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

Abstract

AbstractInvasive fungal pathogens are an increasing problem globally and can cause strong 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. At the sample plot scale, species richness (α–diversity) of the upper tree layer strongly decreased from 1989 to 2021, and the mean cover of elm decreased from 27 to 1% and of ash from 29 to 13%. In the lower tree and shrub layers, elm and ash were replaced by other, mainly shade-tolerant, tree species. The cover and richness of the shrub layer increased in previously elm-dominated stands but not in ash-dominated stands. The extensive loss of canopy cover in elm stands caused a larger change in upper tree layer 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 toward hornbeam or small-leaved lime, and wetter plots turned toward 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.