M.G. Le Duc

A study of succession on lowland heaths in Dorset, southern England: changes in vegetation and soil chemical properties

Lowland heaths are high-profile ecosystems for conservation action in England, but they are under threat from invasion by Betula spp., Pinus sylvestris, Pteridium aquilinum, Rhododendron ponticum and Ulex europaeus. Ten heathland sites in the Poole Basin area of Dorset, where succession to one or other of the above species had occurred, were studied to examine the changes in vegetation and soil chemical properties. A series of hypotheses to explain (i) vegetation successional trajectories, and (ii) soil chemical properties associated with vegetation change were tested using multivariate models (DECORANA - vegetation; CANOCO - vegetation and soil). A range of pathways was found along which heathland communities move during succession, with some successional pathways remaining closer to heathland than others. The Betula spp. succession caused the greatest changes in terms of the vegetation present. The Ulex europaeus and Pinus sylvestris trajectories retained some typical heathland species. Different soil nutrients were found to increase along different successional pathways, which were associated with the different successional species invading. Pinus sylvestris had similar soil nutrient concentrations to those of the heathland. Sodium concentrations increased during the Rhododendron ponticum succession. Concentrations of ammonium-nitrogen and nitrate/nitrite-nitrogen increased during the Pteridium aquilinum and Ulex europaeus succession. The Betula spp. had the greatest effect on the soil nutrients with increased pH, extractable phosphorus and exchangeable calcium. The results are discussed in relation to practical heathland management and the restoration of heathland after succession. Specifically, it is more sensible and cost-effective to restore heathland on sites where Pinus sylvestris successions have occurred, than where Betula spp. successions have occurred.

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

Abstract 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. Abbreviation: RDA = Redundancy Analysis. Nomenclature: Stace (1997).

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. Nomenclature: Stace (1997) for higher plants and ferns; Smith (1978) for bryophytes. Abbreviations: RDA = Redundancy analysis.

Moorland vegetation succession after the control of bracken with asulam

Abstract A range of moorland sites in the North York Moors, North Yorkshire, UK, where bracken control with the herbicide asulam has been carried out, has been re-surveyed. The trajectory of vegetation change at each site has been analysed using a range of techniques. Vegetation change after spraying appears to depend on three things—the species present at the time of control, the initial effectiveness of treatment and the amount of subsequent disturbance (mainly by sheep). Where grazing pressure was low, there was little invasion or spread of plants, except on sites with an initial moderate cover of Vaccinium myrtillus . Where high sheep activity disturbed the litter layer, invasion by the moss Campylopus introflexus characteristically occurred, followed by an increase in dominance by grasses, especially Deschampsia flexuosa . Where grazing pressure was high enough, it appeared that bracken regeneration was slowed, otherwise the bracken canopy and the stand recovered. However, in the timescale of the study, sufficient variety and cover of other species remained after the canopy regenerated to prevent the complete progression to vegetation characteristic of untreated areas.

Changes in the field-layer of Wytham Woods – assessment of the impacts of a range of environmental factors controlling change

Abstract Question: What are the impacts of factors controlling patterns of change in woodland field-layer vegetation, through time? Location: Wytham Woods, Oxford, UK (51°41′ N, 1°19′ W). Methods: Species occurrence was monitored in 163 permanent plots three times between 1973 and 2002 and related to management compartment and history. The dataset was analysed using a combination of ordination methods, univariate and multivariate analysis of variance and HOF modelling. Results: The two key findings of this paper are: (1) that the species composition of ancient woodlands (often cited as a conservation goal), may be moving along an ‘undesirable’ trajectory, from a conservation viewpoint; and (2) that the impact of a synergistic effect of changing canopy dynamics, nitrogen deposition and a rising deer population has resulted in an increase in grass species, at the expense of woodland herbs. These findings have significant implications for the practical conservation of similar habitats across Europe. Conclusions: The analytical methodology presented here allowed detailed assessment of the potential roles of a range of environmental factors controlling change. The methodology may be applicable to other long-term studies of vegetation, and is particularly appropriate for identification of cross-site drivers, such as pollutant load and climate change. Nomenclature: Stace (1997).

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. Nomenclature: Stace (1991).

