Restoration of bracken-invaded Calluna vulgaris heathlands: Effects on vegetation dynamics and...

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Restoration of bracken-invaded Calluna vulgaris heathlands:Effects on vegetation dynamics and non-target species

Inger Elisabeth Marena,b,*, Vigdis Vandvika, Kristine Ekelundc

aDepartment of Biology, University of Bergen, Allegaten 41, N-5007 Bergen, NorwaybDepartment of Natural History, University of Bergen, Allegaten 41, N-5007 Bergen, NorwaycThe Heathland Centre, Lygra, 5912 Seim, Norway

A R T I C L E I N F O

Article history:

Received 27 August 2007

Received in revised form

2 January 2008

Accepted 24 January 2008

Available online 10 March 2008

Keywords:

Asulam

Conservation management

Gratil

Northern heathlands

Organic farming

Pteridium aquilinum

A B S T R A C T

The coastal heathlands of north-western Europe are endangered habitats of great conser-

vation value. Invasion by bracken Pteridium aquilinum is a major challenge for conservation

and restoration of these heathlands, including the under-studied northern regions. Today,

the herbicide asulam is the most widely applied bracken control measure, but increasing

focus on organic farming and nature conservation calls for alternative, preferably mechan-

ical, approaches. In a 7-year replicated field experiment in western Norway, we investigated

efficiencies of the four bracken control measures asulam, Gratil, annual cutting and bian-

nual cutting, in restoring the characteristic heathland vegetation structure and species

composition. We specifically tested herbicide effects on diversity and composition of

non-target species. Effects of treatments over time were evaluated by repeated measures

ANOVA, and for multivariate data, Principal Response Curves. Our results show that UK

based control methods are largely applicable to bracken at its northern limit in the Euro-

pean heathland habitat. Asulam resulted in the fastest reduction in cover but cutting

proved equally efficient long-term. Community compositions progressed towards desired

heathland vegetation, but successional trajectories differed. Asulam had unintended

effects on a number of heathland species not predictable by species characteristics or func-

tional groups. Gratil failed to have any long-term effects. In summary, cutting is as efficient

as herbicide application in reducing bracken, and more so in restoring northern heathland

vegetation over time.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

The lowland heathlands of Europe are endangered habitats

of considerable conservation value (Gimingham, 1992; Aar-

restad et al., 2001), now protected under the EU Habitats

Directive (92/43/EEC). These are man-made cyclic vegetation

systems where secondary succession is manipulated by

management, and major threats are abandonment leading

to woodland encroachment, development leading to frag-

mentation, pollution and subsequent loss of diversity, and

invasion of species such as bracken [Pteridium aquilinum (L.)

Kuhn]. The recent spread of bracken is recognised as a seri-

ous threat to the unique qualities of heathlands as it elimi-

nates characteristic ericoid shrubs, graminoids and forbs by

changing the successional dynamics of these semi-natural

systems (Watt, 1955; Gimingham, 1972; Kaland and Vandvik,

0006-3207/$ - see front matter � 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.biocon.2008.01.012

* Corresponding author: Department of Biology, University of Bergen, Allegaten 41, 5007 Bergen, Norway. Tel.: +47 55588136; fax: +4755589667.

E-mail addresses: [email protected] (I.E. Maren), [email protected] (V. Vandvik), [email protected](K. Ekelund).

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ava i lab le at www.sc iencedi rec t .com

journal homepage: www.elsevier .com/ locate /b iocon


1998; Marrs and Watt, 2006), yielding habitats of low conser-

vation, agricultural and recreational value (Lowday and Mar-

rs, 1992a).

Bracken is the most widely distributed Pteridophyte on

earth and the only terrestrial fern to dominate large areas

outside woodlands in temperate climates (Marrs et al.,

2000). Its clones can occupy several hectares, expanding

quickly in areas where there is good oxygen supply in the soil

and with soil depths >20 cm. An extensive rhizome network

works as a store for carbohydrates and contains large num-

bers of dormant buds (Lowday, 1984). Once bracken is estab-

lished, slowly-decaying litter (Ghorbani et al., 2006) and

potentially toxic compounds (Dolling et al., 1994; Dolling,

1996) may inhibit seed germination, establishment and

growth of many characteristic heathland species, including

Calluna vulgaris. It is one of a diverse group of species that

are able to expand under grazing by combining the ability to

maintain dominance at high density with avoidance of graz-

ing (Tryon, 1941; Page, 1976, 1994; Marrs and Watt, 2006).

