Effects of bracken [Pteridium aquilinum (L.) Kuhn] control...
Transcript of Effects of bracken [Pteridium aquilinum (L.) Kuhn] control...
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Effects of bracken [Pteridium aquilinum (L.) Kuhn] control treatments on the dynamics of bracken litter and its nutrients, and the potential consequences for species diversity
*R H Marrs, K Galtress, #Tong, C. , +S J Blackbird, T J Heyes and M G Le Duc
Applied Vegetation Dynamics Laboratory School of Biological Science University of Liverpool PO Box 147 Liverpool L69 3GS and +Department of Earth & Ocean Sciences University of Liverpool 4 Brownlow Street Liverpool L69 3GP #Present address, Department of Ecology and Environmental Sciences Inner Mongolia University 010021Huhhot Inner Mongolia China *Corresponding author: [email protected] 37 http://appliedvegetationdynamics.co.uk 38
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Tel +44 (0) 151 794 4752 Fax +44 (0) 151 794 4940 October 2003
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Bracken (Pteridium aquilinum (L. Kuhn) is an invasive, clonal fern which often reduces species diversity and
conservation interest. It can produce a deep litter layer that impedes the establishment of new species. There
have been few studies on bracken litter and its dynamics, especially during bracken control. This study
examined the effects of bracken control treatments applied over seven years on bracken litter distribution,
nutrient compartmentation and litter turnover and their relationship with developing species diversity.
Bracken litter was > 2000 g m-2 in untreated plots, and control treatments reduced this quantity to varying
degrees. The distribution of nutrients in the soil-plant profile suggested that (i) cations were rapidly leached
from the standing fronds, C, N, P and Ca amounts were greater in the soil profile with the greatest organic
matter, whereas K and N were greater in the mineral fractions., and (iii) the rhizomes are a major store of
nutrients, especially Ca and Mg, with 30% of the total amount in the profile found there.
Cutting twice/year was the most successful treatment in reducing bracken litter. This treatment was also
associated with a significant increase in bracken litter turnover and mass of non-bracken vegetation. Other
treatments also reduced bracken litter mass, and accelerated litter turnover and development of non-bracken
vegetation, but not to the same extent. There was a significant amount of nutrients released by the cutting
twice/yr treatment; in absolute terms large amounts of C and N were released, but when expressed as a
percentage of the total amount in the system, between 19-26% of the total Ca and Mg in the system was
released from the rhizomes. Although, some of these released nutrients were taken up by the developing
vegetation, there was a net loss to the system as a result of bracken treatment. This release has implication in
terms of reducing C stocks and also potential loss of Ca and Mg, which will enhance acidification.
Species diversity was greater where bracken litter had been reduced, but there was an important interaction
with sheep grazing. Where bracken litter mass was low, there was increased diversity in the sheep grazing
treatment, whereas only Deschampsia flexuosa was associated with the ungrazed treatment.
The study provided further evidence that litter is an important above-ground component of the bracken
habitat which must be considered in the restoration of sites with a dense bracken cover. Cutting twice/yr was
the most effective treatment in reducing bracken and litter cover, but it also released considerable quantities of
nutrients into the system, greater than the uptake by developing vegetation and their litter. The consequences of
this are a release of C to the atmosphere and cations (especially Ca and Mg) available for leaching, although
they may also be taken up by soil microbes. Irrespective, there is a potential dilemma between controlling a pest
species, which has evolved to sequester nutrients and potential long-term degradation.
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Key-words: moorland, litter turnover, nutrient compartmentation, decomposition, restoration, land
management
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Bracken (Pteridium aquilinum (L.) Kuhn)1 is an invasive, clonal fern which covers large tracts of land in the
UK (Pakeman and Marrs 1992). It is a particular problem in upland and marginal areas (Pakeman et al. 2000a).
Its spread over recent decades has been partly blamed on a combination of (i) a decline in harvesting for soap,
livestock bedding and thatch and (ii) a general reduction in its management (Pakeman et al. 2000b). Bracken is
a problem for many land users. It can have a substantial commercial impact through obstructing agricultural
and forestry practices, it is poisonous and carcinogenic to browsing livestock (Marrs et al. 2000), and it presents
a risk to human health by harbouring ticks, which act as vectors for Lyme disease. Although records indicate
that bracken may not have increased dramatically in total abundance compared to historical values, it has
replaced substantial areas of plant communities regarded as having greater conservation and amenity value
(Pakeman et al. 2000a). It poses a threat to biotopes of international importance that are subject to Biodiversity
Action Planning, such as lowland heaths and heather moorland, of which the UK contains ca. 20% and 70% of
the European stock respectively (Thompson and Usher 1991, Dolman and Land 1995). The canopy of bracken
fronds, which emerges in spring, shades out most competitors and the thick litter layer, which develops
underneath, hampers the growth of other species (Marrs et al. 2000). The uncompacted dead fronds and petioles
compete with other species for space, while older compacted litter appears to hinder their germination (Paterson
et al., 2000). In woodlands, it may also delay tree regeneration (Pakeman et al. 2000a, Marrs et al. 2000).
Moreover, it has been predicted that the area of land dominated by bracken will increase under the reduced
stocking densities encouraged under Agri-Environment schemes, and under global warming (Pakeman et al.,
2000a).
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Bracken’s success as a weed depends to a large extent on the size of its perennial rhizome network, which
acts as a large store of carbohydrates, nutrients, and perennating buds for summer frond production (Watt 1976;
Marrs et al. 1992, 1993, 1998a). These large reserves make bracken eradication from areas of extensive
invasion almost impossible with available control methods (Marrs et al.,, 1998b). The major treatments
currently used are application of the selective herbicide asulam (product Asulox, active ingredient, methyl (4-
aminobenzenesulphonyl) carbamate) and mechanical treatment (usually cutting), or combinations of the two.
Currently, in the UK asulam is applied by aerial spraying on between 5000 - 8000 ha each year (Pakeman et al.
000a).
A great deal of experimental work has been done to test the efficacy of different combinations of these
control treatments, although much of this has concentrated on their effects on frond cover and biomass and on
1 P. aquilinum is referred to as bracken throughout; nomenclature follows Stace (1997) for higher plants and Corley & Hill (1981) for bryophytes.
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the relationship between these two parameters and species diversity. There has been very little research into the
effect of bracken control on litter dynamics and its relationship between bracken frond production and
developing understorey vegetation, even though bracken litter disturbance or removal is known to increase the
establishment rate of other species (Lowday and Marrs 1992,Mitchell et al. 1999). This paucity of information
was highlighted as the reason for the very poor predictive estimates of bracken litter produced by a computer
model (REBRA) designed to predict the outcome of bracken control treatments at Cavenham Heath in Suffolk
(Paterson et al. 2000). Where decomposition rates have been measured they are slow; estimates of the
exponential decay rates for a Cumbrian site indicated that it would take between 11 and 13 years for almost
complete decay to occur (Ling-Zhi Chen and Lindley 1981).
