010; Orr et al., 2008),

and House et al. (2010) state that maintaining “vegetation cover is a key win–win

management strategy that will reduce erosion and loss of soil carbon, and protect a

variety of services such as the continued delivery of a high quality water resource.”

This will not necessarily be achieved by maintaining business-as-usual management

practices, but require adaptation, possibly by allowing natural succession (Marrs et al.,

2000). If management practices continue, and emergency authorisations for the spraying

of asulam are rejected, an alternative, narrow-spectrum herbicide needs to be developed

that meets robust environmental standards (Berget, 2012), or otherwise biological

control needs to advance past trialling (Pakeman and Marrs, 1992; Lawton, 1988).

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Conservation is about tackling the problems associated with anthropogenic exploitation

and degradation of the environment (Pullin et al., 2013; Kapos et al., 2009). Scrutiny has

never been greater regarding the decision-making process and project implementation,

and Pullin and Knight (2009) believe questions like “are conservation interventions

effective?” and “are they doing more harm than good?” will be asked more often in the

future. Therefore future decisions for the PDNP, the UK uplands, and the wider

environment, need to be based on sound evidence, and be unambiguous, unequivocal,

and (as far as possible) future-proofed.

4.9 Concluding summary

“Landscape is not a static background which we inhabit, but the interaction of a society and the habitat it lives in, and if either man or the habitat changes then so inevitably must the resulting landscape. Landscape = habitat + man.”

Nan Fairbrother (1970) From the Landscape Strategy 2008 of the Peak District National Park Authority

Bracken has a worldwide distribution except Antarctica, and is found in a broad range of

environmental conditions where it is often viewed as an invasive weed. In the UK

uplands it encroaches into habitats of greater conservation and economic value,

although, at appropriate levels, it is a beneficial component of the landscape mosaic.

Mapping in ArcGIS recorded almost 310 ha of bracken in the 2005 sample, an increase

twice the per annum increase from 1990 estimations (1.8% p.a.) than increases

observed in the sub-sample between the 1960s and 2005 (0.9%), although within

accepted estimation of spread. This suggests that bracken spread has increased in recent

years, a conclusion that concurs with unpublished work (McAlpine, 2013) of mapping in

the PDNP between 1989 and 2005. However bracken extent may also have been under-

estimated in earlier years, in which case bracken’s expansion may have been more

regular and gradual.

Since 1990 bracken control by asulam spraying has remained constant, however heather

moorland burning has increased. This implies that increased burning, possibly combined

with the effects of climate change, has facilitated increased bracken spread more

recently. The implications of a ban on spraying asulam, the most effective control, are

accelerated encroachment, especially under scenarios of future uplands warming. This

therefore makes the discussion on successional paths relevant.

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Hopefully this study provides a beneficial contribution to ‘the bracken problem,’ adds to

the pool of evidence-based knowledge, and informs future studies, stakeholder

decisions, and upland polices. Several decisive areas for future research have been

recommended.

Sheep amongst bracken, the PDNP, July 2013 (by author)

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5. Annex

“At the end of the day… it will be the ethics and desires of the people, not their leaders, who give power to governments and NGOs or take it away. They will decide if there are to be more or fewer reserves, and choose whether particular species live or die”

Edward O. Wilson (2002) From The Future of Life

5.1 Introduction

The original proposal was for an assessment and comparison of bracken (Pteridium

aquilinum) areal extent in the 1940s and 2005 at elevations above 350 m in three upland

English regions - the Peak District National Park (PDNP), the North York Moors National

Park, and the Lake District National Park. My previous unpublished study found discrete

stands above 350 m in the PDNP had spread significantly more than accepted

estimations (McAlpine, 2013). The aim was to compare bracken distribution in the

uplands for the study periods, and describe changes in distribution by altitude, aspect

and slope, and describe the patterns of change.

By creating zones of elevation, and taking into account aspect and slope, it was hoped

that a more specific examination of the change in bracken distribution might highlight

where bracken is likely to spread, the dynamics involved, and whether this spread may

have been influenced by climate change (e.g. altitudinal increases in bracken’s historic

range).

However, this proposal evolved into the finished study: An Assessment of the Extent,

Distribution, and Change of Bracken (Pteridium aquilinum) in the Peak District National

Park, a one-region study in the PDNP which offers greater detail and more robust data

than would a broader, multi-region study.

