Factors Affecting the Restoration of Heathland and Acid Grassland on Pteridium aquilinum–Infested Land across the United Kingdom: A Multisite Study

The variability in the success of Pteridium aquilinum (bracken) control and vegetation restoration has been highlighted as a major issue in the United Kingdom. Experiments were set up at four different regional locations to assess bracken control at the national scale and the impact of restoration practices at the local scale. Bracken control treatments (cutting once or twice per year, a combination of cutting and asulam spraying, and asulam in year 1) were combined with site‐specific treatments designed to restore appropriate heathland or acid grassland vegetation. This article considers the effects on the developing understorey vegetation, testing the following hypotheses: (1) local differences between sites would affect community change; (2) treatments applied to control Pt. aquilinum (same at all sites) influences community change; and (3) treatments applied at the individual site level to restore vegetation influences community change toward the target vegetation. There were a considerable number of spatial effects. It is, therefore, difficult to develop a one‐size‐fits‐all policy for vegetation restoration within a national Pt. aquilinum control strategy. Few bracken control treatment effects were found, and, where they were detected, it was only at single sites. Thus, the development of target vegetation requires a combination of control and restoration treatments that take into consideration the aspects of that site. Only three species, Deschampsia flexuosa, Galium saxatile, and Campylopus introflexus, increased as a direct effect of the control treatments. Vegetation restoration was most successful in the cutting‐twice‐per‐year plots, the treatment with the greatest reduction in Pt. aquilinum cover.     

The effects of cutting pre-treatments on the performance of asulam on bracken (Pteridium aquilinum)

The performance of asulam was assessed from a field experiment in which bracken (Pteridium aquilinum) was subjected to a variety of cutting pre-treatments prior to spraying with the herbicide. Both cutting bracken twice in the year prior to asulam application, and spraying frond regrowth 10 wk after a single mid-June cut, improved the performance of asulam. Summer cutting removes nutrients and dry matter from the system and reduces frond height, but increases frond density and the number of active buds on the rhizome system. The enhanced efficiency of asulam on pre-cut plots may be due to any one of these factors, or a complex interaction between some (or all) of them. In areas of tall bracken a cutting pre-treatment has the important practical advantage of reducing the height of the canopy, making the subsequent use of ground spraying equipment easier.     

The effects of cutting and herbicide treatment on Pteridium aquilinum encroachment

Abstract. Pteridium aquilinum (bracken) encroachment is an important factor in the loss of certain habitats in the United Kingdom. However, no information exists as to whether prevention of encroachment is a cost‐effective strategy for Pteridium management. Conventional methods for the control of Pteridium (cutting, asulam application) were tested at one site (Levisham) to quantify their ability to prevent or delay encroachment and to affect the vigour of the Pteridium at the edge of the stand. The effects of encroachment and asulam application on the vegetation present were monitored at a second site (Ramsley), where techniques commonly used for moorland restoration were employed in combination with asulam application. Cutting once per year or a single application of asulam delayed the advance of the Pteridium front. At Levisham, the untreated front advanced 2.7 m in 5 yr, while in the same period the cut front advanced 0.88 m and the sprayed front was 1.5 m behind its initial position. At Ramsley, the untreated front invaded 1.8 m in 5 yr, and the sprayed front was again 1.5 m behind its starting position. Both spraying and cutting reduced frond biomass, frond cover and rhizome biomass. Herbicide spraying prevented the loss of Calluna vulgaris, though the restoration treatments had little effect. The merits of a balanced targeting of control on encroaching fronts or Pteridium at the stand level are discussed for different situations.     

Control of Pteridium aquilinum: meta-analysis of a multi-site study in the UK

Background and Aims

A great deal of money is spent controlling invasive weeds as part of international and national policies. It is essential that the funded treatments work across the region in which the policies operate. We argue that experiments across multiple sites are required to validate these programs as results from single sites may be misleading. Here, the control of Pteridium aquilinum (bracken) is used as a test example to address the following four questions. (1) Does the effectiveness of P. aquilinum-control treatments vary across sites? (2) Is the best treatment identified in previous research (cutting twice per year) consistent at all sites, and if not why not? (3) Is treatment performance related to P. aquilinum rhizome mass, litter cover or litter depth at the various sites? (4) Does successful P. aquilinum control influence species richness?

