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International audience; Tree microhabitats (cavities, conks of fungi, bark features) play an important role for both rare and specialized species biodiversity; their preservation should therefore be targeted by biodiversity-friendly forest practices. However, when compared to other old-growth characteristics like deadwood or large trees, tree microhabitats have only recently been studied so related scientific knowledge is still relatively limited. Defining target values for microhabitat densities in managed forests is mostly based on expert knowledge rather than quantitative empirical data. We compared the densities of microhabitat-bearing trees between managed forests, where wood is still harvested, and strictly protected forest reserves, where harvesting has been abandoned, in 17 French forests (222 plots) located in both lowlands and mountain regions. We found that microhabitat densities are generally higher in strict forest reserves than in managed forests and that this difference is mainly driven by standing dead and large living trees. Though scarce, standing dead trees over-contribute to the difference observed while large trees played a lesser but significant role. In addition, contrary to results obtained for other old-growth characteristics (such as deadwood volumes), the difference between managed and strict forest reserves was higher in mountain than in lowland forests. For individual microhabitats, five out of eleven microhabitats in mountains and only one in lowland (woodpecker cavities) were significantly more numerous in strict forest reserves than in managed forests. Finally, total microhabitat density and density of specific microhabitats such as cavities and bark features increased with time since the last harvest. This increase was also mainly supported by standing dead microhabitat-bearing trees. Compared to previous studies in comparable contexts, the densities we estimated were generally higher; however, such comparisons could only be made for the most documented microhabitat types. Our results support the idea that management abandonment favours the abundance and diversity of microhabitats. However, microhabitat dynamics remain poorly known and only long-term monitoring will help understand underlying mechanisms of recruitment. In the meantime, our results may inspire forest managers in their application of daily biodiversity-friendly practices.

International audience; Over the past centuries in Western Europe, the long history of forest management has shaped both landscape- and local-scale forest structures. In France, a network of strict forest reserves was created in the 1950’s to serve as a reference for nature conservation and forest dynamics. However, few studies to date have quantitatively compared the structural characteristics between managed stands and the reserves, where management has been (more or less recently) abandoned. In addition, the time needed for old-growth characteristics to restore themselves in strict forest reserves remains poorly known. We analysed the differences in stand characteristics between managed and unmanaged stands in 17 French forests (233 plots) located in both lowland and in mountainous regions. We quantitatively showed that overall basal area, very large trees and deadwood features were significantly higher in unmanaged forests, though this pattern was mainly found in the lowland forests while in mountainous regions, managed and unmanaged forests showed far fewer differences. In addition, most structural characteristics that we higher in unmanaged forests also increased with the time since last harvesting. Compared to the remaining old-growth forests elsewhere in Europe, the strict forest reserves in France often appear less mature, since they are still recovering from centuries of intensive management. Our results also show lower levels than those observed in other European studies on the restoration of old-growth characteristics over time; this indicates that it will take French reserves a very long time to reach values comparable to old-growth references in Europe. Our study constitutes one of the first references on unmanaged forests in France and may serve practitioners in their everyday management.

SUMMARYSustainable forest management aims to produce wood while preserving habitats for biodiversity, which is particularly challenging for vertebrates with local and landscape scale requirements, such as birds or bats. Managers need additional scientific evidence to help them balance conservative and integrative management methods. In this study, the relative influence of management abandonment, stand structure and landscape features on bird and bat communities in 14 managed and unmanaged forests in France is evaluated. Total birds and bats richness, richness for forest and threatened birds and edge-specialized bats significantly increased with total deadwood quantities. Richness of generalist, omnivorous and cavity-nesting birds was higher in unmanaged stands and richness of gleaner bats was positively influenced by the density of standing deadwood. By contrast, landscape variables had little influence on the different ecological groups but did have effects on individual species. Though the effects showed relatively limited magnitude, this study supports the value of deadwood and the importance of management abandonment for forest vertebrates. This study confirms that for integrated conservation strategies to be fully efficient they must be complemented by designating strict forest reserves because some target species groups depend on structural features found only at sufficient levels in those areas.