Beyond the fragmentation debate in forest planning: how do habitat amount and spatial arrangement matter for saproxylic beetle diversity?

Abstract

In managed forests, intensive silvicultural practices reduce the density/diversity of deadwood and tree microhabitats at the forest stand scale. This negatively affects biodiversity, especially saproxylic beetles which dependent upon these old-growth attributes. At the landscape scale, forest management plans lead to a spatial heterogeneity of these attributes which can be perceived as a source of fragmentation by many saproxylic species. However, the influence of saproxylic resources distribution at large scales received little attention to date. More particularly, the relative importance of quantity vs. fragmentation per se (spatial configuration of resources independently from quantity) on biodiversity is currently a controversial issue.

In this study, we addressed this latter question within a temperate hardwood forest located in the north of France. Our aim was to evaluate how the quantity and/or configuration of saproxylic resources considered at different spatial scales, could affect local α-diversity of saproxylic beetles with focus on the response of several ecological guilds.

First, we calibrated predictive models of saproxylic resource abundance (cavities, polypores and deadwood) based on field data and using forest stand characteristics as predictors (e.g. stand age, species trees composition). From this spatial distribution mapping of resources across the whole forest, we selected 8 landscape windows (500 ha) characterized by a similar amount but contrasting configuration of resources. In each of these landscape windows, we set up 6 sampling 1-ha plots consisted of two flight-interception traps. In total over 13,000 individuals were captured during 4 months representing 353 species. We used generalized linear mixed models to assess the effect of amount/configuration of cavities, polypores and deadwood at three spatial scales (1-ha plot, 26-ha buffer and 500-ha window) on the α-diversity of three ecological species guilds (cavicolous, fungicolous and lignicolous beetles, respectively).

We found that species richness of all ecological guilds did not significantly respond neither to the quantity nor the configuration at any spatial scale. In contrast, the abundance of cavicolous and fungicolous species responded positively to amount of cavities and polypores, respectively but only at 26-ha buffer scale. These results emphasis that amount of saproxylic resources rather than configuration seems to be a key factor for maintaining diversity at fine scale, in particular by increasing the abundance of individuals - i.e. population sizes - for several ecological guilds. Further analyses are carrying out to evaluate the relative contribution of α- and β-diversity to the regional γ-diversity, and thus provide important information to identify efficient conservation strategies for saproxylic biodiversity in managed forest.