5.2 The evolution of the study

5.2.1 Site selection

The original idea of the multi-region study was to constrain as many physical factors as

possible including altitude, aspect, and slope. At an elementary level this would comprise

finding bracken in a number of places at the same elevation, on a similar slope and with

similar aspect. It would then be logical to look at the same elevation and slope with a

different aspect, and the same aspect with a different slope, and so on. In essence the

number of samples would increase dramatically to fulfil the multitude of scenarios.

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Time constraints would have allowed for only a coarse assessment across the three

national parks, therefore one park was sampled comprehensively. The PDNP was

selected because a) imagery for 2005 was in-hand, b) it allowed comparisons to be made

with earlier work (McAlpine, 2013), c) the PDNP is representative of English uplands in

its ecosystems, management and threats, and d) it was the most proximate. Helpfully,

this removed regional climatic differences as a variable, although, of course, local and

micro-climatic variations exist within the PDNP (as they do elsewhere).

5.2.2 Variables and the elevation boundary

The use of 1 km2 Ordnance Survey grids has been used in other studies (Birnie et al., 2000;

Pakeman et al., 1996) and is essentially random compared to groundcover. From 75 1 km2

random grids, 44 were rejected and 31 were selected on the study pre-requisite of

containing bracken (see Methods). ArcGIS was used to assign mean elevation, slope and

aspect to a trial grid in which bracken was observed. Whilst trialling the strategy it

became apparent that within each grid was a potential diversity of variables. For

example Figure 5.1 illustrates the issue: the two-dimensional image of a grid with a

mean 350 m altitude masks the reality uncovered in three dimensions; topographic

heterogeneity in which bracken is present at elevations ranging from 256 to 417 m, and

on at least three opposing aspects and a range of slopes.

Whilst elevation is a vital component of the final study, the 350 m limit was abandoned.

Instead elevation was graded zonally (see Method) and bracken presence could be

assessed and compared by its distributions in zones of altitude, aspect and slope. This

discovery prompted various trains of thought, as it appeared that a 1 km2 grid may be

too coarse to constrain variables (mean slope and aspect for Figure 5.1 would be

meaningless). For example one thought was to map whole squares and delineate areas of

bracken and assign the variables to discrete portions for a multivariate analysis. Another

option considered was to map a large number of discrete stands (perhaps use a point

sample to identify stands to map), and determine change and compare to a larger

number of variables.

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Figure 5.1 The differences between two- and three-dimensional images: the diversity of elevation, aspect and slope is apparent in a three-dimensional image but not in a two-dimensional image

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The chosen method in this study, in which all the bracken observed was mapped in the

2005 sample and sub-sample study grids is considered the most robust (see Method). It

avoided introducing new parameters, and resulted in a useful areal estimation of

bracken extent in the PDNP in 2005 (62 km2), and interesting comparisons by altitude,

aspect and slope for the 2005 sample, and between study periods in the sub-sample.

5.2.3 Study period selection

The study hope to assess change over the longest possible period, however imagery for

the 1940s and 1950s in the PDNP was not of suitable quality in which to confidently

identify bracken in sufficient study grids. At the PDNP Authority in Bakewell a decision

was made to use imagery taken in 1966, 1968, and 1971, which were of better quality

and covered a good proportion of the 31 1 km2 study grids previously selected. In the

end just 13 grids from the 1960s could be confidently mapped.

5.3 Mapping

5.3.1 Mapped data

Mapping may be regarded as the straightforward, prosaic element of the study. Yet it is

the study. All the subsequent data are derived from the hundreds of thousands of

individual points mapped. Until the data are copied from attribute tables in GIS one is

totally unaware of the extent mapped, their potential significance, and the trends that

may emerge (e.g. of bracken apparently favouring certain altitudinal ranges). Thus it is a

nervous and exciting time when the data are merged, tabulated and analysed.

The maps showing how bracken has spread during the study period, its gains and losses,

are interesting and illustrate temporal bracken distribution at a local scale. However,

this study concentrated on total change, which enabled a robust assessment from which

projections could be made and trends emerged. Site-specific change is more the result of

a single random selection although the change that occurs is valid, yet in combination

the individual study grids generate the bigger picture.

The maps provide an ancillary benefit; they are pictorial representations of change,

easily understood, and thus useful tools for stakeholders. Figure 5.2 shows a mapped

grid from the sub-sample with the 1960s mapped extent overlaying the 2005 mapped

extent. Figure 5.3 demonstrates the mapped periods separately, and the gains and losses

of bracken extent from the 1960s to 2005.

