Vegetation responses to pathogen-induced tree loss – Swedish elm and ash forests revisited after 32 years

Abstract

Abstract Invasive fungal pathogens are an increasing problem globally and can cause devastating effects on forest ecosystems. In this study we contrast vegetation surveys in eutrophic elm (Ulmus glabra) and ash (Fraxinus excelsior) forests in southern Sweden, conducted just prior to the arrival of Dutch elm disease (DED) in 1989, and then again in 2021, several years after ash dieback (ADB) began. Mean cover and species richness (α-diversity) of the upper tree layer strongly decreased from 1989 to 2021. In the lower tree layer, loss of elm and ash was compensated by an increase in other tree species. The cover and richness of the shrub layer increased in previously elm-dominated stands but not in ash-dominated stands. Canopy recovery was mainly dominated by shade-tolerant tree species which, especially in the previously ash-dominated stands, implies a successional shift. The extensive loss of canopy cover in elm stands caused a larger change in species composition and increased compositional variability (β-diversity) between plots when compared to the ash stands. The direction of the changes in tree layer composition between the surveys varied with soil moisture and nutrient availability. While beech increased in less eutrophic plots, more nutrient rich plots changed towards hornbeam or small-leaved lime, and wetter plots turned towards alder and bird cherry. Hence, our results indicate increased compositional diversity and alternative successional pathways for community reorganization following DED and ADB. Future research will reveal if these pathways will later merge or further split.