Deadwood volumes matter in epixylic bryophyte conservation, but precipitation limits the establishment of substrate-specific communities

Abstract

Deadwood is a habitat for numerous biota and serves as an indicator of forest biodiversity. Bryophytes significantly contribute to deadwood communities and epixylic bryophytes are particularly threatened. Additional to deadwood volumes, their occurrence depends on climatic conditions. However, the interactive effects of deadwood and climate on epixylic bryophytes have hardly been assessed. Here, we analyse these effects based on 8 143 bryophyte specimens collected on 510 logs in 51 Austrian forests. We found that annual precipitation sum and deadwood volume explained 67%, 90%, and 82% of the variation in moss, liverwort and total species richness, respectively. Segmented regression indicated several breakpoints in these relationships, especially at c. 60 m3/ha of deadwood and 900 mm, respectively 1700 mm of annual precipitation. Epixylic bryophyte diversity increased strongly up to the deadwood threshold, but only moderately with still higher volumes. Nine hundred mm of annual precipitation was a macroclimatic limit for deadwood-specific bryophyte species. Below this value, obligate epixylic bryophytes were lost and liverworts were rare, even at sites with high deadwood amounts. At the wettest sites, the overall number of species decreased while deadwood-specific specialists further increased in number. The close tie of epixylic bryophytes to macroclimate may constrain the efficiency of deadwood management measures and suggests considerable impacts of climate change. A drier and warmer climate will probably decrease species richness and change the composition of epixylic bryophyte communities, with the most characteristic species, especially liverworts, facing the highest risk.