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Figure 5.2 A 1km2 study grid in which is shown mapped bracken from 2005 (dark) overlaid with mapped bracken from the 1960s (light).

Figure 5.3 The mapped extents for the 1960s and 2005, and the losses and gains in bracken extent from the 1960s to 2005 in a 1km2 study grid

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5.3.2 ArcGIS

Mapping was undertaken on desktop computer in ArcGIS 10.0, and on a laptop in ArcGIS

10.1. However, these are not compatible and rather than simply saving a map in one to

work in the other, shapefiles need to created and imported into a new map: “Once you

open and save an existing map document … using ArcGIS 10.1, the map can no longer be

opened with earlier versions of ArcGIS because it will now reflect the new functionality

added at 10.1. Similarly, new documents you create with 10.1 also cannot be opened in

earlier versions of the software” (ArcGIS, 2014). This is inconvenient and time

consuming, and it is suggested that ArcGIS would be greatly improved if the versions

were compatible as they are on other (free) GIS platforms (e.g. Saga GIS, Grass GIS).

5.4 The Results

The results appear to support the method used, as they both demonstrate trends and

present nothing that is outlandishly unreasonable. The zones used were successful, the

amount of data derived substantial yet manageable. They are viewed as of interest and,

hopefully, of value.

Limiting the ranges would have resulted in data that was too coarse, and extending them

would have created far too many variables, consisting of smaller areas (in which there

would be less confidence of accuracy), for the timescale of this study. An extensive study

could include more aspects, and narrow the altitudinal and slopes zones, and introduce

more variables such as surrounding habitats, edaphic conditions, and substrates.

5.5 Bracken: further discussion

‘The bracken problem’ in the UK is viewed in terms of the impact it has on upland

habitats, species, and economies. It is also interesting to consider the issue in wider

contexts:

The value of bracken to other species

Should the effects of albedo be considered when making decisions on the

successional direction from bracken stands: restoration to early-successional

habitats, or forwards by natural (and/or induced) succession?

bracken toxicity and the hydrological cycle

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5.5.1 The value of bracken

Bracken has been acknowledged in this study as a valuable component of the landscape

mosaic at ‘appropriate’ levels, and for having conservation value (e.g. Marrs and Watt,

2006). But its value is considered low in comparison to the habitats into which it spreads

(e.g. Alday et al., 2013; Reed et al., 2009; Cox, 2007). Pakeman and Marrs (1992)

described its encroachment of heather moorland as “disastrous” for hitherto-

endangered birds like grouse and merlin (Falco columbarius). However bracken retains

value for a diversity of species groups (expanding on Table 1.1):

Mammals: bracken contributes shelter for fox (Vulpes vulpes) earths, badger (Meles

meles) setts, and rabbit (Oryctolagus cuniculus) burrows (Pakeman and Marrs, 1992).

Red (Cervus elaphus) and fallow (Dama dama) deer, and smaller mammals can hide

within bracken (Marrs and Watt, 2006; Nicholson and Paterson, 1976). The following

mammals also found in bracken habitats include: roe deer (Capreolus capreolus),

shrew (Sorex spp.) hedgehog (Erinaceus europaeus), wood mouse (Apodemus

sylvaticus), field vole (Microtus agrestis), and bank vole (Clethrionomys glareolus)

(Nicholson and Paterson, 1976).

Birds: bracken is regarded as of minimal ornithological value, and some birds

actively avoid it (e.g. red grouse, curlew (Numenius arquata), merlin, golden plover

(Pluvvialis apricaria), hen harrier (Circus cyaneus), short-eared owl (Asio flammeus),

and green-shank (Tringa nebularia)) (RSPB, 2012), for example 33 bird species

breed in heather moorland, 25 in acid grassland, compared to 15 in bracken

(Ratcliffe, 1977). Birds found in bracken habitats include: whinchat (Saxicola

rubetra), warblers (Phylloscopus spp.), ring ouzel (Turdus torquatus), tree pipit

(Anthus trivalis), nightjar (Caprimulgus europaeus), wrens (Troglodytidae spp.), and

twite (Carduelis flavirostis) (Pakeman and Marrs, 1992).

Reptiles: In bracken litter reptiles such as the common lizard (Lacerta vivipara),

sand lizard (Lacerta agilis), smooth snake (Coronella austriaca), and adder (Vipera

berus) can be found hibernating, although they require nearby open ground for

basking and courting (Marrs and Watt, 2006).