The lack of information on litter distribution and dynamics is surprising since it is known that litter
abundance affects frond development (Watt 1956, 1969, 1970), rhizome distribution (Watt 1976) and
vegetation development after control (Lowday and Marrs 1992). Litter depth can have both positive and
negative effects on frond development. Deep litter tends to delay frond development, presumably as a result of
reduced surface and soil temperatures, but it can also help protect emerging fronds from early frosts emergence
(Watt 1956, 1969, 1970). The depth of the rhizomes under the ground can also be affected by litter depth. The
rhizomes rise towards the surface and in some cases actually grow through the litter layer (Watt, 1976), and
where the litter is removed in such situations the rhizomes near the soil surface are more readily affected by
frost (Snow and Marrs 1997). Removal of the litter layer has also been shown to promote the establishment of
new vegetation in previously species-poor vegetation under dense bracken (Lowday and Marrs 1992).
The mode of action of bracken control treatments will have both direct and indirect effects on bracken litter,
and there are important differences in the way that the treatments act. Mechanical treatment (especially cutting)
has the most direct effect because, although the cutting process severs the fronds just above ground level, it is
not selective, and any litter present will also be shredded and compacted to a greater or lesser extent. This
treatment accelerates the comminution phase of decomposition (Swift et al. 1979). However, there are also
indirect effects. Cutting is applied continuously for several years to reduce the rhizomes’ carbohydrate and
nutrient reserves and is targeted to coincide with the maximum transfer of carbohydrates and nutrients from the
rhizomes to the developing fronds, and to precede the fronds redirecting resources back into the rhizomes (late-
July in the UK). As this process is continued, the frond production, and hence inputs to the litter, should be
reduced each year (Marrs et al. 1998b). On the other hand, asulam is usually applied once in late July-early
August, when the fronds are relatively unlignified, so that herbicide enters the tissues easily, and when the
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rhizomes have become the major sink for photosynthate and nutrients (Paterson et al. 2000). The asulam is
translocated in the phloem to meristematic areas in the rhizomes (ie short shoots with frond-bearing buds),
where it prevents further frond development. Usually, there is a very good kill of frond-producing buds on the
rhizome, resulting in a 95-99% reduction in frond production in the year after spraying, although there can be
rapid recovery of frond biomass if follow-up treatments are not applied (Marrs et al. 1998b). Thus, after
spraying there should be a reduction in inputs to the litter to almost zero in the year after spraying, followed by
an increase. No information is available on whether litter turnover changes after these relatively drastic
treatments are applied.
This aim of this study was to quantify the effects of a selected suite of bracken control treatments on the
mass, compartmentation, turnover and nutrient content of bracken litter in a long-term experiment designed to
investigate integrated bracken control and moorland regeneration methods at Hordron Edge in Derbyshire, and
relate the litter dynamics to the species diversity of the developing moorland vegetation
Methods
The experiment was set up in 1993 to test a range of bracken control and vegetation restoration treatments at
Hordron Edge in Derbyshire (National grid reference, 4213 3870; Longitude and Latitude, 1041’W, 53023’N).
This experiment was set up in 1993 and uses a randomised block design with split-split plots in three replicate
blocks in a bracken patch that was wholly covered with dense bracken litter ca. 30 cm deep (Le Duc et al.
2000). The main-plot treatments were: (1) control (untreated), (2) cutting (flail cutter trailed by ATV) once per
year (cut once/yr), (3) cutting twice per year (cut twice/yr), (4) asulam application in first year only (asulam),
(5) asulam application in first year followed by single cut in second (asulam + cut), (6) cutting in first year
followed by asulam in second (cut + asulam). The split-plot treatments were: (1) no sheep exclosure, sheep
grazed at approximately 0.5 ha-1 (Pakeman et al. 2000b), and (2) sheep exclosure, ungrazed.
Each year a range of measures was made: (1) summer bracken frond biomass (June and August), (2)
bracken litter cover and depth (June) and (3) number, identity and % cover of species in the vegetation (June)
(detailed methods are available in Le Duc et al. 2000). For this part of the analysis floristic data collected in
2000, ie the summer immediately preceding the litter sampling were used. Three additional diversity measures
were calculated from the floristic data for each split-plot: (1) Shannon-Weiner index, (2) equitability and (3)
Simpson’s Index. As all measures gave similar results, only species number and Shannon-Weiner index are
presented here.
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Field sampling of litter, vegetation and soils 1
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On 12th October 2000, seven years after the experiment was started, a 0.25 m2 quadrat was located at pre-
selected random co-ordinates in each plot (10m x 10m), and the following samples taken in order: (1) the
standing, senescent fronds were cut at ground level (denoted the uncompacted bracken litter fraction); (2) the
remaining vegetation (biomass + necromass including compacted bracken litter) was cut at ground level; (3)
two 15-cm deep soil cores were taken using a Dutch bulb auger (5.08cm diameter). These cores were then
divided into three sub samples; fibrous organic material (A00 horizon), dark, amorphous organic material (A0
horizon) and mineral material (A1 horizon). All samples were returned to the laboratory. The vegetation
samples were sub-sampled randomly and sorted into bracken litter (denoted the compacted bracken litter
fraction) and non-bracken plant vegetation\litter. These sub-samples and all other samples were then oven dried
at 850C for 48 h and weighed. All data were converted to g m-2 for each fraction.
Calculation of bracken litter turnover rates
A litter turnover index (K) was calculated as the current year’s litter input (L, g m-2) divided by the total above-
ground (i.e. standing + compacted) crop of litter (XL, g m-2) (Swift et al. 1979). Because of limitations of the
available data for calculating L, two estimates of L (L1 and L2) were made, resulting in two turnover rates (K1 =
L1/XL, K2 = L2/XL) for each quadrat. For control and non-cut treatments L1 was estimated using August frond
biomass data. For the cut once/yr treatment L1 was estimated using June + August frond biomass data and for
the cut twice/yr treatment L1 was estimated as June + August frond biomass + the mass of senescent fronds in
October. Thus, L1 is an estimate of the maximum possible bracken litter input during 2000. L2 was estimated for
control and non-cut treated split-plots using the mass of senescent fronds in October. For the cut once/yr
treatment L2 was estimated using the mass of senescent fronds in October + the June frond biomass, and for the
cut twice/yr treatment, L2 was estimated using the mass of senescent fronds in October + June + August. Thus,
L2 is an estimate of the minimum possible bracken litter input during 2000.
Estimation of nutrient concentrations
The soil and litter samples were ground to a fine powder using a Kika-Labortechnik A10 mill. C and N were
estimated using a Carlo Erba Instruments NC2500 elemental analyser. Samples were then digested using the
hydrogen peroxide: sulphuric acid digestion reagent recommended by Allen (1989). P was estimated
colorimetrically by ammonium molybdate-stannous chloride methods; K and Na were estimated by emission
spectrophotometry and Ca and Mg by absorption spectrophotometry (Allen 1989). Data were expressed on a
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mass basis (g per unit mass of tissue\soil) or on an area basis (g m-2). C:N and C:P ratios were calculated for
some of the nutrient pools.
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Data analysis
Initially, data were analysed by analysis of variance using a randomised block split-plot experimental design
(PROC ANOVA, SAS 1989). Percentage cover data were arcsin-transformed (arcsine(√x%/100) and all other
data except the diversity indices were transformed using 1n(Y+1) (Sokal and Rohlf 1995). Kendal rank
correlation coefficients (PROC CORR, SAS 1989) were calculated between (i) diversity, biomass and litter
variables, (ii) bracken litter and biomass variables and (iii) turnover rates and both nutrient pools and derived
ratios. Regressions, linear and quadratic regressions, were fitted to some relationships using PROC REG (SAS,
1989).