Changes in climate and land use may favour the spread of

bracken (Marrs et al., 2000). For example, bracken may benefit

from reductions of extensive livestock grazing (cattle effec-

tively trample and also eat some bracken; Williams, 1980;

Page, 1982; Pakeman et al., 2005), as well as from recent in-

creases in airborne nitrogen deposition and use of artificial

fertilizers (bracken occurs on relatively fertile soils; Miles,

1985).

One of the keys to bracken control lays in exhausting the

rhizome reserves of buds and carbohydrates (Braid, 1937; Wil-

liams and Foley, 1976; Lowday and Marrs, 1992a; Pakeman and

Marrs, 1994). Traditionally, bracken was kept in check by graz-

ing and cutting, and fronds used as bedding for livestock.

Experiments confirm that biannual cutting can be an efficient

control measure (Lowday and Marrs, 1992a, b; Marrs et al.,

1993, 1998a; Le Duc et al., 2007). In the 1960s herbicides be-

came widely used in the UK and at present asulam is the most

common means of controlling bracken in Europe (Petrov and

Marrs, 2000). Recently, asulam’s long-term efficiency has been

questioned and multiple follow-up treatments are often rec-

ommended (Stewart et al., 2005; Marrs and Watt, 2006; Pak-

eman et al., 2007). Chemically-based practices also create an

ethical dilemma for nature conservation, as well as for farm-

ers aiming to produce e.g., meats, dairy products and honey

for the increasing organic market.

Norway harbours the northern 1/3 of Europe’s coastal

heathlands, characterized by long and unbroken histories of

management by means of burning and year-round grazing

by sheep of the Old Norse breed (Kaland and Vandvik, 1998;

Prøsch-Danielsen and Simonsen, 2000; Vandvik et al., 2005).

In contrast to their European Union counterparts, which are

protected under the Habitats Directive, these northern heath-

land habitats lack national legislative instruments to ensure

their conservation. Bracken is absent from the northernmost

heathlands, but south of ca. 62�N it is expanding in heathland

areas. However, bracken control has never been studied sys-

tematically in these northern heathlands. Asulam is not a le-

gal herbicide in Norway and has never been tested here, but a

two-year study of the herbicide Gratil in western Norway

(Skuterud, 1998) reported effects comparable to those of asu-

lam in the UK, but with less negative impact on non-target

vegetation. Both herbicides were therefore included in this

study.

Most evaluations of different control measures have fo-

cused on reducing bracken cover and on restoring heather

and a few other flagship species (Pakeman and Hay, 1996;

Marrs et al., 1998a, 2000; Mitchell et al., 1998; Pakeman

et al., 1998, 2000; Britton et al., 2001). In contrast, nature con-

servation and organic farming may have more demanding

criteria for restoration success, particularly regarding the

fates of non-target species (Cadbury, 1976; Pakeman et al.,

1997), the rate and direction of revegetation (Pakeman et al.,

2007), biodiversity, species composition and other ecosystem

characteristics. This calls for more detailed head to head

comparisons of different control strategies, in particular dif-

ferent mechanical cutting schemes vs. different chemically-

based strategies (Stewart et al., 2005). In this study we com-

pare the efficiency of different mechanical and herbicide

bracken control practices. Our main focus is restoration of

the heathland community and potential impacts of herbi-

cides on the species composition and diversity. We ask: (1)

How effective are treatments in reducing bracken? (2) How

successful are these different control measures in restoring

heathland structure and community composition? (3) Do

the herbicides affect structure, diversity, or composition of

the non-target community? To address these questions we

performed a replicated before–after control-impact design

experiment and analysed responses over seven years using

repeated measurements ANOVAs, redundancy analysis

(RDA) and principal response curves (PRC).

2. Materials and methods

2.1. Study area

The island of Lygra is situated at 60�42’N, and 5�5’E, in the

Lurefjorden fjord basin, approximately 20 km inland from

the coast, 40 km north west of Bergen (Fig. 1). Hard and slowly

eroding bedrock gives rise to nutrient-poor soils. Climate is

oceanic with relatively small differences between June;

12.0 �C and January; 2.0 �C mean temperatures, the 1600 mm

of precipitation is relatively evenly distributed throughout

the year (Førland, 1993) and there is a long growing season

(ca. 220 days). Lygra is dominated by Calluna/grass heaths with

mires/Salix shrubs in wetter areas. Parts of the semi-natural

rangeland have been under continuous management (burn-

ing, grazing, and turf and heather cutting) up until today, cre-

ating a mosaic heathland of different successional stages

(Vandvik et al., 2005). The area is grazed by 0.1 cow/ha in sum-

mer and by 0.8 sheep/ha in winter (Samson Øpstad; unpub-

lished data), which is comparable to stocking level in other

heathland areas (see Hulme et al., 2002). Bracken has in-

creased and by 2004 it had invaded ca. 30% of the area. Exper-

iments were carried out in two adjacent areas; A; invaded by

bracken and B; no bracken present. In both areas, ericoid

shrub species such as C. vulgaris, Vaccinium myrtillus and V. vi-

tis-idaea occurred throughout, in combination with common

graminoids, forbs and mosses. Nomenclature follows Lid

and Lid (1994) for vascular plants, Smith (1990) for mosses

and Krogh et al. (1980) for lichens.

