Peter Lawrence

The epidemiology and severity of ash dieback (ADB), the disease caused by the ascomycete fungus Hymenoscyphus fraxineus , has been linked to a variety of site conditions; however, there has been a lack of analysis at an individual tree scale. Symptoms of ADB were scored on ca. 400 trees of Fraxinus excelsior (ash) in permanent sample plots during two successive years in a UK natural woodland reserve. Using comprehensive plot records maintained since 1945, and detailed spatial records updated since 1977, we assembled an array of potential explanatory variables, including site environment factors, ash tree density, previous and present tree condition and near neighbourhood summary statistics (NNSS), such as species mingling and size dominance. Their impact on the severity of ADB of focal ash trees was tested with generalised linear mixed effects models (GLMM). The severity of ADB was much greater in the lower slope parts of the site with moister soils and least in a managed area subject to tree thinning in the previous 35 years. Severity of ADB had a negative association with focal ash tree prior relative growth rate over a period of a decade immediately before the disease was detected at the site. Greater ADB severity was also significantly associated with smaller diameter at breast height of ash trees. Additionally, ADB was significantly positively associated with a greater proportion of heterospecific trees amongst the six nearest neighbours of the focal tree. Synthesis. The relationship of the severity of ADB disease with site environment, tree condition and neighbourhood is complex but nevertheless important in the progression of the disease. The findings suggest some silvicultural interventions, such as thinning to increase the vigour of retained ash trees, might reduce the impact of ADB.

Spatial heterogeneity of species (beta diversity) is an important attribute of ecological communities, but is less frequently considered when assessing restoration success than other aspects of diversity (gamma and alpha). Differences in beta diversity between restored and natural sites may arise due to differences in environmental heterogeneity. We used a nested sampling design to survey plant communities and environmental conditions (elevation, redox potential and metrics of topography) on four pairs of restored and natural saltmarshes. We assessed whether there were differences in both alpha and beta diversity between natural and restored sites and analysed their environmental drivers. Topography was an important driver of plant alpha diversity and beta diversity on saltmarshes. The effects of topography were partly indirect, mediated though changes in redox potential, but topography also influenced plant communities independently of both elevation and redox. Restored saltmarshes were less heterogeneous in topography than natural marshes. This reduced topography was reflected in lower beta diversity; plant communities 1 m apart in natural marshes were as dissimilar as those found 20 m apart in restored marshes. Large‐scale topographic manipulation carried out at one site a decade after initial restoration successfully increased topographic heterogeneity and increased beta diversity when surveyed 3 years after manipulation. These changes were still evident when resurveyed after a further 2 years. Synthesis and applications . Increasing environmental heterogeneity can improve restoration outcomes by increasing beta diversity on restored sites. The effect of environmental heterogeneity is likely to be particularly strong within intertidal habitats such as saltmarshes, where small changes in topography can determine whether a species can occur at a given location. Topographic manipulation is a feasible post‐restoration technique that can be applied to ensure restored saltmarshes better meet policy targets of biological, physical and functional equivalence with natural marshes.