Multivariate analysis using CANOCO for WINDOWS (ter Braak and Šmilauer 1998) was employed
to determine which factors were influencing species composition. This was done using two datasets: Analysis 1
used the floristic data collected in 2000, the summer preceding the sampling, and Analysis 2 used all the
floristic data collected between 1994 and 2002. In both analyses, a Detrended Correspondence Analysis (DCA)
was used initially to measure gradient lengths; in both cases the gradient length was < 2.5, and accordingly the
linear Redundancy Analysis (RDA) was selected for all further analyses (ter Braak and Šmilauer 1998). Species
data were 1n(Y+1) transformed, the downweighting option for rare species was not used, and significance was
tested using a Monte Carlo test with 999 permutations. The resultant biplots were produced using
CANODRAW (ter Braak and Šmilauer 1998).
In Analysis 1 the Forward Selection procedure was used to select those environmental variables (eg
bracken litter depth and grazing) that accounted for a significant proportion of the explained variation (P <
0.05). A final RDA was then done including just those selected environmental variables. The eigenvalues were
of the four axes were: λ1 = 0.215, λ2= 0.075, λ3 = 0.194 and λ4 = 0.097. The first axis and the model were both
significant (P < 0.001) with F-values of 9.031 and 6.731 respectively.
In Analysis 2, a RDA was done with bracken litter cover alone as the constrained variable, and with all
other experimental treatments and time factored out as covariables. The eigenvalues were of the four axes were:
λ1 = 0.109, λ2= 0.111, λ3 = 0.090 and λ4 = 0.068. The model was significant (P < 0.002) with an F-value of
145.28.
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Results
Effects of treatment on litter mass, turnover, frond production and non-bracken vegetation For all bracken variables and biomass of non-bracken vegetation there were significant differences because of
the bracken control main-plot treatments, but no significant effect of exclosure. There was only one significant
main-treatment x sub-treatment interaction, involving the August frond dry mass.
Bracken control treatment had a significant effect on the mass of uncompacted, compacted and total bracken
litter (P < 0.01) (Table 1). Cutting and asulam-treatment plots had significantly less of bracken litter than
untreated plots; plots with the combined treatments were not significantly different from untreated plots (Table
1). Cutting twice/yr significantly reduced uncompacted and total bracken litter mass compared to all other
treatments, and for mass of compacted bracken litter, cutting twice/yr also produced the greatest reductions,
these being significantly lower than for all other treatments except cutting once/yr.
Treatment in most cases increased the mass of non-bracken vegetation relative to untreated plots (P < 0.05)
but asulam + cutting was an exception. Cutting twice/yr resulted in the greatest increase (Table 1). Cutting,
especially twice/yr significantly increased the bracken litter turnover (P < 0.001). For K1 (based on maximum
possible litter input), cutting once/yr and twice/yr significantly increased turnover compared to untreated plots
and asulam treatment. The combination treatments were intermediate (Table 1). For K2 (based on minimum
possible litter input), only cutting twice/yr resulted in a significantly faster, being between three to twenty times
faster than all other treatments.
Bracken control treatments also reduced summer frond mass: the untreated and asulam+cut treatments had
the greatest mass, other asulam treatments were intermediate, and the cutting treatments had the lowest mass
(Table 1).
Effects of treatment on species diversity and the relationship between diversity, biomass and litter turnover
No significant differences were found between bracken control main-plot treatments and their interaction with
the grazing sub-treatments on total species number/plot or Shannon-Weiner index. However, grazing
significantly increased diversity compared to the ungrazed treatment: grazed mean = 9.3 species/plot versus
ungrazed mean = 6.3 species/plot; LSD (P<0.05) = 2.38, F1,12 = 7.22; P < 0.05 and for the Shannon Weiner
index a grazing mean = 2.05 versus ungrazed mean = 1.37; LSD (P<0.05) = 0.459, F1,12 = 10.36, P < 0.01.
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Both species number and Shannon Weiner index showed a significant negative relationship with all bracken
litter biomass variables (P < 0.05). An example is illustrated in Fig. 1a.. Non-bracken biomass showed no
significant relationship with any measure of diversity. The litter turnover rates gave conflicting results, with K1
showing no significant relationship with any of the species diversity measures but K2 showing a significant
positive relationship (P < 0.05) with total number of species (Fig. 1b)
Each of the bracken litter variables showed significant positive relationships with each other and with
summer frond variables (rk > +0.41, P < 0.001), and significant negative relationships with turnover rates and
with non-bracken vegetation mass (rk > -0.42, P < 0.001). Non-bracken vegetation mass also showed significant
negative relationships with summer frond mass (rk > -0.35, P < 0.01) but significant positive relationships with
turnover indices (rk > +0.55, P < 0.001) (Table 2).
Relationship between treatments, environmental variables and community composition
The biplot for analysis 1 (Fig. 2a), based solely on the study year revealed a diversity gradient along axis 1,
corresponding positively with increasing bracken litter depth. As litter depth decreased, there was a transition
from a bracken-dominated community at the positive end of axis 1 through to an acid heath/grassland at the
negative end. Grazing was extremely influential on axis 2, with grazed plots having a greater positive score and
ungrazed ones having a negative one. Where bracken litter mass was low and sheep grazing occurred, there was
a relatively diverse mixture of grasses, herbs and moss species typical of an acid heath/grassland community,
whereas ungrazed plots were dominated by Deschampsia flexuosa. Where bracken litter mass was high, grazing
had a lesser impact, but on ungrazed plots typical heath species (Erica tetralix and Vaccinium vitis-idaea) were
found, possibly because these species were grazed in preference to the less palatable bracken where sheep were
present. The inclusion of Agrostis castellana in the species data set indicated immigration from adjacent plots
where it had been sown as a nurse; it is unlikely to have been present in the seedbank before 1993 (Le Duc et
al., 2000).
The biplot for analysis (Fig. 2b) based 10 years data, where all environmental variables have been factored
out other than bracken litter cover, shows more or less the same pattern, with almost all species being placed at
the negative end of the bracken litter cover gradient. There was a strong gradient on axis 2, reflecting the
influence of grazing. Here the only species positively associated with bracken litter was the liverwort
Lophocolea bidentata. Galium saxatile was positioned at the negative end of the bracken litter gradient, but this
may reflect a response to increased light rather than litter per se.
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Effects of treatments on nutrient distribution, fluxes and relationship with turnover 1 2 3
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The nutrient concentrations for both the litters and the soils (Table 3) were within the ranges quoted by Allen
(1989) for similar materials. When expressed on a mass basis there were differences in the concentrations of
the different elements. The compacted bracken litter had marginally greater concentrations of N, C and P than
the standing litter, but the standing litter had much greater concentrations of K, Mg and Na (~x2), and
especially Ca (x13) than the compacted litter. This result suggests that as the litter changes from standing to
compacted form, N, C and P amounts increase perhaps as a result of decomposition processes, but the cations
are reduced as a result of leaching. The non-bracken vegetation had slightly greater concentrations of N
(slightly) and P (x2), approximately similar concentrations of K, and lower concentrations of C, Ca, Mg and Na
than the standing bracken. The concentrations within the soil profile separated elements into two groups; C, N,
P and Ca reduced through the profile indicating that they are associated with the levels of organic material
present; K, Mg and Na increased through the profile indicating that there are large amounts associated with the
mineral matrix.