B I O L O G I C A L C O N S E R V A T I O N 1 4 1 ( 2 0 0 8 ) 1 0 3 2 – 1 0 4 2 1033


2.2. Experimental design

2.2.1. Herbicide controlAsulam [methyl(4-aminobenzenesulphonyl)carbamate] is a

selective post emergent systemic herbicide which controls

several annual graminoids, ferns and broad leaved weeds in

crop fields by entering the rhizome and accumulating in both

active and dormant buds, causing death (Veerasekaran et al.,

1976). Bracken frond biomass declines sharply during the two

years following treatment but without further management

some of the dormant buds and parts of the rhizome survive

and it may expand again (Pakeman et al., 2005). Exposure to

asulam has shown to inhibit growth of mosses (Rowntree

et al., 2003) and as it is a mobile herbicide with high potential

for leakage into ground or surface waters, effects on non-tar-

get species are likely and need to be investigated further. Gra-

til WG 75 (amide sulforon), classified as a sulfonyl urea, is

used against Rumex spp. and Ranunculus spp. in fields. It

assimilates through foliage and roots, preventing the forming

of certain amino acids essential for growth. It does not leak

into ground water, but is poisonous for aquatic organisms

and can cause long-term damage to aquatic environments

(Whitford et al., 2002; Bayer CropScience, 2007). Its systemic

effect on the acetolacetate-synthesis terminates growth with-

in 48 h. Wilting of bracken is seen within 2–4 weeks (Skute-

rud, 1998).

2.2.2. Experimental design and samplingExperimental square plots of 25 m2 were established in two

areas, A; invaded by bracken and B; no bracken present. The

Fig. 1 – Location of the island of Lygra in the fjord system of Western Norway.

1034 B I O L O G I C A L C O N S E R V A T I O N 1 4 1 ( 2 0 0 8 ) 1 0 3 2 – 1 0 4 2


latter area was included to enable us to test for herbicide ef-

fects on the heathland community, independent of the effect

of removing bracken (see below). Plots were placed at least

10 m apart, creating a buffer zone to avoid effects of airborne

herbicides (Marrs et al., 1989; Gove et al., 2007). Three

0.5 m · 0.5 m permanent quadrats were randomly placed

within each plot. In mid June of year T0 quadrates were ana-

lysed for % cover of all species of vascular plants, bryophytes

and lichens, functional groups (graminoids, forbs, ericoid

shrubs and mosses), and environmental parameters (topogra-

phy, slope, and aspect). Treatments applications were started

in T0 and the quadrats were reanalyzed yearly in July/August

of T1–T6.

Treatments were randomly allocated to plots within each

area according to Table 1. The experimental design was ex-

panded successively over the first three years, yielding a bal-

anced design of four replicate plots · five treatments in area A

and four replicate plots · three treatments in area B. In all cut-

ting treatments bracken stems were cut app. 20–30 cm above

ground, not affecting the non-target vegetation, and newly

cut bracken fronds removed. After initial testing of different

cutting techniques (motorised vs. scythes) a long handled

scythe was selected as the most efficient tool for our area.

In area A, follow-up cutting of emerging bracken fronds in

the chemically-treated plots was performed yearly in late July

following the methods of Lowday and Marrs (1992a).

2.3. Data analysis

The final experimental setup is a balanced BACI (before–after,

control-impact) design with four replicate plots per treat-

ment, and the general statistical approach chosen was re-

peated measures ANOVA (Sokal and Rohlf, 1995) for cover

data and its multivariate counterpart principal response

curves (PRC; van den Brink and ter Braak, 1998; van den Brink

and ter Braak, 1999; van den Brink et al., 2003) for species

composition data. The general model is

y ¼ treatmentþ timeþ treatment� time

with the predictor variables ‘treatment’ and ‘time’ included as

factorial variables in all analyses and with different univari-

ate or multivariate response variables (bracken or functional

group cover; species composition) depending on the research

questions. Because of the successive extensions of the study

(see above), starting years differed between plots (1997,

1999) resulting in time-series of different lengths. This was

accounted for by analysing for effects of treatments as a func-

tion of time since treatment, rather than of actual Julian

years. This could have been problematic, e.g., if there had

been temporal trends of change in the vegetation in our study

area due to climate, successional dynamics, or other broad-

scale factors. We tested this by analysing among-year varia-

tion in control plots, and found no significant temporal

trends; hence the differing starting years are unlikely to have

introduced biases in our results.