Methods

Pteridium aquilinum‐control treatments were monitored for 10 years using six replicated experiments and analysed using meta-analysis. Meta-regressions were used to explore heterogeneity between sites.

Key Results

The effectiveness of treatments varied between sites depending on the measure used to assess P. aquilinum performance. In general, cutting twice per year was the most successful treatment but on some sites other, less expensive treatments were as good. The effectiveness of treatments at different sites was not related to rhizome mass, but the effectiveness of most applied treatments were inversely related to post-control litter. Effective treatment was also associated with high species richness.

Conclusions

It is concluded that successful development of national weed control programs requires multi-site experimental approaches. Here, meta-analyses demonstrate that variation in effectiveness between sites could be explained in part by pre-specified variables. Reliance on data from a single site for policy formulation is therefore clearly dangerous.Key words: Bracken, Pteridium aquilinum, weed, control, herbicide, litter dynamics, species diversity, meta-analysis     

Effects of the litter layer of Pteridium aquilinum on seed banks under experimental restoration

Questions: Does the litter layer of Pteridium aquilinum (bracken) act as a barrier to certain species in the seed bank? Does bracken control/restoration treatment affect seed transfer through the litter layer? Location: Five experiments at three sites across the UK covering two major vegetation types; acid‐grassland and heath‐land. Methods: At each experiment a range of bracken control and vegetation restoration treatments were applied for about ten years. The seed bank was sampled in both the bracken litter and the soil. The cover (%) of each species in the vegetation and the bracken litter abundance (cover and depth) was also estimated. Results: The bracken litter layer acts as an inert barrier as it contained a large proportion of seeds available in the litter‐soil profile (38%– 67% of the total). Bracken litter depth and cover also influenced significantly the seed bank composition in both the bracken litter and the soil. These effects were site‐specific, and species‐specific. The application of treatments changed significantly the balance between seed inputs and outputs in the bracken litter layer for some species. This was either a positive or negative response relative to the untreated control plots. Conclusion: For heathland and acid‐grassland restoration, the bracken litter layer may be an important seed source, but it must be disturbed particularly before seed addition.     

Vegetation development on moorland after control of Pteridium aquilinum with asulam

Abstract. A range of moorland sites in the North York Moors National Park were surveyed where Pteridium aquilinum has been sprayed once with the herbicide asulam between 1 and 7 yr before. The regeneration of both P. aquilinum and the moorland vegetation were studied. The latter was variable and much slower than reported for lowland grassland or heathland, with a large proportion of the developing plant cover consisting of mosses, especially Campylopus introflexus. This slow rate of revegetation was primarily due to the paucity of micro-sites offered for germination in dense P. aquilinum litter, the poor establishment of seedlings in this litter and its slow breakdown. The consequences of this slow and variable development of vegetation to moorland restoration are discussed.     

Temporal Responses of Propagule Banks during Ecological Restoration in the United Kingdom

Successful ecological restoration is expected to be accompanied by change in the propagule bank. We tested for temporal change in the soil propagule bank at two Bracken (Pteridium aquilinum)‐infested sites in the United Kingdom (acid grassland and heathland), each with replicate experiments. A combination of bracken control (cutting, spraying, and combinations) and vegetation restoration treatments (seeding, fertilizer, harrowing) were applied. Soil propagule banks were sampled in 1998, 4–5 years after the start, and in 2003 after a further 5 years. We used univariate and multivariate statistical methods to investigate the response of the propagule bank to experimental treatment in space and time. Few effects were found in 1998, but after a further 5 years, the propagule bank size and composition changed significantly, implying that propagule bank development lags behind vegetation development. The effect of treatment on the propagule bank differed within sites. Thus, ecosystem development is occurring at different speeds and directions even in closely adjacent areas. Coupling between the propagule bank and vegetation changed between sampling times with either some new coupling or decoupling. At both sites, the propagule bank contained propagules of the target community and thus propagule bank development during restoration provides increased potential for vegetation recovery. However, it can take a considerable time for management effects to be detected in the propagule bank. Moreover, the effect or speed of effect is spatially variable. Continuing application of restoration treatments is recommended at heathland where there is a deep bracken litter layer.     