When these date were expressed on an area basis the amounts of nutrients in the litter pools were in the
order compacted bracken litter > non-bracken litter > standing bracken litter, and the soil pools were A1 > A0 >
A00, reflecting the different mass of the fractions per unit area. The total amount of nutrients within the
untreated ecosystem (n=6, exclosure treatments pooled) were estimated in g m-2 (15 cm depth) as: C =
10278±1151; N = 455±45; P = 75±7; K = 724±67; Ca = 60±7; Mg = 68±6; and for Na = 81±6.
There were no significant effects of applied treatment (P < 0.05) on nutrient concentration in any soil or
litter fraction when expressed on a mass basis. On an area basis, however, there was again no differences
between the soil fractions, alone or combined, but there were significant effects of bracken control treatment on
the nutrient content in the three litter pools (Table4), there were no exclosure effects. When the litter data were
tested in pooled form, only the bracken litter (standing + compacted) showed significant effects of bracken
treatment.
The effects of the bracken treatments varied for each element in terms of significance and the same
responses were not found for the impact of the respective treatments, ie as the effect was not solely a function
of the mass of the fraction. For the two bracken litter components the greatest amount of all elements were
usually in the untreated plots, although for some elements the asulam+cut or cut+asulam treatments were the
greatest. Usually, the order was untreated > asulam+cut > cut + asulam, cut once/yr > Cut twice/yr. In all cases
the Cut twice /yr treatment was the lowest. These results were essentially mirrored in the non-bracken
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vegetation; the untreated vegetation had the lowest amounts of all nutrients, and there were greater amounts in
the treated plots generally increasing through to the cut 2/yr treatment (Table 4).
Significant differences were found between the two types of bracken rhizome for C, Ca and Mg (Table 3),
with the short shoots having a greater C and Ca concentration and the long shoots a greater Mg concentration
when expressed on a mass basis, but all elements had a significantly greater amount in the long shoots
compared to the short shoots when expressed on an area basis. A substantial fraction of the total nutrient pool
was found within the rhizomes for all elements except K (Table 3). This increased in order N < C < K < P <
Mg and Ca, and for Ca and Mg, 30% of the total mass of nutrients within the ecosystem was found in the
rhizomes.
The effects of bracken control treatments had a significant effect on the mass of long shoots and the N
concentration (mass basis) in short shoots, but no significant effect on mass of short shoots or total rhizome
mass (Table 5a). When expressed on an area basis bracken control treatment reduced the N, C, Mg, Ca and K
content, but not P or Na (Table 5b). The general treatment responses were in order untreated > combined
treatment > asulam > Cut once/yr > Cut twice/yr.
The correlation between the two turnover indices and the nutrient concentrations confirmed that the mass of
the litter fractions were the main factors controlling bracken litter turnover. There were no significant
correlations with any soil fraction (mass or area) or for the concentration of any litter fraction when the data
were expressed on a mass basis. There were significant correlations between both rate constants and the mass of
the standing litter (r = -0.647, P <0.001 with K1, r = -0.740, P <0.001 with K2), compacted litter (r = -0.542, P
<0.001 with K1, r = -0.419, P <0.001 with K2), and the non-bracken litter (r = 0.551, P <0.001 with K1, r =
0.560, P <0.001 with K2). When expressed on an area basis, there were significant correlations between the
turnover rates and the element content of each of the litter fractions (standing litter – all negative, r > -0.308, P
<0.02; compacted litter – all negative, r > -0.0.625, P < -0.001; non-bracken litter – all positive, r > 0.376, P<
0.01). There were also no significant correlations between either turnover index and C:N and C:P ratios.
The fluxes of nutrients released or taken up were estimated by calculating the difference between (1) the
amount of nutrient in the rhizomes, bracken litter and non-bracken litter pools in treated plots from (2) the
amounts in comparable untreated plots (Table 6). Overall here was in general a release of most elements except
Na from the rhizomes, and the amount released increased with increasing success of treatment, with the Cut
twice/yr treatment releasing the greatest amount. For this treatment the amount released by element was in
order C > N > K >Mg > Ca > P, but as a percentage of the amount present in the ecosystem it was Mg > Ca > P
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> K > N > C. For Ca and Mg between 19-26% of the total amount in the system was released during the 7
years of cutting twice/yr.
In contrast the fluxes between litter pools were substantial only for C and N with a release from the bracken
litter and uptake by the non-bracken litter. Again the magnitude was dependent on treatment success with the
Cut twice/yr treatment releasing most C and N from bracken litter and taking up most in the non-bracken litter.
Although some of this release could be accounted for by uptake by the developing non-bracken vegetation, this
did not account for all of it (Table 6). .
The implication of this increase nutrient supply would be that the vegetation that develops in low litter
treatments would be species of more fertile soils. There was no significant correlation between score on axis 1,
the axis constrained on bracken litter cover and Ellenberg N value (Hill et al. 1999), but there were a significant
number of species present with Ellenberg values greater than that of Calluna (N=2) (Fig. 3).
Discussion
Bracken is an important late-successional invasive species that causes problems for land managers who wish to
restore early-successional communities with a high conservation interest (Pakeman and Marrs 1992, Marrs et
al. 2000). Whilst it has been appreciated for some time that the deep litter layer produced by bracken impedes
re-establishment of many species (Lowday and Marrs 1992), there have been few studies of the impact of
bracken control strategies on litter dynamics and nutrient pools. This study is the first to assess how successful
bracken control treatments are in reducing bracken litter, and their effects on litter turnover, nutrient pools and
species diversity.
Two major findings of this study were derived from the distribution of nutrients within the bracken soil
system. First, there was evidence that for some elements at least there are substantial pools in the recent
standing litter that have not been resorbed into the rhizome. Moreover, the concentrations of the cations reduces
as the new standing litter moves into the older compacted form, suggests large losses\fluxes through leaching.
Second, a major finding of this study was that large amounts of nutrients and especially P, Ca and Mg were
stored in the rhizomes as a proportion of the total amount in the system. This result implies that bracken
preferentially sequesters these nutrients within the rhizome system, where a large pool is developed. The large
amount of Ca and Mg found in the tissues was very surprising and may reflect an evolved mechanism to
survive in very acidic, nutrient-poor conditions.
13
Impact of control strategies on bracken litter mass, nutrient cycling and turnover 1 2 3
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All bracken control treatments impinged on the distribution of litter and nutrients. However, of those control
strategies assessed, cutting twice/yr was most effective in reducing bracken litter. This was not surprising
considering that previous studies on a variety of bracken infested sites have indicated that this treatment tends
to be the most successful at reducing frond productivity by depleting rhizome carbohydrate and nutrient
reserves (Le Duc et al. 2000, Marrs et al. 1998a). However, unique to this study was the evidence that some
treatments could impact on the bracken litter decomposition rate itself, refuting the hypothesis that bracken
litter inputs and standing crop are maintained proportionately as treatment reduces frond mass. We accept that
the litter turnover indices used provide only a crude assessment of decomposition rates (Swift et al. 1979) and
they do not consider the contribution of continued frond senescence to the litter inputs through the season.
