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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; Riparian habitats are transitional zones where strong environmental gradients shape community. To prevent flood risks and channel migration on managed rivers, civil engineering techniques have been widely used. Recently, ecological restoration of rivers has become a major issue. However, given the alteration of natural disturbance regimes induced by human infrastructures, the real added-value of these restoration actions is questionable. Thus, a major challenge is to better understand whether changes in abiotic conditions induced by human activity influence the response of plant communities to environmental gradients. Studying a highly degraded large river, we evaluated the effect of the elevation and soil texture gradients on plant functional composition and assessed whether human-mediated environment gradients, achieved through maintenance and restoration measures, shape community structure. In the summer of 2017, we sampled 17 geomorphic surfaces, mostly gravel bars, along the Rhône River and its tributaries that were either repeatedly cleared (brush clearing vs plowing), newly reprofiled or naturally rejuvenated by high flows. The results show shifts in trait values with elevation and convergence in plant traits with increasing proportion of fine sediments. The co-occurrence of species with contrasting traits was higher in highly disturbed environments, revealing the importance of rejuvenation processes. However, the influence of both environmental gradients was mediated by human activity. For maintenance measures, plowing was better able to promote species diversity and limited biotic homogenization along environmental gradients. Among the three geomorphic surfaces, naturally rejuvenated bars were the most stressful environments, hosting distinct functional assemblages, while communities on newly reprofiled banks were in the same ecological trajectories as repeatedly cleared bars. To promote an effective ecological restoration of riparian zones, (i) a greater variability of the minimum flow is needed, (ii) bedload transport restoration should be a priority and (iii) reprofiled banks should better mimic the landforms of natural river margins.

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.

Extending the network of strict forest reserves is one of the conservation measures promoted by the French National Strategy for Biodiversity improvement. According to the scientific literature, strict forest reserves may help preserving a part of the biodiversity that is threatened by forestry. However, this strategy is based on poor knowledge in the temperate context and available studies may suffer from methodological shortcomings. The national-scale project named "Forest management, Naturalness and Biodiversity" aims at quantifying the effects on forest structure and biodiversity of management abandonment in strict reserves. Based on a meta-analysis of worldwide literature and original data from 213 study plots set up in 15 forest sites throughout France - around half of the plots in forests left unmanaged for from 8 to 148 years (mean: 46 years)- , we analyzed the response of 7 taxonomic groups to management abandonment. The meta-analysis demonstrates that forestry affects total richness of saproxylic taxa worldwide, which is verified in our national dataset. However, management abandonment per se is not always the best explanation of the differences between managed and unmanaged forests, but other variables, notably linked to deadwood, better explain the observed patterns for saproxylic groups. For other taxa, the response is weaker and depends more on structural features than on management abandonment. In terms of policy, our project has allowed methodological advances thanks to the development of inventory and remote sensing protocols, as well as statistical methods. The dataset we have gathered is also a first comparison of structure and biodiversity between strict forest reserves and managed forests for France. This network may therefore constitute a first basis for long term biodiversity monitoring in French forests.

International audience; Forest biodiversity assessment relies mostly on indirect indicators based on elements of forest structure used as a surrogate for species. However, most of those indirect indicators used in national and international evaluation processes lack scientific evidence documenting their quantitative link with the biodiversity they are supposed to assess. In addition, those for which the link is established generally concern only one taxon and are rarely considered at the community level. In this context, tree microhabitats – defined as tree-borne singularities such as cavities, conks of fungi or bark characteristics – have recently gained attention as a potential forest biodiversity indicator for species that depend on them for at least a part of their life cycle and that are difficult to detect. However, like most other potential indicators, precise quantitative information on the link between biodiversity indices and microhabitat variables remains scarce. We explored the link between the richness of three taxonomic groups (bats, birds, and saproxylic beetles) and microhabitat indices using a nation-wide multi-taxonomic sampling design comparing 213 plots located inside and outside forest reserves. We hypothesized that the positive effect of setting aside forest reserves on biodiversity conservation was indirectly due to the increase in the proportion of large structural elements (living trees and standing deadwood) which in turn favoured the quantity and diversity of microhabitats. We also hypothesized that microhabitat indices would be better indicators than large and standing dead tree densities, two classical and often used indirect forest biodiversity indicators. We first analysed the response of total species richness and species richness of different groups and guilds (e.g. red-listed species, forest specialists, cavity dwellers...) to several microhabitat indices (density, diversity) to identify the best microhabitat metric. We then used confirmatory structural equation models to assess the indirect effects of management, large structural elements and microhabitats on the diversity of the target species. We first showed that microhabitat indices based on diversity better described biodiversity than indices based on abundance. Structural equation models confirmed that setting aside forest reserves increased the quantities of large trees and standing deadwood, which in turn drove higher quantities and diversity of microhabitats. This significantly increased the richness of several taxonomic groups including notably total richness and cavity dweller richness of both birds and bats. However the magnitude of the link between microhabitat indices and species richness was relatively small. Conversely, the biodiversity of saproxylic beetles was not driven by microhabitat indices and, for this taxon only, models involving direct effects of large structural elements or management performed better. Tree microhabitats appear to be an indicator for part of the taxonomic groups we analysed, but do not constitute a universal biodiversity indicator. They rather have a complementary role compared to large structural elements. Currently, their moderate correlation with biodiversity, as well as thei sensivity to other factors not taken into account in this study (e.g. observer effects), question their transferability to management and policy. Finally, this study calls for replicable and standardized multicriteria methods to validate biodiversity indicators.

National audience; Nous présentons l'état de nos réflexions et les leçons tirées sur la base du projet GNB du point de vue du rôle respectif des gestionnaires et des chercheurs. C'est aussi l'occasion de présenter GNB et certains de ses résultats.

National audience; D'envergure nationale, l'objectif principal du projet "gestion forestière, naturalité et biodiversité (GNB)" est d'étudier l'impact de l'arrêt de l'exploitation forestière sur la structure des peuplements et sur la biodiversité.

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.