Recovery of Moorland Vegetation after Aerial Spraying of Bracken (Pteridium aquilinum (L.) Kuhn) with Asulam

Bracken (Pteridium aquilinum) is a major weed of seminatural vegetation in Great Britain, as in many other countries. As a consequence, large areas have been subject to control with the intention of restoring the former vegetation. The use of aerial spraying with asulam, a narrow spectrum, carbamate herbicide, has become a common method of control. However, its long‐term efficacy has not been assessed in terms of either controlling the bracken or in producing suitable restored habitats. This study undertook such an assessment by comparing the results of a 2002 survey of the sites subject to bracken control by aerial spraying of asulam with previous (1990/1991 and 1994) surveys of the same sites. This showed that a single application of asulam was effective in eradicating bracken (<1% cover remaining) on a third of sites. However, on 10% of the sites, the bracken had regenerated completely (cover >80%) and on the remainder it was still present in patches (>20% of quadrats), often at high density. More than half the sprayed sites had seen good recovery of moorland vegetation, the target of the restoration, because they were now classified as having upland heathland vegetation within the National Vegetation Classification. Considerable amounts of bracken control are grant aided as part of agri‐environment schemes. These schemes should be adapted to encourage good practice, namely, intensive follow‐up treatment by spraying any emerging fronds, and to encourage treatment of previously sprayed areas rather than spraying of new areas in order to protect previous investment of grant aid.     

Vegetation re‐establishment on land previously subject to control of Pteridium aquilinum by herbicide

Abstract. Vegetation development after conventional bracken control with herbicide in the uplands is often slow and rarely results in the development of the target community. Two experiments were established on contrasting sites to investigate cost‐effective methods for the re‐establishment of vegetation dominated by Calluna vulgaris (heather), which is the usual target community seen as the desired replacement for Pteridium aquilinum in the uplands by many land managers. At one site, where grazing intensity was low and shelter was sufficient to prevent rapid Pteridium litter loss, the necessary treatments to establish Calluna were to disturb the litter and add a source of Calluna seeds. A difference between continued Pteridium control or a relaxation of control was not observed over the time‐scale of the experiment. At the second site, where grazing intensity was high and shelter minimal, vegetation dominated by Rumex acetosella had developed on a thin but dense litter layer. To enhance Calluna regeneration it was necessary to prevent sheep grazing, disturb the remaining litter layer and add a source of Calluna propagules. A nurse crop hindered Calluna establishment due to its vigorous growth. At neither site was an initial application of fertilizer an aid to Calluna establishment. It is possible to devise methods for the re‐establishment of Calluna at sites where previous attempts at Pteridium control alone have not had the desired effect on vegetation development. However, the precise choice of methods used is dependent on the grazing management at the site. High levels of livestock grazing prevented Calluna establishment, but some trampling may aid litter breakdown. The ability to adjust stocking rates and access to the site by machinery are critical factors to take into account in producing an effective strategy for long‐term Pteridium control and its replacement by target vegetation.     

Resistance of Pteridium aquilinum to attack by non-adapted phytophagous insects

Fresh bracken Pteridium aquilinum L. Kühn or crude bracken extracts deterred 9 non-adapted phytophagous insect species from feeding or settling. Feeding deterrent activity to 5 out of 7 insect species was present in crude bracken extracts at all times over the growing season and was significantly highest in May. Sequential isolation and purification of deterrents indicated that the sesquiterpene pterosin F (6-chlorethyl-2,5,7-trimethyl-indan-1-one) occurred in sufficiently high concentrations in late May/early June bracken frond (24–28 mg/kg fresh wt) to be partially responsible for the deterrent effects of crude bracken extracts to two insect species. A number of other fractions had feeding deterrent activity, but these were not characterized or quantified.     

The Distribution of Silicon Deposits in the Fronds of Pteridium aquilinum L.