Repeated measures ANOVA was used to analyse for effects

of different bracken control treatments on bracken cover and

on the cover of ericoid shrubs, graminoids, forbs, and bryo-

phytes (question 2), using data from the bracken-invaded area

A. The analyses were run using the procedure MIXED in SAS

9.1., cover data were arcsine transformed to normalize vari-

ances, a first order autoregressive covariance structure across

time were used, and error degrees of freedom were estimated

by the Satterthwaite approximation (SAS Institute, 2003). In

cases of significant treatment effects, differences in the least

square means among treatments were tested by post hoc

tests, with Bonferroni correction (SAS Institute, 2003).

Redundancy analysis (RDA) (RDA; ter Braak, 1987) was

used to quantify and visualise effects of the different treat-

ments on the species composition of the community (ques-

tion 2). The effects of asulam on the non-target community

(question 2) were tested by principal response curves (PRC;

van den Brink and ter Braak, 1999; van den Brink et al.,

2003). PRC is the multivariate equivalent of repeated mea-

sures ANOVA, and analyses the community response through

time to one or more treatments relative to a control. It is a

partial RDA where treatments and time are included as facto-

rial variables in a model analysing the effects of the

time · treatment interaction while including time as a covar-

iate to control for any overall temporal trends. Treatment ef-

fects (Cdt) quantify the compositional difference between

treated plots and controls at each sampling date, and tempo-

ral trends can be visualised by plotting Cdt against time. The

species scores (bk) can be interpreted as the affinity of each

species with this overall effect; species with high positive val-

ues follow the overall community response, species with high

negative values respond in the opposite way, and species

with values near zero do not respond to the treatment. PRCs

Table 1 – Overview of treatments applied to area A (60–90% bracken cover before treatment), and area B (without bracken),four replicates of each treatment x area, a total of 32 plots (96 quadrats)

Treatment Area T0 Treatment details

Asulam A*, B 1997/1998 Sprayed with 4 kg a.i. ha�1 asulox July 31st

grazers excluded for 90 days after spraying**

9Annual cutting A 1997/1998 Bracken cut annually in late July

No treatment A, B 1997/1998 No treatment applied

Biannual cutting A 1999 Bracken cut biannually in mid June and late July

Gratil A*, B 1999 Sprayed with 0.06–0.08 kg a.i. ha�1 Gratil August 1st

Grazers excluded for 7 days after spraying**

T0 = the year treatment was initiated.

* Bracken regrowth cut annually in late July following the methods of Lowday and Marrs (1992a).

** Following the prescriptions for use of these herbicides.



can be tested by Monte Carl permutation tests where whole

time-series are permuted freely within areas, and changes

in treatment effects through time can be evaluated in

sequential tests for each sampling time, permuting the data

freely within areas. The analyses were run on log-trans-

formed percentage cover data with down-weighting of rare

species (Leps and Smilauer, 2003). Bracken was not included

in the RDA or PRC analyses, but was plotted on the diagrams

for illustrative purposes. 999 permutations were run in all

permutation tests. The software packages CANOCO 4.5 and

CanoDraw 4.0 (ter Braak and Smilauer, 2002) were used for

analyses and ordination diagrams.

3. Results

3.1. Reducing bracken cover

While untreated plots showed only small non-significant

variations in bracken cover over the seven years of recording,

all experimental bracken control treatments had strong but

variable effects on bracken cover (repeated measures ANOVA:

‘treatment’ effect, d.f. = 17.4, F = 67.9, p(F) = <0.0001; ‘treat-

ment’ · ‘time’ interaction, d.f. = 226, F = 15.9, p(F) < 0.0001;

Fig. 2). Herbicides resulted in the fastest reduction of bracken:

Under the asulam treatment bracken cover decreased by 99%

0

10

20

30

40

50

60

70

80

90

100

T0 T1 T2 T3 T4 T5 T6

Years since onset of treatment

Mea

n %

cov

er o

f bra

cken

in q

uadr

ats

0

10

20

30

40

50

T0 T1 T2 T3 T4 T5 T6Years since onset of treatment

Mea

n %

cov

er o

f eric

oid

shru

bs in

qua

drat

a

b

Fig. 2 – (a) Mean cover (%) of bracken in experimental plots over time. Within each year, filled symbols signify treatments that

are significantly different from untreated plots, and different shades signify that the treatment is significantly different from

treatments above and below it in the graph at that point in time (P < 0.05 after Bonferroni correction). (b) Trends in mean cover

