European Ash (Fraxinus excelsior) as a Functional Keystone Species Shaping Woodland Flora in the UK

Ash dieback disease, caused by the pathogenic fungus Hymenoscyphus fraxineus, is now widespread in the UK. The pathogen has caused substantial loss of European ash over recent decades, the long-term consequence of which is complex to assess. While no higher plant species are exclusively associated with ash, widespread ash mortality is predicted to affect woodland floral biodiversity because of the functional traits uniquely associated with this species: namely the greater light penetration through the canopy, rapid foliar decomposition and nutrient cycling abilities. In woodlands where ash is frequent, vascular plant communities may undergo considerable compositional changes after loss of ash trees. At present little is known regarding how ash trees contribute to plant community and functional diversity at a fine scale across different woodland types. Using data from 1274 survey plots characterized in the Woodland Survey of Great Britain alongside plant functional trait data, we examined the local relationship between ash trees and plant community diversity and functional trait distribution. We show that ash trees are significantly associated with key plant functional traits and diversity indices and that this functional association is not typical of other dominant canopy species in the UK. Specific leaf area, nutrient and pH requirements and community diversity show significant correlations with ash basal area across various woodland types. Additionally, effects of ash appear to interact with soil pH resulting in a greater structural effect of ash upon plant community composition in lower pH soils. These findings support previous suggestions that ash functions as a keystone species with respect to nutrient cycling and plant community composition with potentially stronger influence on lower pH soils. Consideration should be given to these ecological roles when monitoring and addressing the impacts of widespread ash mortality from dieback disease across Europe.