Abstract
Abstract Global change scenarios project drastic modifications in tree species range and an increase in exotic tree plantations. Subsequent tree species substitutions may alter habitat conditions for biodiversity. We measured substitutability between tree species for early deadwood colonisers, through a sentinel log approach, i.e. through the experimental exposure of paired down deadwood (DDW) pieces to native beetles in native stands. We compared two native/substitute tree species pairs: one conifer pair composed of a rapidly expanding exotic species (Douglas fir) and a declining native species (silver fir), and one deciduous pair composed of two native species, one expanding (sessile oak) and one retreating (beech) at the regional scale. The effects of expanding exotic and native trees on beetle communities were not in line with expectations. Species assemblages in Douglas fir DDW were indistinguishable from those in native silver fir DDW and did not contain fewer species. Assemblages were not more generalist on average in substitutes than in substituted trees: we did not note any decrease in species richness of functional groups to the detriment of specialist species. Moreover, species richness and abundance were higher in substitute oak than in native beech DDW, confirming that species from the regional pool were able to colonise oak, even though it is a minor tree species at the regional level. Large‐scale monitoring schemes including multi‐taxon, multi‐year and multiple native/substitute pairs would further our knowledge of the generic effects of tree species substitution on biodiversity and ecosystem functioning.
Highlights
Climate change is expected to greatly affect species distributions in the near future (Thuiller et al, 2005)
Mean numbers of saproxylic beetle individuals and species per bag were similar for both conifers, but significantly higher for oak than for beech (Table 1)
Douglas fir deadwood held more total species over the sampling design, in line with the Biotic Resistance Hypothesis suggested for herbivores (Lombardero et al, 2012)
Summary
Climate change is expected to greatly affect species distributions in the near future (Thuiller et al, 2005). Atlas cedar (Cedrus atlantica) have increasingly been promoted in western European forestry in place of native conifers (Schmid et al, 2014) because of their resistance to drought, their yield and their wood quality. These changes in native tree species’ ranges due to both the influence of climate change and the forestry-induced expansion of exotic tree species are likely to alter habitat conditions for forest biodiversity (Felton et al, 2013), example through the tree species composition of dead wood available for saproxylic biodiversity (Ulyshen et al, 2018). Host tree specificity among saproxylic species is not so narrow: for instance, 75% of French saproxylic beetle species for which data are readily available, are not specific to a single host species but are known to use at least two tree species (Bouget et al, 2019)!
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