Long-term experimental management in Swedish mixed oak-rich forests has a positive effect on saproxylic beetles after 10\xa0years

Oskar Gran,Frank Götmark

doi:10.1007/s10531-019-01736-5

Abstract

Secondary succession in protected oak-rich temperate forests reduces variation in habitats and leads to denser, shadier sites. Long-term experimental studies of the effects of conservation management alternatives are needed for such forests. Here we present a rare follow-up study of the response of beetles (a highly diverse taxon with many red listed species) to conservation thinning, an action that could favour biodiversity. We previously harvested about 25% of the tree and shrub basal area in a treatment plot, and no trees and shrubs in a nearby matched minimal intervention plot, in each of eight oak-rich (Quercus robur and/or petrea) forest reserves. After two seasons, thinning had led to an increased number of species of both herbivorous and saproxylic beetles. In the present study, we examined the 10-year response of the beetle groups at the same sites. For herbivorous beetles, the initial positive effect of thinning on the number of species had disappeared after 10 years, presumably because of regrowth. In contrast, saproxylic beetles showed a further positive response after 10 years, increasing in the number of species by a third compared to before thinning. We found no change in species composition of either group due to the thinning, but many saproxylics were unique to thinning plots. Overall, our results suggest that in mixed oak-rich forests, saproxylic beetles seem to benefit from conservation-oriented thinning for at least 10 years.

Highlights

Introduction and objectivesGlobally, forests protected for biodiversity and related conservation values have increased; ‘primary forest’ decreased, the total global protected forest area increased from 7.7% in 1990 to 16.3% in 2015 (Morales-Hidalgo et al 2015)
149 species were classified as oak saproxylics, 50 as herbivores, and 23 as red listed saproxylics
The effect of conservation thinning on oak saproxylic beetle species numbers In our previous study, we found a short-term positive effect of thinning on oak saproxylics (Franc and Götmark 2008), which may be caused by increased openness

Summary

Introduction

Forests protected for biodiversity and related conservation values have increased; ‘primary forest’ decreased, the total global protected forest area increased from 7.7% in 1990 to 16.3% in 2015 (Morales-Hidalgo et al 2015). The scientific and management communities pay little attention to research within ‘strict’ forest reserves, even though they are potentially very useful for long-term research (see Götmark et al 2015). Decisions on habitat management are usually taken by reserve staff based on own experiences (Pullin et al 2004; Cook et al 2010) or on general advice from researchers; long-term, controlled experiments are needed to clarify potentially misleading short-term effects (Tilman 1987) and the management options available (Davies et al 2008; Götmark 2013). With long-term results, the advice from research will be more reliable—this is especially true for forests, which develop slowly

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Results

Discussion

Conclusion