Impacts of salvage logging to community assembly: Evidence for stochastic extinctions from functional space
The amount of forest affected by stand-replacing natural disturbances has increased over recent decades and it is expected to increase further. The practice of removing the resulting dead or injured trees (i.e. post-disturbance logging) has become increasingly controversial because of its impacts on biodiversity. Taxa most seriously affected by post-disturbance logging are those dependent on dead-wood legacies – particularly saproxylic beetles, wood-inhabiting fungi and epixylic lichens and mosses. According to the species-energy-hypothesis the loss of saproxylic taxa due to post-disturbance logging may be explained by the loss of specific dead-wood resources. However, if a resource is reduced, species may disappear – based on theory – either due to limiting similarity as competition among scarce resources has increased or due to environmental filtering towards a specific type of remaining resources. Empirical evidence for which of these assembly mechanisms determine local species richness after post-disturbance logging is scarce. We conducted species surveys of vascular plants, bryophytes, lichens, birds, saproxylic beetles and wood-inhabiting fungi in experimentally logged storm-felled coniferous forest stands in Central Europe. We used species ecological traits combined with phylogenetic trees to estimate the impact of post-disturbance logging to standardized effect sizes of mean pairwise functional-phylogenetic distance.
All saproxylic taxa except epixylic bryophytes tended to display significantly higher species richness on unlogged plots. In contrast, we could not detect any significant response in species richness of non-saproxylic taxa to post-disturbance logging. However, non-metric multi-dimensional scaling revealed distinct responses in community composition of both saproxylic and non-saproxylic taxa. Communities of saproxylic beetles, epixylic lichens and epixylic bryophytes displayed a clustered pattern on unlogged plots, indicating environmental filtering. Communities of wood-inhabiting fungi displayed an over dispersed pattern on unlogged plots, indicating limiting similarity. All saproxylic groups were altered towards more random assembly mechanisms either due to displaying decreasing competition or decreasing environmental filtering on logged plots. Furthermore, post-disturbance logging increased environmental filtering to communities of breeding birds and epigeal bryophytes but no significant impacts were found for epigeal lichens and vascular plants. Our results suggest that post-disturbance logging alters natural processes of community assembly in early successional stages of natural disturbed forest stands, even of species groups which do not directly depend on deadwood resources. The direction of shifts in assembly mechanisms of deadwood related species groups may reflect stochastic extinctions from functional-phylogenetic space.