Response of ground vegetation and epiphyte diversity to natural age dynamics in a Central European mountain spruce forest

Question Natural forest age dynamics is often more or less cyclic, with profound temporal changes in stem density and tree size, tree age structure, deadwood frequency and the abundance of canopy gaps. We investigated the response of ground and epiphyte vegetation to the natural forest age dynamics of an old-growth spruce forest focussing on (1) the influence of stand age-related shifts in forest structure and related changes in soil conditions on the diversity and composition of plant communities, (2) differences in the species turnover of cryptogamic epiphytes and ground vegetation in relation to forest age development, and (3) the importance of later (advanced) forest development stages for characteristic epiphyte communities. Location High-montane old-growth spruce forest (Picea abies (L.) Karst.) on Mt. Brocken, Harz Mountains, Germany. Methods Five defined forest development stages (regeneration to decay) were investigated with five 100-m2 plots for each stage, in which we studied forest structure, ground vegetation and the epiphytes of living trees and dead trunks. Results The ground vegetation did not significantly change across the forest development stages. Epiphyte diversity on dead standing trees markedly increased towards later stages, with the highest diversity in the over-mature and decay stages. Diversity of epiphytes on lying trunks was highest in early development stages. Trees in decay as well as regeneration stage included a set of characteristic epiphytes, being rare or absent in the other stages. Conclusions Deadwood, i.e. trees and lying trunks and their epiphytes, outlast the oldest individuals of the tree layer and are still present in the early stages of the following forest generation. Epiphyte diversity on lying deadwood was higher in young than late forest development stages. Habitat continuity presents a fundamental difference to managed forests; it promotes species with dispersal limitations, which are common among cryptogamic epiphytes.