Beetle Diversity – Land Sparing or Land Sharing? : the effects of timber extraction and restoration method on saproxylic beetle assemblages in mature managed pine forests

Abstract: The impoverishment of saproxylic beetle communities is a growing concern for management of boreal forest biodiversity. Past research has found active restoration methods, such as creating deadwood and conducting prescribed burnings, to have a short-term positive effect on the species richness and compositional diversity of saproxylic beetle assemblages. However, comparatively little work has been done on the effect of combining these with timber harvests. To efficiently allocate conservation efforts, the relative merits of combining or separating management for these two goals must be understood. Efficiently combining the two could help offset opportunity costs, make restoration efforts self-financing, or reduce goal conflicts between landowners and regulatory agencies. This is conceptualised according to a land-sparing/land-sharing model; wherein land-sparing entails management for biodiversity and timber production on separate lands, and land-sharing combines management for these goals across forest lands. The aim of this study is to examine how the diversity and composition of post-restoration saproxylic beetle assemblages is affected by retention level and choice of restoration method. Twelve stands were examined, each exposed to one of four treatments: prescribed burning with 100% or 50% retention level, and deadwood creation with 100% or 50% retention level. Trunk emergence traps were placed on deadwood in every stand. Treatment, i.e. restoration method and retention level, was found to have a significant effect on both saproxylic beetle abundance (p = 0.003) and species richness (p = 0.03). However, the direct effect of treatment on the underlying population could not always be separated from the effect of sampling intensity and no effect exclusively dependent on restoration method or retention level could be isolated. Treatment was found to have a significant effect on the composition of saproxylic assemblages (p = 0.03), albeit with low explanatory power (R2 = 0.35). Although treatments that included burning and/or partial retention tended to have similar assemblage composition, prescribed burning with 100% retention level maintained higher variation between replicates and more unique species than any other treatment. This study concludes that, although land-sharing strategies might be situationally advantageous, large-scale implementation is unlikely to act as an effective substitute for the complex and highly variable ecological legacies of mimicking natural disturbance regimes and burning untouched stands.