The occurrence of silicon in mature fronds of Pteridium aquilinumwas investigated using light and scanning electron microscopyand X-ray micro analysis. The heaviest deposits were locatedin the outer tangential walls of the epidermis of the petiole,but the upper epidermal cells of the costal regions of the laminaealso accumulated small quantities. No discrete bodies were associatedwith these deposits. However, the mid-ribs of the laminae exhibitedsome long siliceous fibres associated with the luminae of thestrengthening tissues. The significance of these accumulationsin relation to deposition mechanisms and possible carcinogeniceffects are discussed. Pteridium aquilinum L., bracken, frond, microanalysis, silica     

Lignin signatures show the effects of changes in heather and bracken cover on the composition of organic matter in a moorland soil profile

Bracken (Pteridium aquilinum) is aggressively displacing heather (Calluna vulgaris) on many moorlands in Britain. We investigated the use of lignin derivatives to identify the distribution of soil organic matter (SOM) derived from bracken in moorland soil profiles formed under heather. Phenylpropanoids extracted from recently senesced litters, roots and SOM, using alkaline CuO oxidation, showed distinct signatures for bracken and heather, with vanillyl moieties dominating bracken litter extracts and vanillyl and syringyl dominating heather litter extracts. Ratios of vanillyl and syringyl concentrations characterised the SOM derived from heather and bracken better than the concentrations of the individual moieties. The analysis showed up to a depth of 5 cm under pure bracken cover, and at the interface between heather and bracken, the SOM was largely derived from bracken litter but below that depth SOM was apparently derived from heather. The use of these methods to identify the plant origin of SOM not only enables understanding the effects of changing vegetation cover on organic matter dynamics in moorland soils but could also facilitate management techniques in moorland/heathland restoration which involve the removal of comparatively nutrient-rich SOM derived from bracken. Received: 30 November 1997 / Accepted: 11 April 1998     

The morphology of bracken (Pteridium aquilinum (L.) Kuhn) in the North York Moors—a comparison of the mature stand and the interface with heather (Calluna vulgaris (L.) Hull) 1. The fronds

The encroachment of bracken (Pteridium aquilinum (L.) Kuhn) into areas previously dominated by heather represents a threat to the ecology, agricultural economy and landscape value of many UK upland areas, including the moorland of the North York Moors National Park. The morphology of bracken, within a mature stand and at several bracken-heather interfaces, has been studied at a number of sites within the National Park. Differences have been found in the frond growth of bracken in a mature stand, at stationary stand margins, and at advancing stand margins where bracken is encroaching into heather. Frequency of fronds present on bracken rhizome growing at a stationary stand margin close to the interface with heather (1–2 m behind the boundary) are approximately the same as those found within a mature stand. At advancing margins (again 1–2 m behind the boundary), maximum frond densities were often found to exceed those present in either a mature stand or at a stationary margin. Frond numbers decline rapidly at the stand margins as distance from the stand increases. This is especially true where the front is stationary and bracken is not encroaching into heather at a significant rate. Maximum frond heights in a mature stand consistently exceed those at stand margins (even 1–2 m into the stand) and are greater at stationary margins than at advancing margins. Outlying fronds at the edges of bracken stands are generally present in greater numbers, and further into the area dominated by heather, where the margin is advancing. Heights of outlying fronds fall as distance from the bracken stand increases, as does stipe length. Fronds at the edges of bracken stands emerge each spring before those further into the stand and are therefore particularly vulnerable to frost damage. Outlying fronds are not, however, the first to emerge. Early emerging fronds reach their maximum height and eventually become senescent before later emerging fronds. Whilst most fronds emerge before the end of June a few fronds continue to emerge throughout the summer. Frond densities close to the edges of bracken stands (1–2 m into the stand) are comparable to those in a mature stand. At advancing stand margins frond densities generally exceed those in a mature stand, suggesting that a large number of potential entry points for foliage-applied herbicides are available for bracken control at the stand margins. The ratio of potential uptake points to biomass of rhizome is also greatest at the edges of the stand, and the canopy 1–2 m into the stand is usually almost completely closed. It is possible therefore, the efficacy of herbicides could be improved by the use of small scale applications, using tractors or hand-held sprayers, close to the margins of bracken stands.     

