High tree host diversity buffers the effect of Fraxinus excelsior dieback on epiphytic bryophytes composition in the Białowieża Primeval Forest

Invasions are one of main drivers transforming the functions of forest ecosystems. The invasion of alien fungus Hymenoscyphus fraxineus is still reducing the abundance of Fraxinus excelsior throughout temperate Europe. F. excelsior is a tree species belonging to the group of foundation species for numerous epiphytic species. We studied the effects of F. excelsior decline on epiphytic bryophytes in the Białowieża Primeval Forest. In this forest human interference is limited, allowing us to register the natural dynamics of ash-dependent bryophyte communities. F. excelsior decline was discovered in the Białowieża Primeval Forest in 1998, and in 2016 we resurveyed a historical survey of epiphytic bryophytes, i.e. shortly before the dieback process started. Using ordination methods and mixed-effect models we assessed shifts in epiphyte bryophytes composition over time and amongst the plots with (i) historical and recent presence of F. excelsior, (ii) with recent extinction of F. excelsior, and (iii) absence of F. excelsior both historically and recently, as well as at the level of alternative tree hosts employing the paired Mann-Whitney and t-tests. F. excelsior dieback did not influence the species composition of bryophytes associated with this tree host. Despite the drastic reduction in living F. excelsior trees (85%), overall the species composition, species richness and Shannon index of F. excelsior-dwelling bryophytes did not shift significantly between two sampling periods. Similarly weak changes over time we reported for the bryophytes’ community weighted means of ecological indicator values. Equally subtle temporal shifts in epiphytes’ biodiversity were observed amongst the plots with the presence, absence, and extinction of ash, likely due to the relatively high diversity of available alternative hosts. F. excelsior-associated epiphytic bryophytes were able to exploit other niches in the microhabitat-heterogeneous Białowieża Forest ecosystem, and thus far have not suffered a reduction in biodiversity parameters at the scale of our survey. High diversity of alternative host tree species, with particular emphasis on the occurrence of pioneer trees (i.e. B. pendula and P. tremula), may maintain the epiphytic bryophyte communities, which themselves may be able to act as a source for the recovery of F. excelsior-affiliated epiphyte populations.