Dead wood (DW) provides a critical habitat for thousands of wood-dependent (saproxylic) species in forests. However, intensification of forest management has heavily reduced the amount and diversity of DW. This has resulted in many saproxylic species being threatened and has caused a situation where interventions aiming at increasing DW might be necessary to support its associated biodiversity. Examples of such interventions include felling, girdling, creation of high stumps, leaving of crowns, logs and trees during harvest operations, and restoration burnings. Although the evidence base on how effective different interventions aiming at increasing DW volumes grows, there is a lack of reviews on the topic. We have therefore conducted a full systematic review with meta-analysis, where we have synthesized the current state of knowledge drawn from replicated experimental studies into solid quantitative evidence of the effects of DW manipulation on forest biodiversity.
Our review included three interventions, all compared against sites with no intervention: creation of DW, addition of DW, and prescribed burning; we used studies conducted in set-asides and production forests in boreal or temperate regions. Relevant outcomes included abundance and richness of saproxylic insects, beetles, ground insects, wood-inhabiting fungi, and cavity-nesting birds. Studies were mainly selected from a recent systematic map1 but additional studies were identified through updated searches. 91 studies were included in our systematic review, 58 conducted in northern Europe, a majority in coniferous forests and the dominant species group was beetles. A subset of stand-scale studies (37) were used in meta-analysis and, although this evidence base was heterogeneous, it revealed that DW amount had a significant positive effect on the abundance and richness of saproxylic insects and fungi. No negative effects were found for any of the species groups. All three intervention methods revealed a positive effect on saproxylic insect abundance, burning showing a tendency for the strongest effect; this was despite the fact that DW volume was, in general, twice as high where DW had been added or created, as compared to burning. Weighted meta-regression revealed only one significant covariate: the positive response of saproxylic insect richness to DW enrichment decreased with increasing amounts of DW, indicating a log(Species)~log(DW) relationship similar to the Species Area Relationship.
Overall, our results support that it is beneficial to create DW, regardless of method, to increase biodiversity for DW-dependent species. However, there is a need for more long-term studies, studies in more regions (e.g. Russia and Asia) and more species groups (e.g. lichens, vascular plants and bryophytes).
1Bernes C, Jonsson B, et al. What is the impact of active management on biodiversity in forests set aside for conservation or restoration? A systematic map. Environmental Evidence. 2015;4:25.