Frond senescence begins in the lowermost pinnae shortly after peak frond biomass in late July/early August,
and continues until complete frond death at the time of the first autumn frost (Pitman 1989). Some litter input is
also likely as a result of frond death and damage during the summer, and through physical damage from grazers
or harsh weather. While these unmeasured inputs are likely to be small in comparison with measured inputs,
further work is needed to quantify them. Moreover, no measurements were made of the decomposition rates of
fractions other than the standing bracken fronds, and given the development of non-bracken vegetation in the
treated plots, this is a major weakness, In addition, we only sampled at one point and differences inferred from
untreated plots; further work to derive a time course of change would be valuable. However, the constraint on
studying these, and other processes, is that these experiments were set up for other purposes, and there is a limit
to the damage that we can inflict on them by additional experimentations that involves destructive sampling.
Nevertheless, the result that cutting twice/yr significantly increased turnover rate relative to all other
treatments and the measurements of nutrient fluxes both indicates that this treatment is effective in accelerating
litter turnover, a transfer of nutrients between bracken litter pools and those of other species, and implies much
faster nutrient cycling. Deep stands of bracken litter lock large quantities of nutrients out of effective
circulation, and her we have shown that large amounts can be released, indeed release from bracken pools were
greater than uptake by developing vegetation and their litter. For the most successful treatment, relatively large
amounts of nutrients were released from bracken litter. In absolute terms, bracken management released C and
N but in proportionate terms very large amounts of Ca and Mg were released. This may have substantial
impacts on the vegetation change with a move to eutrophication, and the long-term sustainability of the site, as
these elements are very susceptible to leaching. There is also the intriguing possibility that management to
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provide environmental\conservation benefits will have a C cost, which is outside the spirit of the Kyoto
Protocol. Here we had a net potential release of 1010 g C m-2 over 7 years or 144 g C m-2 y-1. This must be
viewed in context of the C sink in a north Pennine catchment of 15.4 g C m-2 y-1 ( Worrall et al. 2003), and the
predicted sink of 170 reducing to 70 g C m-2 y-1 within 50 years (Karjalainen et al. 2002). A more detailed
knowledge of C fluxes in conservation management treatments to control bracken, and their change through
time, are clearly needed.
An important issue was that there were few treatment effects on litter quality in terms of nutrient
decomposition; all treatments effects were brought about through the effect of the size of the litter pool. This
implies that the comminution of the litter through the passage and operation of the machinery (Pakeman et al.
2000b) is the over-riding factor controlling the decomposition of the bracken litter at this site, operating by
exposing a greater surface area to decomposing organisms Swift et al 1979). We hypothesised that there could
be important interactions with the resource quality of the litter input, which may be affected by treatment. We
might have expected that in untreated bracken the senesced fronds would have a lower nutrient concentration as
a result of enhanced nutrient translocation to the rhizome and would be added to the litter in autumn when
temperatures are cooler. In cut plots, at least a fraction of the standing fronds would have at best a limited
withdrawal of carbohydrates and nutrients into the rhizomes at the time of cutting, and they would be added to
the litter in mid summer when temperatures are higher. We would have expected these combinations of litter
quality and temperatures to influence turnover (Hobbie 1996, Anderson and Hetherington 1999; Hector et al.
2000). However, we found no evidence of resource change in the bracken litter pools of the various treatment.
The grazing sub-treatment had no significant effect on any of the litter pools. This was not surprising
because the sheep stocking levels at Hordron Edge are low in line with Environmentally Sensitive Area
prescriptions. Higher grazing pressures might have increased litter turnover through trampling acting as a low
level continuous cutting treatment, and through additional nutrient cycling via urine and faeces (Wardle et al.
2002).
Relationship between species diversity/composition and above-ground bracken components
The negative relationships between bracken litter variables and the vegetation that has colonised in 7 years,
confirms that persistent deep bracken litter is a major constraint on vegetation establishment following bracken
control (Lowday and Marrs 1992, Mitchell et al. 1997, 1999, Marrs et al. 2000). However, the results also
indicate that reducing bracken litter alone is not sufficient to promote diverse vegetation. The ordination biplots
illustrated, however, that although reducing litter is influential in restoring high diversity, a low grazing
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pressure was also needed. Nutrients released during treatment may also help to promote species of more fertile
soils. Reduced bracken litter led to increases in Galium saxatile, an herb intolerant of grazing and occurring on
infertile soils (Pakeman et al. 2000b). In the unglazed vegetation with low bracken litter, D. flexuosa
dominated, suggesting that fertility was increased in these plots (Diemont and Heil 1984). Low level sheep
grazing had a positive effect on diversity, increasing the cover of a large number of species typical of acid
grassland, probably because moderate grazing created colonisation gaps and decreased shade by limiting the
growth of tall invasive herbs, such as Chamerion angustifolium (Bakker 1998).
Litter depth was identified as the main environmental variable influencing species diversity, and this
suggests that litter is more important than summer frond biomass. This tentative result confirms the conclusions
of a field validation of a model designed to predict the outcome of bracken control treatments on revegetation
(REBRA) (Paterson et al. 2000). Paterson et al. (2000). suggested that the principal reason for poor vegetation
recovery following bracken control is that the slowly decomposing litter provided few opportunities for
seedling germination and subsequent establishment.
Relationship between species diversity and bracken litter turnover
The positive relationship between K2 and species density, and the finding that cutting twice yr-1 resulted in the
greatest biomass of non-bracken vegetation and fastest turnover indices, indicates that there may be positive
feedback between bracken litter decomposition and increasing diversity. Although some studies have found
decomposition to be insensitive to changes in species diversity (Blair et al. 1990, Rustad, 1994), others have
demonstrated an increased turnover rate with increasing diversity due to synergistic non-additive effects of litter
mixing (Salamanca et al. 1998, Bardgett and Shine 1999, Anderson and Hetherington 1999, Hector et al. 2000,
Wardle et al. 2002). For instance, Anderson and Hetherington (1999) found that the mass loss of 1:1 mixtures
of Calluna and bracken litter was 10% greater over the same time period than that of litter of either species on
its own, and in Hector et al’s. (2000) study on acid grassland, mass loss of litter of individual species within
diverse mixtures was greater than that expected based on their losses when alone. However, considering that no
correlations were found between K1 and any measure of species diversity, the results from this study here are
inconclusive. This may result partly from the crude nature of the methodology, the over-riding effect of
comminution within this initially mono-specific litter layer, but it may also be a consequence of the complex
nature of the potential synergistic interactions. Translocation of both nutrients and inhibitory compounds
between different species’ litters can occur by diffusion or fungal hyphae (McTiernan et al. 1997) and, while
movement of nutrients could ameliorate nutrient limitation in the decomposer community, movement of
16
inhibitory compounds could slow this effect (Hector et al. 2000). Hector et al. (2000) proposed this as a reason
why their results indicated that the actual mixture of species involved was probably more important than the
level of diversity alone in influencing decomposition rates. The toxic compounds found in bracken, which make
its fronds unpalatable to most grazers, may presumably also make its litter unpalatable to many decomposers
(Marrs et al. 2000).
