Local dispersal sources strongly affect colonization patterns of wood-decaying fungi on spruce logs

Coarse woody debris (CWD) is an important habitat for many species in forest ecosystems. However, forestry has decreased the abundance of CWD so that many wood-dependent species have become threatened. To alleviate this problem, guidelines for a more biodiversity-oriented forestry focus on increasing CWD in managed forests. Unfortunately, how this increase is to be allocated on a landscape scale is not well understood. The present study reports an experiment in which freshly cut logs of varying sizes were placed in stands with contrasting abundance of natural CWD and subsequently varying pools of wood-inhabiting species. The first six years of colonization by wood fungi show that local abundance and composition of the fungal flora strongly influenced colonization. Higher species richness was observed in CWD-rich sites, and several species were more frequent on the experimental logs at CWD-rich sites. The strong within-site effect is interpreted as resulting from high spore deposition from the local species pool. This is supported by spore deposition estimates of Fomitopsis rosea, a red-listed species that only occurred on experimental logs at the CWD-rich sites. F. rosea had a 9-180 times higher spore deposition at the CWD-rich sites compared to the CWD-poor sites. The species richness and composition on small logs differed from that of large logs with higher richness on the latter. The results strongly suggest that restoration efforts would be more efficient if directed toward sites close to CWD-rich sites and that preferably large logs should be created.