Potential impacts of the loss of Fraxinus excelsior (Oleaceae) due to ash dieback on woodland vegetation in Great Britain

Abstract

The non-native fungus Hymenoscyphus fraxineus which causes ash dieback is now established across much of Europe including the UK. The disease may potentially kill large numbers of Fraxinus excelsior (ash) trees in infected areas. Ash woods tend to be relatively rich in vascular plants and the composition of the flora might be expected to change if F. excelsior is lost and the environmental conditions (levels of shading, nutrient addition/recycling) change. We explore this possible scenario for the UK, using the floristic tables from the UK National Vegetation Classification (NVC) to identify for analysis eight ash-constant woodland types (where F. excelsior is a constant species) and four woodland types where F. excelsior is a frequent species. From these 12 communities, we identify 58 ground flora species that may be described as being ash woodland-associated species, including five species with some level of conservation protection. Changes in the ground flora are likely to be driven initially by increased light due to the opening up of the canopy as F. excelsior is lost, followed by increases in the shrub layer and eventually a closing of the canopy by other tree species. Using existing knowledge of plant species traits and habitat preferences (regeneration strategies, Ellenberg light values and Grime CSR scores) and community composition from NVC tables, we predict how the vascular plant community of ash-woodlands may change over time if F. excelsior is lost. We show that ash dieback could drive substantial changes in the ground flora community composition of currently ash-dominated woodlands.