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The potential mechanisms whereby cutting and grazing can impact on bracken litter abundance and the
establishment of other species are summarised in Fig. 3; this illustrates the complex nature of the possible
interactions involved. For instance, several possible routes are presented whereby cutting can cause a reduction
in litter abundance. Frond production, and therefore litter input, may be decreased through weakening of
rhizomes via reduced photosynthate translocation, or by reduced shading of the understorey or increasing the
nutrients, and increasing competition from other vegetation. Grazing may enhance decomposition by
encouraging an increase in species diversity or by trampling, mimicking the effects of cutting on turnover
(Wardle et al., 2002). Further experimental work is needed to separate these complex interactions.
Acknowledgements - We thank DEFRA for supporting this long-term project, Neil Taylor and Jeremy Archdale
for permission to work on their land and Dr S Paterson for assistance with the field work.
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FIGURE LEGENDS 1
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Fig. 1 Relationship between species number and (a) uncompacted bracken litter mass, (b) bracken litter
turnover rate based on minimum possible input; the linear regression and variation accounted for by
the regression (r2) and significance are shown; * = P < 0.05, ** = P < 0.01; *** = P < 0.001.
Fig. 2 RDA biplot of the most abundant species and explanatory variables from: (a) Analysis 1, using the
2000 floristic data with the selected significant environmental variables, and (b) Analysis 2, using the
1994-2000 floristic data and constrained on litter cover as axis 1 and all other experimental treatments
and time factored out. Bracken litter is also included as a pseudo-species. Codes for species: Agro cap
= Agrostis capillaris; Agro cas = Agrostis castellana; Agro vin = Agrostis vinealis; Brac lit = bracken
litter; Brac rut = Brachythecium rutabulum; Call vul = Calluna vulgaris; Camp int = Campylopus
introflexus; Camp pyr = Campylopus pyriformis; Care pil = Carex pilulifera; Cham ang = Chamerion
angustifolium; Desc fle = Deschampsia flexuosa; Dicr sco = Dicranum scoparium; Eric tet = Erica
tetralix; Fest ovi = Festuca ovina agg; Gali sax = Galium saxatile; Holc mol = Holcus mollis; Hypn jut
= Hypnum jutlandicum; Loph bid = Lophocolea bidentata; Luzu cam = Luzula campestris; Luzu pil =
Luzula pilosa; Nard str = Nardus stricta; Pleu sch = Pleurozium schreberi; Poly for = Polytrichum
formosum; Pote ere = Potentilla erecta; Pter aqu = Pteridium aquilinum; Rhyt squ = Rhytidiadelphus
squarrosus; Vacc myr = Vaccinium myrtyllus; Vacc vit = Vaccinium vitis-idaea. In (a) grazing
treatments (nominal variables) are shown as the centroids and bracken litter depth as a vector. In (b)
Group 1 consists of Agrostis vinealis, Carex pilulifera, Erica tetralix, Ptilidium ciliare and Vaccinium
vitis-idaea.
Fig. 3 Frequency distribution of species that colonized the experimental site between 1993-2000 based on
Ellenberg N values (Hill et al., 1999).
Fig. 4 Potential mechanisms through which cutting and grazing can influence bracken litter abundance.
Mechanisms for which there is evidence in the results of this study are represented by solid lines.
20
Table 1 The effects of the bracken control treatments on bracken litter and frond biomass, non-bracken vegetation\litter and litter turnover indices after seven years. Mean values are given in two forms; arithmetic means in bold for ease of interpretation, and transformed means (loge x+1) on which the analyses of variance were performed. For the latter the LSD (P <0.05), F-ratio and significance are presented (ns = no significance; * = P < 0.05; ** = P < 0.01; *** = P < 0.001). Means not significantly different from each other are denoted with the same letter.
Treatments LSD (P < 0.05)
F5,10 P
Variable
Untreated Asulam Asulam + Cut
Cut + asulam
Cut 1/yr Cut 2/yr
Litter mass Uncompacted bracken litter
382 5.841 a
157 4.058 b
369 5.568 a
317 5.850 a
86 4.058 b
11 1.806 c
1.132
8.94
**
Compacted bracken litter
1642 7.004 a
589 4.610 b
1445 6.807 a
224 5.018 b
63 3.828 bc
15 2.401 c
1.698
6.11
**
Total bracken litter
2024 7.321 a
746 5.027 b
1814 7.237 a
541 6.083 ab
149 4.891 b
27 2.777 c
2.028
7.08
**
Non-bracken vegetation\litter
143 3.328 ab
749 5.619 bc
186 2.688 a
747 6.433 c
781 6.538 c
1010 6.881 c
2.853
3.93
*
Bracken litter turnover indices
K1 (maximum possible litter input)
0.31 0.329 a
0.43 0.251 a
0.54 0.405 ab
0.53 0.408 ab
1.66 0.872 b
5.40 1.725 c
0.508
12.18
***
K2 (minimum possible litter input)
0.26 0.227 a
0.31 0.256 a
0.31 0.261 a
0.62 0.477 a
0.69 0.503 a
5.40 1.725 b
0.481 14.27 ***
Frond mass August 2000 499
6.123 a
191 3.726 ab
671 6.428 a
201 5.271 ab
162 4.794 b
112 3.667 b
2.389
2.37
ns
Table 2. Correlations between bracken, non-bracken vegetation\litter variables and turnover indices. Kendall’s rank correlation (tau b) coefficients are presented
where significance (P < 0.05) was observed; ** = P < 0.01 *** = P < 0.001.