Effects of restoration treatments on the diaspore bank under dense Pteridium stands in the UK

Abstract. The diaspore bank (seeds of higher plants and spores of ferns and bryophytes) was assessed between 3 and 5 yr after experiments to control Pteridium aquilinum (bracken) and restore appropriate vegetation were initiated at two contrasting locations in the UK. We tested the response of the diaspore bank using univariate and multivariate analysis of variance. The two approaches were complementary and together improved the interpretation of these results. There were considerable differences in the diaspore banks of the two sites and among the experimental locations within sites. Within each experiment there were differences in species composition, with species that were (1) common to both diaspore bank and vegetation, (2) restricted to the diaspore bank and (3) restricted to the vegetation. There is a possibility of increasing the biodiversity of the developing vegetation if some of the species present in the diaspore bank can be germinated. This was especially true for ferns where four species were found in the spore bank which were not present in the vegetation. There were few significant effects of management treatment on the diaspore bank as the experiments had been in progress for only 3 to 5 yr, but a few species had different densities in the different treatments (Betula pubescens, Juncus effusus and some bryophytes). The greatest correlation between vegetation and diaspore bank was found at the top hierarchical level (entire dataset) and this progressively reduced with scale. We interpret this as a landscape/species pool effect: as the scale of the study reduces the correlation between diaspore bank and vegetation also reduces, at least over the time scale of our study. The relevance of these results for restoration ecology is discussed briefly.     

Factors controlling vegetation change in long-term experiments designed to restore heathland in Breckland,UK

Abstract. This paper describes the vegetation change in an 18-year experiment designed to test a range of control of Pteridium aquilinum (Pteridium) and heathland restoration treatments; cutting, asulam application and Calluna vulgar is seeding, in a range of combinations at Cavenham Heath in Breckland, UK between 1978 and 1996. Vegetation change was recorded in two Phases; in Phase I (1978-1986) species biomass was sampled and in Phase II (1986-1996) cover was measured. Initially, Calluna establishment was good in some treatments, especially where Calluna seed was added and Pteridium was controlled. Other plots developed either a grass-heath flora dominated by Agrostis capillaris, Deschampsia flexuosa, Dicranum scoparium, Festuca ovina and Rumex acetosella or were dominated by clonal species such as Calamagrostis epigejos or Carex arenaria. An unconstrained ordination showed significant vegetation change through time and that several treatments influenced the vegetation, especially those involving asulam application. When variation partitioning with constrained ordination was used a different explanation emerged. In Phase I the most important factors were the management treatments applied, elapsed time and spatial factors, with little overlap. In Phase II, elapsed time became irrelevant because the variation time explained overlapped that which could be explained by other variables. The most important of these were management treatments, spatial effects, weather, the amount of bare ground caused by disturbance and Pteridium litter cover (an index of Pteridium recovery). The implications of these results in interpreting vegetation change are discussed.     

The effects of cutting and asulam on numbers of frond buds and biomass of fronds and rhizomes of bracken Pteridium aquilinum

The effects of cutting and the herbicide asulam for the control of a dense uniform stand of bracken Preridium aquilinum were investigated experimentally over a 6 —yr period in Norfolk, England. Bracken is a particularly difficult species to control by either mechanical or herbicidal means because of its extensive and complex rhizome network, containing large reserves of dry matter and a large number of frond buds. Cutting bracken in the summer removed dry matter from the system and reduced frond vigour but did not reduce the number of frond buds to a critical level. Whereas the first few cuts substantially reduced the standing crop of fronds, relatively stable standing crops of fronds were observed in future years despite continued cutting. Asulam caused a 99% reduction in the standing crop of fronds in the year after spraying, but in the absence of further treatment, fronds recovered to pre-spray levels after 6 yr. Asulam caused severe localised damage to frond buds and rhizome apices, but otherwise the rhizome network remained intact. New sections of frond bearing rhizome (short shoots) developed on storage rhizome (long shoots) deep in the soil of both the asulam plots and those cut.     