(%) of ericoid shrubs. Treatments; –X– untreated, –d– asulam + annual follow-up cutting, –j– gratil + annual follow-up

cutting, –�– cut twice yearly, –m– cut once yearly. T0 = year treatment was initiated. (The gratil and cutting twice yearly

treatments lasted five years only, any treatment years where n < 3 are not included).



during the first year after treatment, and the effect persisted

throughout the seven years of the experiment. The Gratil

treatment, with a 98% decrease, was equally efficient short-

term but after four years the bracken started to recover and

it had regained 55% cover after five years. Due to Gratil’s obvi-

ous inefficiency in controlling bracken long-term, the full

analyses of the effects on the non-target community (ques-

tions 2 and 3) are not presented in this paper (although we

note that PRC analyses indicate negative effect on Agrostis

capillaris, Galium saxatile and Luzula multiflora). Effects of the

cutting treatments appeared more slowly, but were equally

efficient as asulam in the long run. Biannual cutting reduced

bracken cover by 75% the first year, and was indistinguishable

from asulam from the second year onwards. Significant ef-

fects of annual cutting did not appear until the second year,

but they increased gradually over time and were indistin-

guishable from asulam after five years.

3.2. Restoring heathland vegetation

Despite high initial bracken dominance, the vegetation was

relatively species-rich with 61 taxa of vascular plants, 23 bry-

ophytes, and two lichens recorded during the seven-year

study. The vegetation was dominated by, in order of decreas-

ing mean cover, P. aquilinum (47%), Rhytidiadelphus squarrosus

(37%), A. capillaris (25%), Hylocomium splendens (18%), Erica tet-

ralix (17%), C. vulgaris (16%) and Potentilla erecta (14%). A num-

ber of less dominant species occurred with high frequency,

notably G. saxatile, Anthoxantum odoratum, Deschampsia flexu-

osa, L. multiflora, Campanula rotundifolia and Carex pilulifera.

Treatments induced considerable changes in species com-

position. The first axis in the treatments · time RDA accounts

for 9.9% of the total compositional variability in area A and re-

flects a general successional trend in the treated plots away

from the controls (Fig. 3). This trend is associated with

increasing abundance of a large number of species including

ericoid shrubs, graminoids, forbs and bryophytes, and

decreasing abundance of only one species, A. capillaris, in

addition to the treated species P. aquilinum. The second RDA

axis (5.4% of the variability) mainly reflects differences in

starting conditions between blocks.

In contrast to the strong floristic responses we found no

overall treatment effects on relative abundances of the dif-

ferent functional groups (forbs, graminoids, ericoid shrubs,

mosses) (repeated measures ANOVAs; P > 0.05 after Bonfer-

roni correction for ‘treatment’ effects and ‘treatment’ ·‘time’ interactions in all cases), suggesting that species with-

in each of these groups respond individualistically to the

treatments. The observed changes in species composition

combine two processes; establishment of new individuals

(from dispersal or soil seed bank) and increased cover of

individuals already present at onset of the experiments.

For ericoid shrubs, which are of particular interest in heath-

land restoration, we found that while these species in-

creased in cover over time in treated plots where they

were initially present (Fig. 2b) colonization of new plots

was rarely observed (only 5 cases during the course of the

study). This increases variability between replicate treated

plots over time, which may contribute to the lack of signifi-

cant responses in terms of functional group cover in our

experiments (e.g., Fig. 2b).

3.3. Effects of asulam on non-target species

The PRC diagrams (Fig. 4) focus on the effects of asulam on

the non-target plant community by contrasting successional

trends in the asulam-treated areas with unsprayed areas. To

evaluate the herbicide effect per se, the annual cutting treat-

ment was used as control in this analysis. The treatment ef-

fects (Cdt) quantify the compositional difference between

plots and controls at each sampling date, and temporal trends

are visualized by plotting Cdt against time (Fig. 4). Here, spe-

cies with high positive bk weights follow the overall commu-

nity response, and are negatively affected by the asulam

treatment; V. myrtillus, Euphrasia micrantha, Achillea millefolium,

Pleurozium screberii, Rumex acetosa, Danthonia decumbens, Poly-

trichum commune and Pseudoscleropodium purum, in addition

to P. aquilinum. C. vulgaris was also negatively affected. How-

ever, the majority of the non-target species actually re-

sponded positively to the asulam treatment. The treatment-

control contrast in these areas (Fig. 4a) reflects the joint effect

of decreased bracken cover and spraying. To tease apart these

effects, we specifically tested the effect of asulam sprayed di-

rectly on the non-target community in the area with no

bracken; B (Fig. 4b). This resulted in significant effects on a

similar number of species, but here the effect of the herbicide

was predominately negative, hence high positive bk weights

for species like A. capillaris, A. odoratum, Veronica officinalis,

G. saxatile, Juncus squarrosus, Circium palustre, Lotus corniculatus,

Ranunculus acris, H. splendens, Viola palustris, Holcus lanatus,

Trientalis europaea and P. erecta. In both analyses, there was a

significant compositional response to spraying in the first

-0.6 RDA axis 1 (9.9%) 0.8

-0.4

RD

A ax

is 2

(5.4

%)