Variable Uncompacted brackenlitter mass (g m-2)
Compacted bracken litter mass (g m-2)
Total bracken litter dry mass (g m-2)
Non-bracken vegetation mass (g m-2)
Litter turnover index (K1)
-0.54 ***
-0.65 ***
-0.65 ***
0.55 ***
Litter turnover index (K2)
-0.42 ***
-0.74 ***
-0.62 ***
0.56 ***
Non-bracken vegetation\litter biomass (g m-2)
-0.53 ***
-0.61 ***
-0.63 ***
-
June frond mass (g m-2)
0.41 ***
0.43 ***
0.45 ***
-0.35 **
August frond mass (g m-2) 0.53***
0.50 ***
0.55 ***
-0.36 **
23
Table 3. Nutrient distribution in the bracken control\ moorland management experiment at Hodron Edge, Derbyshire after seven years treatment: (a) the soil and litter compartments, and (b) the rhizomes. Data are presented as concentrations (C and N = %, other elements = µg g-1) and on an area basis (g m-2, to 15 cm depth). Transformed (loge+1) mean values ± SE (n=36) are presented pooled across all treatments with the arithmetic means in bold. Significance denoted as: - = P > 0.05, * = P < 0.05. ** P < 0.01. *** = P < 0.001. (a) Litter and soil
Soil profile Aoo Ao A1
Element Standing bracken litter
Compacted bracken litter
Non-bracken vegetation\litter
Concentration
C 44.8 3.663±0.150
45.6 3.758±0.109
41.1 3.506±0.179
40.4 3.713±0.023
20.5 3.017±0.056
4.0 1.571±0.042
N 1.5 0.889±0.046
1.8 1.012±0.037
1.9 1.035±0.057
1.8 1.027±0.016
0.7 0.527±0.028
0.2 0.180±0.009
P 531 5.946±0.248
553 6.011±0.196
1125 6.396±0.339
1010 6.874±0.051
843 6.662±0.064
48.2 6.112±0.064
K 5568 8.051±0.346
2453 7.424±0.241
5100 7.803±0.404
2749 7.646±0.165
4341 8.226±0.099
6585 8.760±0.043
Ca 3795 7.735±0.329
292 5.447±0.176
197 4.659±0.303
2264 7.679±0.053
936 6.725±0.098
135 4.716±0.108
Mg 1202 6.673±0.281
459 5.956±0.176
459 5.650±0.293
416 5.980±0.057
435 6.008±0.061
305 5.610±0.085
Na 240 5.158±0.221
144 4.711±0.162
102 4.045±0.283
458 6.090±0.049
734 6.570±0.043
770 6.333±0.029
Area
C 105.1 3.894±0.276
312.2 4.246±0.339
270.6 4.570±0.373
2191 7.571±0.116
2861 7.849±0.077
3234 8.020±0.059
N 3.7 1.219±0.136
12.6 1.634±0.231
13.6 2.113±0.202
97.7 4.482±0.106
9936 4.483±0.084
1.8 5.022±0.065
P 0.1 0.112±0.017
0.6 0.307±0.087
0.8 0.478±0.066
5.5 1.787±0.076
11.6 2.438±0.075
39.3 3.621±0.065
K 1.3 0.681±0.096
1.8 0.641±0.136
3.5 1.180±0.138
14.7 2.495±0.142
61.6 3.927±0.115
549.7 6.244±0.059
Ca 0.9 0.528±0.074
0.2 0.127±0.030
0.1 0.118±0.020
12.4 2.474±0.092
13.2 2.505±0.098
11.2 2.316±0.103
Mg 0.3 0.211±0.030
0.3 0.223±0.056
0.3 0.240±0.031
2.2 1.115±0.052
6.2 1.871±0.076
25.9 3.138±0.084
Na 0.1 0.05±0.008
0.1 0.082±0.024
0.1 0.065±0.012
2.5 1.195±0.059
10.4 2.353±0.058
64.0 4.130±0.051
(b) Element Concentration Area Total rhizome Total expressed
as % of amount in entire profile
Long shoots Short shoots t Long shoots Short shoots t C 42.6
3.776±0.005 44.2 3.812±0.004
4.0***
435 5.969±0.085
156 4.449±0.237 14.3***
591 6.276±0.083
5.7
N 1.2 0.795±0.034
1.2 0.800±0.028
0.1
13.2 2.485±0.101
4.4 1.378±0.136
4.7***
17.6 2.769±0.095
3.9
P 8860 8.916±0.120
6668 8.533±0.155
1.4
9.1 2.101±0.113
2.2 0.906±0.116
10.3***
11.3 2.238±0.103
15.1
K 55525 10.809±0.097
71566 10.838±0.148
0.1
56.0 3.851±0.127
18.7 2.464±0.192
4.3***
74.7 4.147±0.118
10.3
Ca 8128 8.730±0.130
27265 10.06±0.087
6.1***
8.1 1.965±0.120
9.9 1.902±0.170
3.4***
18.0 2.668±0.113
30.0
Mg 27265 9.646±0.089
8620 8.677±0.107
4.9***
17.8 2.744±0.115
2.1 0.967±0.102
12.0***
19.9 2.877±0.107
29.3
Na 658 6.081±0.158
470 5.833±0.161
0.8
0.6 0.432±0.053
0.2 0.147±0.026
6.0***
0.8 0.526±0.057
0.9
24
Table 4. The effects of bracken control treatments on the nutrients in the three litter components (g m-2) in the bracken experiments at Hodron Edge, Derbyshire after seven years treatment. Transformed (loge+1, n=6) means and LSD (P<0.05) values are presented with arithmetic means in bold; means significantly different from each other are denoted by different letters. Significance denoted as: - = P > 0.05, * = P < 0.05. ** P < 0.01. *** = P < 0.001. Compartment Untreated Asulam
+ Cut Cut + Asulam
Asulam Cut 1/yr Cut 2/yr LSD (P<0.05)
F5,10 P
Standing bracken litter
Mass 382 5.841a
369 5.850a
316 5.568a
57 4.058b
86 4.245b
11 1.805c
1.650
8.94
**
C 183 5.103a
176 5.109a
152 4.830ab
74 3.444bc
46 3.523bc
5 1.354
1.480
9.62
**
N 8.5 2.065a
4.9 1.725ab
4.6 1.601b
2.6 1.003bc
1.3 0.782cd
0.2 0.137d
0.784
8.14
**
P 0.20 0.177a
0.20 0.185a
0.18 0.156a
0.10 0.100a
0.05 0.048b
0.01 0.006b
0.107
4.69
*
K 2.8 1.228a
2.2 1.052a
1.8 0.944ab
0.9 0.536bc
0.3 0.257c
0.1 0.069c
0.615
4.96
*
Ca 1.91 1.020a
1.34 0.827a
1.32 0.758a
0.42 0.308b
0.24 0.219b
0.04 0.035b
0.287
18.51
***
Mg 0.39 0.322a
0.37 0.312a
0.45 0.348a
0.22 0.187ab
0.09 0.081b
0.02 0.016b
0.172
6.47
**
Na 0.08 0.079a
0.08 0.077a
0.07 0.081a
0.03 0.032a
0.02 0.023b
0.003 0.003b
0.053
4.14
*
Compacted bracken litter
Mass 1642 7.004a
1446 6.807ab
224 5.018abc
90 4.614bcd
63 3.828cd
15 2.401d
2.247
6.11
**
C 756 6.268a
674 6.073a
111 4.295ab
295 3.971b
30 3.121bc
7 1.746c
2.097
6.78
**
N 35.4 3.200a
25.1 2.860a
4.7 1.465b
9.1 1.380b
1.1 0.667b
0.3 0.232b
1.324
7.87
**
P 1.90 0.818
0.97 0.584
0.09 0.080
0.74 0.330
0.02 0.144
0.01 0.062
0.681
2.4
-
K 4.0 1.249a
4.7 1.537a
0.4 0.292b
1.8 0.601b