The effects of changing weather between 1967 and 1997 on the growth of Corsican pine.

Summary Climate is an important factor influencing the growth of trees, yet effects are often difficult to detect. Here we correlated 16 weather variables derived from a 32-year run from Ness Botanic Gardens with the annual tree ring growth increments for Pinus nigra (Arnold), growing nearby. Various temperature variables and water vapour pressure increased over the 32 years and were correlated with increased tree growth. Temperatures in spring, winter and autumn were more important than those in summer, and there was no relationship with rainfall. This may be because Pinus nigra was originally from the Mediterranean area and may be adapted to dry, warm climates. It may be possible to use dendrochronology of species such as P. nigra to track climatic change on a local scale.

Native dominants in British woodland - a potential cause of reduced species-richness?

Abstract The invasion of native habitats by alien species has received considerable attention. However, in Britain high levels of dominance by a small number of aggressive native plant species may have an equal, or greater, impact on the richness of native woodlands. Here, we examine this hypothesis by modelling the realized niche of native-dominant species along the principal coenocline of British woodlands, and examined niche overlaps with 78 woodland specialist species and two alien species. Four native species had a much greater cover than all other field-layer species, and between them they entirely covered the response range of all other field-layer species, replacing one another along the coenocline. These findings, combined with autecological information suggest that Hedera helix, Mercurialis perennis, Pteridium aquilinum and Rubus fruticosus have the potential to become ‘over-dominant’ and perhaps may impinge on other field-layer species. Our results also identified which field-layer species are likely to be impacted by a change in abundance of each of these dominant-species, and as such, provide a novel quantitative method of risk assessment to aid conservation policy. Understanding how woodland communities remain diverse, even in the presence of aggressive native species, may provide insights into how the impact of exotic invasive species can be managed.

Aliens or natives: who are the ‘thugs’ in British woods?

SummaryThe invasion of native habitats by exotic, or alien, plant species has received considerable attention recently from policy, research, and practical conservation management perspectives. However, a new hypothesis for species dynamics in Britain suggests that a small number of aggressive native plant species (termed ‘thugs’) may have an equal, or greater, impact on native species and habitats than exotic species. Here, we examine this hypothesis using multivariate techniques with field-layer cover data collected during a country-wide survey of British woodlands. Multivariate analysis of these data identified a north-south gradient on the first axis, and that 20 of the 25 National Vegetation Classification woodland types were sampled within the study. The most abundant field-layer species included three of the proposed native ‘thugs’, i.e. Rubus fruticosus, Pteridium aquilinum and Hedera helix in addition to the native woodland indicator species Mercurialis perennis. Variation partitioning was used to compare the relative importance of native field-layer ‘thug’ species with invading alien shrub and tree species relative to other environmental drivers. The variation in the field-layer data-set explained by the three native ‘thug’ species was significant, but they explained a relatively small proportion of the variation relative to other environmental variables (climate, soil, management factors etc.). They did, however, explain almost four times as much variation as the three alien species that were significantly correlated with field-layer species composition (Acer pseudoplatanus, Impatiens glandulifera, Rhododendron ponticum). The results of this analysis suggest that the field-layer of British woodlands is impacted as much by native ‘thug’ species, as it is from ‘aliens’. Concern about the impact of these native ‘thug’ species has been reported previously, but their impact has not previously been compared to the impact of invading aliens. It is hoped that this analysis will do two things, first to act as a sound baseline for assessing any changing balance that should occur in the future, and second, to prompt both ecologists and conservationists to develop woodland management policies based on sound science.

Bracken invasion in Scotland

Summary For the first time, estimates of the amount of sparse bracken on open land, the amounts of bracken under tree cover and the amounts in linear features are made. These data were used to predict the areas of Scotland most at risk from invasion from patch edges, from sparse patches becoming dense and from invasion along linear features. Computer models show that if the climate warms over the next 50 yr as predicted, then bracken growth will increase and the amount of hill land suitable for invasion by bracken will increase. On some sites where bracken is being controlled, it is reduced in cover and other ground vegetation is becoming established. However, we cannot ensure that when bracken is controlled that the control is always effective and that appropriate vegetation will become established. Some generalized rules have been derived to help in the selection of sites likely to show the most benefit from control treatment.