0.6

Pter aquAgro can

Agro capAnth odo

Care pilDant dec

Fest viv

Luzu pil

Call vul

Vacc myr

Vacc v-i

Hylo spl

Hypn jut

Pleu sch

Poly comPseu pur

Rhyt squ

Achi mil

Anem nem

Dact mac

Euph mic

Gali sax

Lotu cor

Oxal ace

Pote ere

Viol pal

Hype pul

T0

T6

T0

T6

T0

T6

T0

T4

Fig. 3 – Multivariate redundancy analysis (RDA) ordination

diagram of species and bracken control treatments through

time for the heathlands at Lygra, Norway. Compositional

change within the different treatments over the course of

seven years (cut twice; five years) is drawn as trajectories

where T0 demarks the year treatment was initiated, and T6

(cut twice; T4) demarks the last year of treatment.

Treatments; –X– untreated, –d– asulam + annual follow-up

cutting, –�– cut twice yearly, –m– cut once yearly. Species

names abbreviations are the four + three first letters of the

genus and species names, respectively.



Fig. 4 – PRC diagrams and species scores (bk) on PRC axis 1, showing the overall impact of asulam on heathland species

composition (treatment effects; PRC axis 1 or Cdt) relative to; (a) unsprayed cut once (controls) in bracken-invaded heath and

(b) unsprayed in heath without bracken. The responses (bk) of individual species are shown to the right: high positive bk

values indicate the species’ response to be well described by the PRC, i.e. decrease in the sprayed areas, high negative values

are indicative of species that increase in sprayed areas. Only species with relatively strong responses are shown, species with

bk values near 0 are not significantly affected by the treatment. *Significantly different from the control, P < 0.05, **Significantly

different from the control, P < 0.001. Species listed above the dotted line to the right are negatively affected by the asulam

treatment, and species below it are positively affected.



years after treatment, an effect that weakened and disap-

peared within 2–5 years.

4. Discussion

4.1. Mechanical versus herbicide treatment; what worksbest and when?

This paper presents the results of different bracken control

treatments on bracken cover, non-target species and vegeta-

tion dynamics to help assess the role of herbicide versus

mechanical control, especially with regard to organic farming

and nature conservation. We found that all treatments re-

duced bracken significantly during the course of the test per-

iod, but that the response rates differed considerably.

Biannual cuttings or spraying with asulam followed by annual

cutting were the most effective means of reducing bracken

cover long-term, confirming that previously reported high

efficiency of these measures (Lowday, 1984; Marrs et al.,

1993, 1998a; Paterson et al., 1997; Le Duc et al., 2007) also

holds true for northern heathlands. Yearly cutting took longer

to take effect, as one might expect, but once it did it was as

effective as cutting biannually. As asulam is not legal in Nor-

way, and as Gratil has been recommended as an alternative to

asulam based on one short-term (2-year) experiment (Skute-

rud, 1998), it was of interest to include Gratil into the experi-

mental protocol. In doing so, we also highlight the

importance of long-term monitoring in researching vegeta-

tional change, as called for by Stewart et al. (2005). However,

our results show Gratil sprayed bracken to regain dense cover

the third year after treatment and can not be recommended

as a bracken control tool. This recovery, in spite of follow-

up cutting, is difficult to explain and should be addressed in

further investigations.

4.2. Do we restore heathland after bracken control?

All bracken control treatments affected species composition,

inducing a shift towards more open and species-rich commu-

nities dominated by ericoid shrubs, graminoids, and forbs

(Fig. 3). We found more species benefiting from the removal

of bracken than suffering from the treatments, such as Hyper-

icum pulchrum, C. pilulifera, V. officinalis, Conopodium majus, L.

corniculatus, Festuca vivipara and Vaccinium vitis-idaea. Some

previous attempts at restoring Calluna heathlands by bracken

control in the UK have resulted in grass-dominated commu-

nities, and simply controlling bracken by herbicides may not

result in conservation or restoration of Calluna heathlands

(Pakeman et al., 1997).This seems not to be the case at our

site. The grazing regime at the site (sheep and cattle) is one

possible explanation as Williams (1980) found bracken re-

growth after asulam application to be considerably slowed

by grazing sheep and cattle compared to sites grazed by sheep

alone. Hence, trampling by cattle can be an important factor

in bracken control, at least in areas where bracken stands

are not too dense. Our use of follow-up annual cutting treat-

ments may also have increased the rate of success in re-

establishing desirable heathland vegetation (see also Lowday

and Marrs, 1992a; Marrs et al., 1998b).

