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

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.

Understory changes in mixed elm stands in response to canopy dieback in Latvia

“…The abundance of AG, however, was related to the mid-part of the gradient, implying that moderate conditions are needed for successful regeneration of canopy species (Pušpure et al 2016). The explicit advance of UG, in turn, appeared related to their plasticity regarding light conditions and moisture (Huenneke 1983, Brunet et al 2023, as highlighted by the ordination (Figure 3).…”

“…Irrespectively of geographic location, half of the stands remained relatively stable in terms of canopy and understory and showed AG and canopy recruitment of elms (Table 1, Figure 3, Supplementary 2, 3), cherishing some hopes for the continuity of elm admixed stands (Jürisoo et al 2019, Brunet et al 2023. The comparable height of AG and UG suggested balanced competition (Pušpure et al 2016, Hytteborn et al 2017, Gleason et al 2018, likely as AG appeared able to benefit from the surplus light provided by the canopy (shelterwood) opening (Figure 3).…”

“…As the studied stands suffered different degree of dieback of canopy trees resulting in canopy opening, light was estimated as the main environmental driver of the understory (Table 3). Complementary to light availability, canopy openings affect microclimate (Brunet et al 2023), particularly the moisture regime in the studied stands, which was estimated as the second principal gradient (Table 3). The mortality of canopy elms and the state of AG differed among the stands (Figures 2, 3; Supplementary 2), thus indicating the effects of local/stand conditions along the identified gradients, particularly considering the occurrence near the periphery of the elms' range (Solheim et al 2011, Jürisoo et al 2019, Czeszczewik et al 2022.…”

“…In half of the stands, the understory has undergone explicit compositional changes, particularly as the stand microclimate has been altered (Brunet et al 2023). The AG preferred specific (moist, medium light and temperature) environmental conditions (Figure 3B, Table 3), which are characteristic in stands after partial canopy dieback (Brunet et al 2014), while the UG species rapidly occupy dieback-induced canopy gaps (Ehrenfeld 1980, Huenneke 1983.…”

“…Previously, gaps in elm stands created by DED and other disturbances have been recolonized by ash, but currently both species are being restricted to understory with low to moderate cover (Peterken and Mountford 1998, Brunet et al 2014, Brunet et al 2023). Accordingly, P. avium and C. avellana, and neighbouring canopy species rapidly colonized openings (Supplementary 2) facilitating the formation of semi-stable shrubland phase (Huenneke 1983, Hytteborn et al 2017, Nestby 2020.…”