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Résumé :
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Among global changes, intensification of forest management is a major issue at the beginning of the 21st century e.g. for bio-fuel production. Past and present pressures on forest resources have led to a drastic decrease in the surface area of unmanaged forests in Europe: they currently represent less than 1% of the total forest area (vs. 13% on the west coast of the United States and 40-52% in Canada). Modifications in forest structure, composition and dynamics inevitably lead to changes in the biodiversity of forest dwelling species. However, the possible gains and losses in biodiversity due to forest management have never been assessed at a pan-European scale. In the current study, carried out within the framework of ALTER-Net, we hypothesized that biodiversity was higher in unmanaged forests although we expected contrasted responses for different taxa and ecological groups. We also identified the gaps in the knowledge concerning the response of biodiversity to forest management in Europe. We used a meta-analytical approach to do this. Meta-analysis is a quantitative review of the literature that combines the results of several independent studies examining the same question. Meta-analysis output accounts for the fact that all the studies are not equally reliable. In this study, we used species richness as a simple, though imperfect, index of biodiversity. The term "forest management" encompassed any anthropogenic pressures related to direct forest resource use (clearfelling, selective felling, any form of tree retention, grazing, planting or drainage). We analysed 51 published papers containing 122 individual comparisons of species richness between unmanaged and managed forests throughout Europe. Species richness was slightly higher in unmanaged than in managed forests but the difference was only marginally significant. However, when we divided the studies according to taxonomic and/or ecological groups, we found both contrasted and more significant responses: (i) bryophytes, lichens, fungi and saproxylic beetles, which mainly depend on deadwood and/or large trees, were penalised by forest management; (ii) on the contrary, vascular plants were favoured by forest management; (iii) the response for carabids or birds was unclear and probably depends on other factors not included in our analysis (e.g. landscape patterns). This study also highlighted the need for more research, especially in temperate and Mediterranean regions and/or for some groups such as mammals or soil invertebrates. To our knowledge, this study has been the first to compare biodiversity in managed and unmanaged European forests using meta-analysis. The conclusions of this study support an active conservation policy, creating new unmanaged forest reserves and encouraging management methods that mimic natural forest dynamics and structures. These actions would contribute to achieving the 2010 biodiversity target. Moreover, we suggest the creation of a coordinated European research network to study and monitor biodiversity of different taxa in managed and unmanaged forests.
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