4.3. Do herbicides affect non-target species?

In the bracken-dominated area the majority of non-target

species were positively affected by asulam, suggesting the

herbicide to have little detrimental effect on biodiversity.

However, this effect confounds two causal factors: the herbi-

cide per se and its effect through the removal of bracken

fronds. Treatments in the area lacking bracken were included

in the experimental setup to tease apart these two effects.

Here, a majority of species were negatively affected, suggest-

ing asulam to have negative effects on the biodiversity of non-

target communities. A dense cover of bracken fronds will act

as an umbrella, somewhat protecting the underlying vegeta-

tion from the full effects of the chemicals. However, the

topography, vegetation cover and bracken density of northern

heaths are very heterogeneous and herbicide application will

unavoidably result in non-target species being sprayed di-

rectly. This calls for caution in herbicide application in heter-

ogeneous heathlands, as found in northern regions, where

non-target communities are intermingled with bracken-in-

vaded heath (Vandvik et al., 2005). The negatively affected

species belonged to different taxonomic and functional

groups, and include the graminoids A. capillaris, Anthoxanthum

odoratum, J. squarrosus and H. lanatus, the forbs V. officinalis, G.

saxatile, L. corniculatus, V. palustris, T. europaea and P. erecta, the

ericaceous dwarf-shrub V. myrtillus, and the mosses H. splen-

dens and Psuedoscleropodium purum. A number of additional

species showed negative trends but occurred too sparsely to

prove statistical significance. V. myrtillus and P. purum were

negatively affected in both areas and might be particularly

sensitive to asulam. Other species found to be asulam sensi-

tive are Digitalis purpurea, Prunella vulgaris, Lychnis flos-cuculi

and Centaurea nigra (Marrs et al., 1986). A particularly interest-

ing group of species are those unaffected or showing a posi-

tive response in area A, yet negatively affected in area B,

such as V. officinalis and L. corniculatus. This apparent shift

can be accounted for by the ‘dual effect’ of asulam in the

bracken-dominated area where the positive effect of its re-

moval cancels out the negative effect of the herbicide per se.

By including this ‘double control-method’ in our experimen-

tal setup, we are able to identify this group of bracken-sup-

pressed, yet herbicide-sensitive species. Such species may

be particularly difficult to restore by chemical control, espe-

cially if repeated spraying is part of the protocol. This study

was carried out on a small scale in vegetation dominated by

common species. Species negatively affected by the herbi-

cides belonged to different taxonomic and functional groups,

making generalisations of potential responses of other

(groups of) species very difficult and need further investiga-

tion. In particular, many ferns are sensitive to asulam (Pak-

eman et al., 2000), and particular care should be taken in

areas with a diverse and/or threatened fern flora.

4.4. Implications for management and conservation

As most studies of herbicides are conducted in the fields of

agriculture, forestry or by the manufacturers, they yield little

information on the effects of these herbicides on semi-natu-

ral vegetation for conservation or restoration purposes. While

much is known about the effects on target species, there is



less information regarding their effects on non-target vegeta-

tion dynamics and on endangered or vulnerable non-target

species likely to be found in heathlands (Marrs, 1985). For suc-

cessful conservation management these effects should be of

most important consideration (Critchley et al., 2004; Bremner

and Park, 2007). For organic farming, which preclude the use

of chemical control, alternative control methods need to be

formulated. Our work shows that biannual cutting retards re-

growth sufficiently for effective control. Annual cutting was

nearly as efficient as biannual cutting after five years, and

may be a more economic option long-term. It is not possible

to eradicate the species permanently; Marrs et al. (1998a)

did not succeed in this even after 18 years of continued brack-

en control, but the population can be kept at a level accept-

able for keeping grazing livestock and conserving the

threatened habitat of coastal heathlands.

All bracken control measures were more efficient in reduc-

ing bracken cover and had longer-lasting effects in this study

compared to studies further south in Europe (e.g., Lowday and

Marrs, 1992a; Marrs et al., 1998a). This could be due to cli-

matic and environmental constraints at the northern brink

of bracken’s distribution in the heathland habitat, suggesting

that bracken control may be easier and less labour intensive

here. On the other hand, bracken shows strong plastic re-

sponses to yearly climate variability (Le Duc et al., 2003). In

northern areas, where temperature is the main limiting factor

for growth, future increases in temperature and growing sea-

son duration could imply range expansion, increasing rates of

bracken invasions and denser bracken stands in already-

invaded areas (Marrs et al., 2000; IPCC, 2007). Future climate

change could therefore result in a greater need for

bracken control measures in the management of northern

heathlands.

