Retention forestry and prescribed burning result in functionally different saproxylic beetle assemblages than clear-cutting

In fire-susceptible boreal forests, clear-cutting has been justified as being a harvesting regime that mimics natural stand-replacing dynamics, despite obvious differences in its biological legacies. Prescribed burning and retention forestry are commonly applied to better maintain naturally occurring legacies, but the community-level effects of different disturbances on the functional characteristics of biota remains still largely unknown. In this study, we investigated the effects of prescribed fire, clear-cutting and retention forestry on the functional properties of saproxylic beetle assemblages in Eastern Finland, using stand-level data from a before-after field experiment with four levels of tree retention (0, 10 and 50 m3 ha−1 and control) and prescribed burning. We analyzed the functional-phylogenetic diversity and a set of species traits that link species to resources. The data include 377 beetle species and 38 549 individuals. Functional-phylogenetic diversity decreased from a random to a clustered pattern after burning and logging with retention trees, indicating environmental filtering of both processes. These effects became more pronounced with increasing logging intensity. Species-level traits that were favored by burning and tree retention were connected to open-habitat conditions and fresh dead wood, whereas clear-cutting revealed a random pattern without reference to specific resources. Our functional approach thus shows that clear-cutting does not mimic the dynamics of wildfire, but leads to different functional composition of species assemblages. Therefore, prescribed burning or wildfire should be incorporated and sufficient amount of trees retained in forest management to conserve functional processes and natural composition of saproxylic species assemblages in boreal forests.