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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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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.
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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.
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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.
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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).
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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.
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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.
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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
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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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Appendix A – continued
Species names Abbreviation
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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