In conclusion, we note that selection of bracken control

measures for heathland conservation, restoration or manage-

ment needs to take into account regional location and topog-

raphy, the desirable future vegetation after control as well as

any special needs of particular land-uses such as organic

farming, habitat conservation or conservation of rare/endan-

gered species. Mechanical control can be relatively efficient,

especially in northern areas when combined with grazing.

These are important issues as it is likely that the use of brack-

en control measures will have to increase if the heathlands of

Northern Europe are to be conserved for the future.

Acknowledgements

We thank P.E. Kaland for initiating the project, V. Dahl, J. Wil-

helmsen and M. Kvamme for field assistance, E. Heegaard for

identifying bryophytes and lichens, B. Helle for technical

assistance, and two anonymous referees for constructive

comments. This work was funded by the Agricultural board

of Hordaland, the University of Bergen, Grolles legat and Ber-

gen Myrdyrkningsforeningsfond.

Appendix A

Species names in full and abbreviated species names; the

four + three first letters of the genus and species names,

respectively, for the 86 species recorded during the seven-year

study. Abbreviations are used in Fig. 3.

Species names Abbreviation

Achillea millefolium Achi mil

Agrostis canina Agro can

Agrostis capillaries Agro cap

Anemone nemorosa Anem nem

Anthoxanthum odoratum Anth odo

Betula pubescens Betu pub

Blechnum spicant Blec spi

Brachythesium rutabulum Brac rut

Bryum spp. Bryu spp.

Calluna vulgaris Call vul

Campanula rotundifolia Camp rot

Campestris spp. Camp spp.

Carex nigra Care nig

Carex ovina Care ovi

Carex pallescens Care pal

Carex pilulifera Care pil

Cerastium fontanum Cera fon

Ceratodon purpureus Cera pur

Circium palustre Circ pal

Cladonia spp. Clad spp.

Conopodium majus Cono maj

Cornus suecica Corn sue

Dactylorhiza macultaa Dact mac

Danthonia decumbens Dant dec

Deschampsia cespitosa Desc ces

Deschampsia flexuosa Desc fle

Dicranella spp. Dicl spp.

Dicranum scoparium Dicr sco

Dicranum spurium Dicr spu

Empetrum nigrum Empe nig

Erica tetralix Eric tet

Euphrasia micrantha Euph mic

Festuca rubra Fest rub

Festuca vivipara Fest viv

Galium saxatile Gali sax

Hieracium pillosella Hier pil

Holcus lanatus Holc lan

Hylocomium splendens Hylo spl

Hypericum pulchrum Hype pul

Hypnum jutlandicum Hypn jut

Hypochaeris radicata Hypo rad

Juncus conglemoratus Junc con

Juncus squarrosus Junc squ

Juniperus communis Juni com

Leontodon autumnalis Leon aut

Leucobryum glaucum Leuc gla

Lophocolea bidentata Loph bid

Lotus corniculatus Lotu cor

Luzula pilosa Luzu pil

Luzula spp. Luzu spp.

Mnium hornum Mniu hor

Nardus stricta Nard str

Oxalis acetosa Oxal ace

Peltigera spp. Pelt spp.

Plantago lanceolata Plan lan



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Pleurozium schreberii Pleu sch

Poa spp. Poa spp.

Pohlia spp. Pohl spp.

Polygala serpyllifolia Poly ser

Polytrichum commune Poly com

Polytrichum juniperinum Poly jun

Polytrichum piliferum Poly pil

Potentilla erecta Pote ere

Pseudoscleropodium purum Pseu pur

Pteridium aquilinum Pter aqu

Racomitrium aciculare Raco aci

Racomitrium languinosum Raco lan

Ranunculus acris Ranu acr

Rhytidiadelphus loreus Rhyt lor

Rhytidiadelphus squarrosus Rhyt squ

Rumex acetosa Rume ace

Rumex acetosella Rume acl

Salix caprea Sali cap

Salix repens Sali rep

Sorbus aucuparia Sorb auc

Taraxacum coll. Tara col

Trientalis europaea Trie eur

Trifolium repens Trif rep

Ulota crispa Ulot cri

Vaccinium myrtillus Vacc myr

Vaccinium uliginosum Vacc uli

Vaccinium vitis-idaea Vacc v-i

Veronica officinalis Vero off

Viola canescens Viol can

Viola palustris Viol pal

Viola riviniana Viol riv



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