Invertebrates: bracken has a relatively small diversity, supporting only 27 “core”

invertebrates that feed either entirely or commonly on above-ground bracken and 11

that eat the rhizome (Marrs and Watt, 2006; Lawton et al., 1987). The only

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Coleoptera identified with bracken is the aptly named bracken chafer (Phyllopertha

horticola) (Marrs and Watt, 2006). The shelter that bracken affords to plants like

common cow-wheat (Melampyrum pratense) and dog violet (Viola riviniana) attract

scarce butterflies, including the pearl-bordered fritillary (Bolorio euphrosyne), small

pearl-bordered fritillary (Bolorio selene), heath fritillary (Mellicta athalia), and high

brown fritillary (Argynnis adippe) (Anderson, pers. comm., 2014; Warren and Oates,

1995).

5.5.2 Albedo and succession

Succession and ‘rewilding’ were discussed in the study, and future climate change

potentially brings the debate to the fore. Reforestation heeds UNFCCC and Kyoto

Protocol policies of climate change mitigation (Arora and Montenegro, 2011; Reyer et al.,

2009), and policies to prevent deforestation (UNFF, 2011).

However albedo (reflectivity) can determine if reforestation contributes to climate

change, rather than mitigating for it (Montenegro et al., 2009; Betts, 2000). In the tropics

evaporative cooling establishes cooler, moister conditions (Galos et al., 2013;

Montenegro et al., 2009). In boreal or taiga zones solar radiation creates warmth

through absorption (Wang et al., 2014; Betts, 2011; Bala et al., 2007). In temperate

climates there is less certainty, and it is disputed whether reforestation causes cooling,

or warming, or, indeed, if there is temperature any change at all (South et al., 2011;

Arora and Montenegro, 2011; Schwaiger and Bird, 2010). The recommended strategies

for climate change mitigation are: increase reforestation and prevent deforestation in

the tropics, whilst desisting from reforestation in the northern latitudes (Arora and

Montenegro, 2011).

Whether the albedo-effect will have any influence on successional policy in the PDNP is

unlikely as the effects are expected to be minimal in either direction, and the likelihood

of albedo-induced warming probably decreases under future climate change scenarios.

5.5.3 Bracken toxicity and hydrology

A further factor which may influence future successional policy is bracken’s toxicity.

Ptaquiloside “is a carcinogenic norsesquiterpene glucoside produced by bracken”

(Rasmussen et al., 2005), and it is a risk to human and animal health (Marrs and Watt,

2006), for example it contributes to the death of 1 million cattle p.a. in Brazil by plant

poisoning (Tokarnia et al., 2002). Following precipitation, it is thought to leach into the

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soil, potentially reaching potable water sources (Ramwell et al., 2010; Rasmussen et al.,

2005). However the amounts are difficult to quantify, being small and concealed by other

pollution within watercourses (Ramwell et al., 2010; Green et al., 2007). Can it be worse

than the effects of bracken herbicide run-off?

This study has been a pleasure to undertake, I hope it has been enjoyable – or at least

enlightening – to read.

Julian McAlpine, August 2014

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7. Appendix Appendix i The taxonomic classification of Pteridium aquilinum (L.) Kuhn (ITIS, 2011; Smith et al., 2006)

Kingdom Plantae—the plants

Sub-kingdom Viridaeplantae—green plants

Infra-kingdom Streptophyta—land plants

Division Tracheophyta—vascular plants

Sub-division Pteridophytina—pteridophytes, ferns and fern allies

Class Polypodiopsida—ferns

Sub-class Polypodiidae

Order Polypodiales

Family Dennstaedtiaceae—rhizomal, mostly creeping

Genus Pteridium

Species Pteridium aquilinum (L.) Kuhn—bracken, eagle fern

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Appendix ii

Climate data for the PDNP (PDNPA, 2004):

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Appendix iii

The top ranked proportion of areas in which bracken cover increased from the 1960s to 2005 in PDNP