Studies on Phytoremediation of Copper Using Pteridium Aquilinum (Bracken Fern) In the Presence of Biostimulants and Bioassay Using Clarias Gariepinus Juveniles

A study was carried out to evaluate the uptake of copper from water containing 10 mg/L copper by Pteridium aquilinum (bracken fern) and Clarias gariepinus in the presence of five plant growth stimulants: Nitrogen: phosphorus: potassium (15-15-15: an inorganic fertilizer), pig, cattle, poultry, and a mixture of pig/cattle manures. A plant growth stimulant differentiated each treatment. A 96-hour bioassay using C. gariepinus was carried out at the end of the experiment to test the efficacy of the clean up by P. aquilinum. The control experiment contained no copper or plant growth stimulant. Fish survival, uptake of copper by P. aquilinum, C. gariepinus, concentration of copper in water, hematology and histopathology of the fish were assessed. Higher concentrations of copper were reported in P. aquilinum than in water or C. gariepinus. Low fish mortality was reported with the highest being 20% in the cattle manure–containing treatment.     

Efficacy of bracken (Pteridium aquilinum (L.) Kuhn) control treatments across a range of climatic zones in Great Britain

Bracken control field experiments were conducted at six locations across Great Britain. The effects of various cutting and herbicide management regimes upon the seasonal dynamics of bracken fronds and rhizomes were examined over a three year period. This enabled a national overview towards bracken control to be constructed. Initially, spraying with asulam was the most effective treatment in reducing frond biomass and density but was least effective in reducing rhizome biomass. Differential reductions in rhizome biomass were observed in relation to cutting frequency, with cutting twice yearly giving superior control. The national trends confirmed a number of previous observations from independent single-site studies; however, others were contradicted. Comparable management options were ranked consistently between the current and former investigation, however, the initial importance of differences in cutting frequencies did not agree between studies. This multiple-site study improves understanding of the consequences of a national bracken control programme by reducing the influence of confounding site-specific factors, and recommendations for the most appropriate bracken control techniques are made. The extent to which individual sites reflect a national trend in response to bracken control is considered and sites are compared. The hierarchy of treatments identified at the national scale was found to apply generally within individual sites. However, several responses which proved significant at the countrywide level were not so clearly defined at the site scale. Cutting once yearly was the only management regime which appeared to give different bracken control between sites. All other treatments gave similar responses between sites. This result was found in the Scottish Borders, during the second year of control, when frond biomass and density (relative to untreated plots) were greater than that recorded at other sites. This contrast was not found in subsequent monitoring. In terms of rhizome biomass depletion, poorer control was achieved following cutting once yearly at the northern sites (Mull, Scottish Borders, Lake District) compared with the southern sites (Clwyd, Breckland, Devon). The implications of experimental results are discussed in relation to increased cost effectiveness of national bracken control programmes.     

Effects of Sodium Selenite on the Growth of Bracken Fern Root Callus

Sodium selenite promotes the growth of bracken fern root callusgrowth over a narrow, low range of concentrations, while higherconcentrations are strongly inhibitory. Pteridium aquilinum, bracken fern, sodium selenite, selenium, callus     

The effects of control on the biomass, carbohydrate content and bud reserves of bracken (Pteridium aquilinum (L.) Kuhn), and an evaluation of a bracken growth model

This paper examines the initial effects of bracken control on frond numbers and biomass, and the biomass, carbohydrate reserves and bud densities of bracken stands cut once per year, twice per year, subject to a single application of asulam or left untreated. The seasonal dynamics of these parameters are displayed; carbohydrate and biomass are both removed from the rhizome system to produce frond tissue, and are replenished at the end of the growing season. Asulam application reduced densities of both active and dormant buds, and both frond biomass and density. It did not significantly reduce rhizome biomass or carbohydrate reserves in the two years after treatment. Cutting, either once or twice per year reduced both rhizome biomass and rhizome carbohydrate reserves, as well as bud densities, though the latter were reduced in proportion to biomass. Cutting twice a year reduced the production of fronds, both in numbers and biomass. The collected data were used to evaluate a model of bracken growth, and subsequently to improve estimates of some of the model parameters. The model simulations of control treatments were compared to field data. The effects of cutting once per year and spraying with asulam were predicted accurately, but the bracken stand was more resilient to cutting twice per year than would be expected from model predictions. The combination of cutting and spraying is discussed as a potential tool in land management and the deficiencies of the model are discussed in relation to the need for future research into the biology of bracken.