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Conventional conservation policies in Europe notably rely on the passive restoration of natural forest dynamics by setting aside forest areas to preserve forest biodiversity. However, since forest reserves cover only a small proportion of the territory, conservation policies also require complementary conservation efforts in managed forests in order to achieve the biodiversity targets set up in the Convention on Biological Diversity. Conservation measures also raise the question of large herbivore management in and around set-asides, particularly regarding their impact on understory vegetation. Although many studies have separately analyzed the effects of forest management, management abandonment, and ungulate pressure on forest biodiversity, their joint effects have rarely been studied in a correlative framework. We studied 212 plots located in 15 strict forest reserves paired with adjacent managed forests in European France. We applied structural equation models to test the effects of management abandonment, stand structure, and ungulate pressure on the abundance, species richness, and diversity of herbaceous vascular plants and terricolous bryophytes. We showed that stand structure indices and plot-level browsing pressure had direct and opposite effects on herbaceous vascular plant species diversity; these effects were linked with the light tolerance of the different species groups. Increasing canopy cover had an overall negative effect on herbaceous vascular plant abundance and species diversity. The effect was two to three times greater in magnitude than the positive effects of browsing pressure on herbaceous plants diversity. On the other hand, a high stand density index had a positive effect on the species richness and diversity of bryophytes, while browsing had no effect. Forest management abandonment had few direct effects on understory plant communities, and mainly indirectly affected herbaceous vascular plant and bryophyte abundance and species richness and diversity through changes in vertical stand structure. Our results show that conservation biologists should rely on foresters and hunters to lead the preservation of understory vegetation communities in managed forests since, respectively, they manipulate stand structure and regulate ungulate pressure. Their management actions should be adapted to the taxa at stake, since bryophytes and vascular plants respond differently to stand and ungulate factors.

Les Bryophytes représentent une partie importante de la biodiversité forestière. Pour étudier l’évolution de leur diversité dans le temps, ou entre modalités de gestion forestière, les protocoles de relevés doivent être adaptés à la diversité des supports colonisés, et reproductibles sans biais. Nous présentons le protocole standardisé que nous avons conçu et testé sur 14 massifs forestiers de plaine et de montagne en France métropolitaine (193 placettes). Sa faisabilité a été évaluée par retour d’expérience des opérateurs, et l’exhaustivité par la part d’espèces captées par le procotole en comparaison à des richesses de référence sur chaque placette. La reproductibilité a été évaluée par un test d’effet opérateur, sur des supports ayant chacun été inventorié par plusieurs opérateurs et fait l’objet d’un relevé de consensus. En combinant un inventaire dirigé par supports prédéfinis et un inventaire complémentaire en plein, notre protocole capte, pour les principaux types de supports, plus de 80 % de la richesse théorique calculée par l’indice Chao2. Augmenter le nombre de supports dans l’inventaire dirigé consommerait du temps pour un gain faible en espèces. L’effet opérateur joue sur le taux de détection des espèces, et peut être soit pris en compte dans les modélisations, soit réduit par entraînement. Notre protocole offre donc un bon compromis entre faisabilité, répétabilité et exhaustivité pour évaluer la diversité bryologique des peuplements forestiers de France métropolitaine dans la strate 0-2 m, dans le cadre de suivis temporels ou de comparaison de modalités écologiques ou de gestion.

Bryophytes are an important component of overall biodiversity in forests, where they colonize a large diversity of supports. In order to assess the evolution of their diversity in time or between different ecological or management contexts, one should apply standardized protocols of data collection, taking into account the diversity of the supports and avoiding any bias to make time or space comparisons possible. Here, we present a standardized protocol to collect bryophyte data up to 2 meters high in temperate forests. We built and tested our protocol within 14 floodplain or mountain temperate forests in France (193 plots). We assessed its feasability by surveying the operators’ feedbacks. The exhaustivity of the protocol was evaluated by comparing the censused richness (either total or per support type) with observed or theoritical (Chao2 index) reference values. The repeatability was evaluated thanks to a designed operator effect study, where a set of supports was inventoried by several operators first separately and then together to obtain a consensual releve as reference. By merging an inventory on predefined supports and a complementary 30 minutes inventory on the whole surface of the plot, our protocol captured more than 80 % of the theoretical Chao2 richness of the main supports. Increas¬ing the number of predefined supports would lead to a very limited gain (no more than one species per support type in average). We showed that the observer effect first and foremost influenced the detection probability of the species, which increased with the familiarity of the observer with the species. This effect could therefore being taken into account by training of the operators, or in the models. We conclude that our protocol offers a good compromise between feasability, exhaustivity and repetability to assess the bryological diversity of temperate forest stands, in order to monitor it or to compare the effects of ecological or management variables on bryophyte diversity., Les Bryophytes représentent une partie importante de la biodiversité forestière. Pour étudier l’évolution de leur diversité dans le temps, ou entre modalités de gestion forestière, les protocoles de relevés doivent être adaptés à la diversité des supports colonisés, et reproductibles sans biais. Nous présentons le protocole standardisé que nous avons conçu et testé sur 14 massifs forestiers de plaine et de montagne en France métropolitaine (193 placettes). Sa faisabilité a été évaluée par retour d’expérience des opérateurs, et l’exhaustivité par la part d’espèces captées par le protocole en comparaison à des richesses de référence sur chaque placette. La reproductibilité a été évaluée par un test d’effet opérateur, sur des supports ayant chacun été inventorié par plusieurs opérateurs et fait l’objet d’un relevé de consensus. En combinant un inventaire dirigé par supports prédéfinis et un inventaire complémentaire en plein, notre protocole capte, pour les principaux types de supports, plus de 80 % de la richesse théorique calculée par l’indice Chao2. Augmenter le nombre de supports dans l’inventaire dirigé consommerait du temps pour un gain faible en espèces. L’effet opérateur joue sur le taux de détection des espèces, et peut être soit pris en compte dans les modélisations, soit réduit par entraînement. Notre protocole offre donc un bon compromis entre faisabilité, répétabilité et exhaustivité pour évaluer la diversité bryologique des peuplements forestiers de France métropolitaine dans la strate 0-2 m, dans le cadre de suivis temporels ou de comparaison de modalités écologiques ou de gestion.