0.1 0.127b
0.04 0.036
0.831
5.53
*
Ca 0.31 0.258a
0.45 0.346a
0.07 0.068b
0.07 0.068b
0.02 0.020b
0.01 0.006b
0.163
7.34
**
Mg 0.72 0.475
0.71 0.485
0.10 0.095
0.42 0.242
0.03 0.031
0.01 0.009
0.378
3.24
-
Na 0.20 0.157
0.18 0.163
0.03 0.029
0.2 0.135
0.01 0.009
0.002 0.002
0.171
2.00
-
Non-bracken vegetation\litter
Mass 143 3.328a
186 2.688a
746 6.462b
749 5.619b
781 6.538b
1009 6.880b
2.028
3.93
*
C 64 2.795a
85 2.234a
330 5.608b
343 4.965b
351 5.740b
449 6.075b
2.464
4.40
*
N 3.5 1.009a
4.3 0.861a
16.9 2.682b
13.7 2.318b
17.0 2.794b
19.9 3.012b
1.169
6.42
**
P 0.17 0.141a
0.22 0.167a
1.03 0.662b
1.28 0.696b
0.79 0.523b
1.00 0.679b
0.443
3.48
*
K 0.7 0.429
1.1 0.494
4.0 1.512
7.2 1.716
3.5 1.354
4.6 1.575
1.030
3.03
-
Ca 0.03 0.030a
0.01 0.012a
0.22 0.190b
0.12 0.119b
0.18 0.154b
0.22 0.202b
0.075
11.61
***
Mg 0.07 0.066
0.07 0.061
0.41 0.333
0.37 0.303
0.37 0.306
0.46 0.368
0.174
6.24
**
Na 0.02 0.017a
0.02 0.017a
0.11 0.105b
0.08 0.075b
0.11 0.103b
0.07 0.070b
0.057
4.63
*
25
Table 5. The significant effects of applied treatments in the bracken control\ moorland management experiment at Hodron Edge, Derbyshire; bracken control treatment on (a) rhizome mass (g m-2) and nitrogen concentration on mass basis (%); and (b) nutrient content on an area basis. Transformed (loge+1) means and LSD (P<0.05) values are presented with arithmetic means in bold; means significantly different from each other are denoted by different letters. Significance denoted as: - = P > 0.05, * = P < 0.05. ** P < 0.01. *** = P < 0.001. (a) n=6 Mass compartment\element
Untreated Asulam + Cut
Cut + Asulam,
Asulam Cut 1/yr
Cut 2/yr
LSD (P<0.05)
F5,10 P
Mass – long shoots 1278 7.151 a
1170 6.999 a
1166 6.994 a
1050 6.823 a
959 6.854 a
497 6.100 b
0646
6.56
**
Mass – short shoots 573 6.297
400 4.855
174 4.515
305 4.841
346 5.729
300 5.196
2.199
1.80
-
Mass – total 1851 7.521
1570 7.266
1340 7.136
1355 7.047
1305 7.162
796 6.570
0.914
2.33
-
N - concentration short shoots
1.6 0.968 a
1.4 0.869 ab
1.1 0.725 c
1.3 0.805 bc
1.1 0.721 c
1.1 0.713 c
0.137
4.20
*
(b) n=6 Mass compartment\element
Untreated Asulam + Cut
Cut + Asulam,
Asulam Cut 1/yr
Cut 2/yr
LSD (P<0.05)
F5,10 P
Long shoots C 555
6.318a 505 6.157a
504 6.157a
436 5.973a
399 5.952a
212 5.259b
0.640
6.74
**
N 21.4 3.073a
17.1 2.801ab
11.0 2.423bc
13.8 2.568bc
9.6 2.249cd
6.0 1.795d
0.593
11.14
***
K 57.3 3.973a
62.0 4.089a
58.4 3.966a
68.9 4.106a
62.0 4.112a
27.0 2.856b
0.991
4.88
*
Ca 10.6 2.290a
6.6 1.928ab
9.0 2.145ab
8.6 1.986ab
9.3 2.104ab
5.0 1.334b
0.786
3.56
*
Mg 24.4 3.168a
17.6 2.884a
22.0 3.001a
13.7 2.575ab
21.5 2.964a
7.8 1.866b
0.654
10.34
**
26
Table 6. Nutrient fluxes (g m-2) from vegetation where bracken control treatments have been applied at Hodron Edge, Derbyshire. Arithmetic means ± SE (n=6) are presented. Fluxes were estimated as the difference between the treated plots and its comparator untreated one. A positive value indicates a release of nutrients from the compartment and a negative one indicated uptake relative to the untreated control. The values for the treatments which effected the greatest flux rates (Cut 2/yr) are also expressed as a percentage of the total nutrient stock in the entire profile Compartment Element Bracken control treatment Asulam +
Cut Cut + Asulam,
Asulam Cut 1/yr Cut 2/yr Cut 2/yr fluxes expressed as % of amount in entire profile
Rhizomes C 129.5±151.9 232.4±94.1 246.1±160.6 262.6±46.3 469.5±58.1 4.6 N 8.4±5.0 18.0±3.9 12.3±5.0 17.6±2.2 21.9±3.5 4.8 P -1.4±1.9 1.1±4.8 -1.7±6.1 4.0±1.6 7.4±.2.5 9.8
K 8.4±24.2 25.8±20.0 13.9±35.4 8.5±19.4 50.3±16.0 6.9 Ca 8.7±5.9 12.3±3.9 9.4±6.4 -0.7±10.4 11.6±4.9 19.3 Mg 7.8±3.7 4.2±4.0 12.2±6.1 4.0±7.8 17.9±4.9 26.3 Na 0.2±0.4 0.5±0.3 0.3±0.5 -0.4±0.5 0.4±0.2 0.5
C 87.9±341.5 674.2±279.6 568.4±251.2 867.5±281.8 925.9±278.4 8.9 N 13.8±16.0 34.5±15.0 32.2±15.0 41.5±15.0 43.4±14.8 9.5 P 1.0±1.1 1.9±1.0 1.3±0.6 2.1±1.0 2.1±1.0 2.8
Bracken litter (both fractions)
K -0.1±3.0 4.5±2.6 4.1±3.0 6.3±2.4 6.6±2.3 0.3 Ca 0.4±0.4 0.8±0.8 1.7±0.6 1.9±0.5 2.2±0.5 3.7 Mg 0.03±0.4 0.6±0.4 0.5±0.3 1.0±0.3 1.1±0.3 3.2 Na -0.001±0.1 0.2±0.1 0.04±0.1 0.2±0.1 0.3±0.1 0.4
C -20.6±77.7 -266.5±76.1 -277.9±107.1 -286.8±54.1 -385.4±56.1 -4.0 N -0.8±4.4 -13.4±4.6 -10.2±5.2 -13.5±2.0 -16.3±3.5 -3.6 P -0.04±0.2 -0.9±0.3 -1.12±0.6 -0.6±0.2 -0.8±0.1 -1.1
Non-bracken vegetation and litter
K -0.4±0.9 -3.3±0.9 -6.4±3.0 -2.7±0.9 -3.8±1.1 -0.5 Ca 0.02±0.02 -0.2±0.1 -0.1±0.1 -0.1±0.1 -0.2±0.03 -0.3 Mg 0.01±0.07 -0.3±0.1 -0.3±0.1 -0.3±0.1 -0.4±0.1 -0.6 Na 0±0.02 -0.1±0.03 -0.1±0.04 -0.1±0.04 -0.1±0.02 -0.1
27
Fig 1
28(a)
-1.0 +1.0
Depth
Grazed
Ungrazed
Brac lit
Gali sax
Pter aquFest ovi
Agro capCamp int
Hypn jut
Rhyt squ
Agro cas
Dicr sco
Desc fle
Pote ere
Agro vin
Eric tet
Vacc vitLuzu pil
Brac rut
Luzu camCamp pyr
Call vul
Pleu schPoly for
Cham ang
Care pil
Axis 2
Axis 1
-1.0
+1.0 (b)
Fig 2
29
7654321
15
10
5
0
Freq
uenc
y
N value
Fig 330
Releases nutrients
Reduces shade
Reduces rhizome reserves
Reduces frond
production
Increases competition
Aids establishment
of other vegetation
Reduces litter
Enhances decomposition
Increases species diversity
Grazing Cutting
31Fig 4