Rank Altitudinal

range Aspect Slope

1 400-450 North 30-35

2 300-350 West 0-5

3 350-400 South 0-5

4 350-400 North 30-35

5 250-300 North 25-30

6 300-350 North 20-25

7 250-300 North 5-10

8 300-350 North 25-30

9 200-250 South 5-10

10 300-350 East 20-25

11 350-400 North 10-15

12 300-350 South 25-30

13 350-400 South 20-25

14 250-300 North 10-15

15 300-350 East 25-30

16 350-400 North 25-30

17 300-350 North 10-15

18 350-400 North 20-25

19 450-500 North 20-25

20 200-250 South 15-20

21 250-300 East 10-15

22 350-400 East 0-5

23 200-250 West 5-10

24 400-450 East 0-5

25 300-350 North 15-20

26 300-350 East 15-20

27 350-400 East 15-20

28 300-350 East 10-15

29 400-450 North 0-5

30 200-250 West 10-15

31 300-350 East 5-10

32 350-400 North 15-20

33 450-500 East 20-25

34 350-400 East 10-15

35 250-300 West 0-5

36 250-300 West 5-10

37 250-300 North 15-20

38 350-400 North 0-5

39 250-300 North 20-25

40 450-500 East 15-20

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Appendix iv

Graphs illustrating trends in summer and winter precipitation (mm), hours of sunlight, and frost free days at the Met Office Sheffield station 1966-2013

0

100

200

300

400

500

600

700

800

Apr-S

ep 1

968

Apr-S

ep 1

971

Apr-S

ep 1

974

Apr-S

ep 1

977

Apr-S

ep 1

980

Apr-S

ep 1

983

Apr-S

ep 1

986

Apr-S

ep 1

989

Apr-S

ep 1

992

Apr-S

ep 1

995

Apr-S

ep 1

998

Apr-S

ep 2

001

Apr-S

ep 2

004

Apr-S

ep 2

007

Apr-S

ep 2

010

Apr-S

ep 2

013

Prec

ipit

atio

n (m

m)

Summer (April to September) precipitation (mm) 1966-2013 with trendline added

0

100

200

300

400

500

600

700

winte

r 1966

winte

r 1969

winte

r 1972

winte

r 1975

winte

r 1978

winte

r1981

winte

r 1984

winte

r1987

winte

r 1990

winte

r 1993

winte

r 1996

winte

r 1999

winte

r 2002

winte

r 2005

winte

r 2008

winte

r 2011

Pre

cip

itat

ion

(m

m)

Winter (October to March) precipitation (mm) 1966-2013 with trendline added

119

0

200

400

600

800

1000

1200

1400

Apr-S

ep 1

968

Apr-S

ep 1

971

Apr-S

ep 1

974

Apr-S

ep 1

977

Apr-S

ep 1

980

Apr-S

ep 1

983

Apr-S

ep 1

986

Apr-S

ep 1

989

Apr-S

ep 1

992

Apr-S

ep 1

995

Apr-S

ep 1

998

Apr-S

ep 2

001

Apr-S

ep 2

004

Apr-S

ep 2

007

Apr-S

ep 2

010

Apr-S

ep 2

013

Ho

urs

Summer (April to September) hours of sunlight, 1966-2013 with trendline added

0

100

200

300

400

500

600

winte

r 1966

winte

r 1969

winte

r 1972

winte

r 1975

winte

r 1978

winte

r1981

winte

r 1984

winte

r1987

winte

r 1990

winte

r 1993

winte

r 1996

winte

r 1999

winte

r 2002

winte

r 2005

winte

r 2008

winte

r 2011

Ho

urs

Winter (October to March) hours of sunlight, 1966-2013 with trendline added

120

0

1

2

3

4

5

6

7

8

9

10

Apr-S

ep 1

968

Apr-S

ep 1

971

Apr-S

ep 1

974

Apr-S

ep 1

977

Apr-S

ep 1

980

Apr-S

ep 1

983

Apr-S

ep 1

986

Apr-S

ep 1

989

Apr-S

ep 1

992

Apr-S

ep 1

995

Apr-S

ep 1

998

Apr-S

ep 2

001

Apr-S

ep 2

004

Apr-S

ep 2

007

Apr-S

ep 2

010

Apr-S

ep 2

013

Day

s

Summer (April to September) frost free days, 1966-2013 with trendline added

0

10

20

30

40

50

60

70

winte

r 1966

winte

r 1969

winte

r 1972

winte

r 1975

winte

r 1978

winte

r1981

winte

r 1984

winte

r1987

winte

r 1990

winte

r 1993

winte

r 1996

winte

r 1999

winte

r 2002

winte

r 2005

winte

r 2008

winte

r 2011

Day

s

Winter (October to March) frost free days, 1966-2013 with trendline added