Changing land use and the spread of 'winning' native or exotic plants are expected to lead to biotic homogenization (BH), in which previously distinct plant communities become progressively more similar. In parallel, many ecosystems have recently seen increases in local species (a-) diversity, yet g-diversity has continued to decline at larger scales. Using national ecological surveillance data for Great Britain, we quantify relationships between change in a-diversity and between-habitat homogenizations at two levels of organization: species composition and plant functional traits. Across Britain both increases and decreases in a-diversity were observed in small random sampling plots (10-200 m 2 ) located within a national random sample of 1 km square regions. As a-diversity declined (spatially in 1978 or temporally between 1978 and 1998), plant communities became functionally more similar, but species-compositional similarity declined. Thus, different communities converged on a narrower range of winning trait syndromes, but species identities remained historically contingent, differentiating a mosaic of residual species-poor habitat patches within each 1 km square. The reverse trends in b-diversity occurred where a-diversity increased. When impacted by the same type and intensity of environmental change, directions of change in a-diversity are likely to depend upon differences in starting productivity and disturbance. This is one reason why local diversity change and BH across habitats are not likely to be consistently coupled.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. British Ecological Society is collaborating with JSTOR to digitize, preserve and extend access to Journal of Applied Ecology. Summary 1. 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. 2. 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. 3. 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; CANOCOvegetation and soil). 4. A range of pathways was found along which heathland communities move during succession, with some successional pathways remaining closer to heathland than others. 5. 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. 6. Different soil nutrients were found to increase along different successional pathways, which were associated with the different successional species invading. 7. 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. 8. The results are discussed in relation to practical heathland management and the restoration of heathland after succession. Specifically, it is more sensible and costeffective to restore heathland on sites where Pinus sylvestris successions have occurred, than where Betula spp. successions have occurred.

Summary1. The role of prescribed burning of vegetation to manage fire risk is controversial in a variety of situations worldwide. It is becoming more topical (i) as a result of potential global warming where the risk of wildfire might increase and (ii) because fire might affect the various ecosystem services provided in a different way. Where prescribed fire is used, ecologists need to know the impact on biodiversity (post-fire recovery) and on provisioning and regulating services such as water collection and carbon sequestration. Here, we assess the effect of prescribed burning on plant community composition and its component species at the regional scale of the Peak District, where the moorland vegetation is severely degraded. 2. Species cover (%) was assessed on five moors with respect to elapsed time since prescribed burning and vegetation height. A stratified random method was used to choose burn patches covering a range of ages since burning; quadrats were then sampled randomly within these patches over a 3-year period. Detrended correspondence analysis was used to relate species composition to significant environmental variables, and variation partitioning was used to assess their relative contribution. Response curves were produced for the major species with respect to elapsed time since burning and vegetation height. 3. The species ordination produced two gradients, (i) a continuum from a graminoid-dominated vegetation to one dominated by Erica tetralix, Vaccinium myrtillus and Rubus chamaemorus and (ii) a post-fire growth response of the dominant species, Calluna vulgaris. Species composition was more highly correlated with vegetation height than elapsed time since burning. The environmental variables explained 15AE2% of the variation. 4. Calluna vulgaris was the only species to show an increasing response after burning; all others showed an increase immediately after burning, but then they either decreased or showed a unimodal ⁄ skewed response. Most other species were restricted to vegetation <40 cm height and 20-25 years after burning. 5. Synthesis and applications. We found two major results of importance to policy makers and land managers: (i) that prescribed burning maintains species diversity in the immediate post-burn phase, and (ii) as the vegetation ages and increases in height, most species disappear and the vegetation becomes dominated by C. vulgaris. From a policy perspective, prescribed burning (or some other disturbance) is needed to maintain burning and a no-burn policy will result in a low-diversity, C. vulgaris-dominated vegetation. As vegetation height is the easiest measure for land managers to use in judging when to burn, we recommend moorland vegetation be burned before it reaches 25 cm in height to maintain the pre-burn complement of plant species. If the rotation allows the vegetation to become much taller (>40 cm), then most species will be lost and they will have to colonize after subsequent fires from the seedbank or from the surrounding area.

No abstract

Summary 1.Molinia encroachment has been viewed as a major threat to moorland conservation in the UK and elsewhere in Europe. In England and Wales agri-environment schemes are in place that aim to reduce Molinia caerulea and encourage the development of dwarf shrub vegetation. We tested a range of management treatments to achieve these objectives in two regions (the North Peaks and Yorkshire Dales) in England. 2. Within each region, the same experiment was carried out on two types of moorland vegetation, Molinia -dominated 'white' moorland and a mixture of Molinia and Calluna vulgaris 'grey' moorland. Burning, grazing and herbicide (glyphosate) treatments were applied in factorial combination at each of the four sites (two regions × two moor types). The responses of both vegetation and individual species were assessed. In addition, on the white moors two techniques for Calluna re-establishment were investigated, (i) removal of Molinia litter by raking and (ii) application of Calluna seed. 3. The data were analysed using a combination of univariate and multivariate analysis of variance to identify trends in this complex data set. 4. The only treatment that had consistent effects in the univariate analysis of variance was glyphosate application, which had similar effects on Molinia at all study sites. There was little difference between the use of low and high application rates (0·27 and 0·54 kg ai ha). There was little impact of herbicide use on other moorland species. Some species were affected on some sites in some years, but there were no consistent effects. Tentative identification of species that responded positively, negatively and erratically to glyphosate application was made. 5. Greater Calluna seedling densities were found in the plots where herbicide was applied, the Molinia litter was removed and seed was added. However, after initial colonization, there was a reduction in Calluna seedling densities as the Molinia recovered. This indicated that disturbance, seed addition and follow-up management are required for successful Calluna establishment. 6. There were significant differences in community response between both the regions and moorland types. The Dales had a relatively greater contribution of grassland species than the Peaks, where the grey site had a relatively greater dwarf shrub component. 7. Burning had little effect on community composition but both grazing and herbicide application had important effects. Grazing of the grey sites, even at the very low levels used in this study, tended to push the communities towards bog-moorland vegetation, but little effect was found at the white sites. Glyphosate treatment tended to push communities towards acidic grassland at the Dales grey site but not at the Peaks. Successional change was also noted, with marked change between the third and fourth year and again between the fifth and six year. Grey sites showed the greatest temporal change. 8. Synthesis and applications . In terms of Molinia control and subsequent restoration of dwarf shrubs, there was...