Aim : The global knowledge of the distribution and proportion of forests with high conservation values such as mature forests in metropolitan France is scarce, focuses only on a few specific well-known sites, and has never been tackled countrywide and evaluated with a robust multivariate statistical approach. Here, we modelled French forest reserves9 time since the last harvesting operation, then inferred the current statistical distribution of mature forests (i.e. forests over 50 years without harvesting) in metropolitan France. Location : Metropolitan France Methods : We used inventories of forest stand structure issued from forest reserves and managed forests to calibrate a generalised linear mixed model explaining the time since the last harvesting with selected stand structure attributes of maturity, combined with environmental variables. We chose the time since the last harvesting as a proxy for forest maturity, assuming older unmanaged plots to host older trees. We then projected this model on an independent nation-wide dataset issued from the National Forest Inventory. Thus, we obtained an updated estimation of the proportion and a rough distribution of mature forest stands (i.e. abandoned for over 50 years) in metropolitan France. Results : We found that the structural variables that best characterized the time since the last harvesting were high basal area of very large trees, high volumes of both standing and downed deadwood, high diversity of tree-related microhabitats and more marginally diversity of decay stages. Variables translating legacy of past forest management also characterised more mature plots, which displayed lower volumes of stumps and high density of coppices. Our projection gave an estimated 3% of French forests over 50 years without harvesting and a promising proportion of French forests that could reach interesting levels of maturity in the near future. Main conclusions : Our model showed that the time since the last harvesting is a good proxy for forest maturity in French temperate forest. It highlighted the stand structure attributes that are key in characterising those forests with high conservation values. We found that forests with higher levels of maturity seem to be located in more inaccessible places notably mountainous areas. Our projection gives the first robust statistical estimate of the proportion of mature forests in metropolitan France and may serve to report on the status of mature forests.

National audience

Aims: Primary forests are critical for forest biodiversity and provide key ecosystem services. In Europe, these forests are particularly scarce and it is unclear whether they are sufficiently protected. Here we aim to: (a) understand whether extant primary forests are representative of the range of naturally occurring forest types, (b) identify forest types which host enough primary forest under strict protection to meet conservation targets and (c) highlight areas where restoration is needed and feasible. Location: Europe. Methods: We combined a unique geodatabase of primary forests with maps of forest cover, potential natural vegetation, biogeographic regions and protected areas to quantify the proportion of extant primary forest across Europe's forest types and to identify gaps in protection. Using spatial predictions of primary forest locations to account for underreporting of primary forests, we then highlighted areas where restoration could complement protection. Results: We found a substantial bias in primary forest distribution across forest types. Of the 54 forest types we assessed, six had no primary forest at all, and in two-thirds of forest types, less than 1% of forest was primary. Even if generally protected, only ten forest types had more than half of their primary forests strictly protected. Protecting all documented primary forests requires expanding the protected area networks by 1,132km2 (19,194km2 when including also predicted primary forests). Encouragingly, large areas of non-primary forest existed inside protected areas for most types, thus presenting restoration opportunities. Main conclusion: Europe's primary forests are in a perilous state, as also acknowledged by EU's “Biodiversity Strategy for 2030.” Yet, there are considerable opportunities for ensuring better protection and restoring primary forest structure, composition and functioning, at least partially. We advocate integrated policy reforms that explicitly account for the irreplaceable nature of primary forests and ramp up protection and restoration efforts alike.

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; The "Plateforme Biodiversité pour la Forêt" works to build a large biodiversity monitoring system in metropolitan french forests. The idea is to mobilize funders and to make existing monitoring system managers work together to fill the current gaps.; Une réflexion sur la mise en place d'un vaste réseau de suivi de la biodiversité en forêt a été lancée par la Plateforme Biodiversité pour la Forêt, en France. L'idée est de mobiliser les financeurs et gestionnaires des réseaux de suivis existants tout en comblant les lacunes actuelles.

International audience; Managing forests to preserve biodiversity requires a good knowledge not only of the factors driving its dynamics but also of the structural elements that actually support biodiversity. Tree-related microhabitats (e.g. cavities, cracks, conks of fungi) are tree-borne features that are reputed to support specific biodiversity for at least a part of species' life cycles. While several studies have analysed the drivers of microhabitats number and occurrence at the tree scale, they remain limited to a few tree species located in relatively narrow biogeographical ranges. We used a nationwide database of forest reserves where microhabitats were inventoried on more than 22,000 trees. We analysed the effect of tree diameter and living status (alive or dead) on microhabitat number and occurrence per tree, taking into account biogeoclimatic variables and tree genus. We confirmed that larger trees and dead trees bore more microhabitats than their smaller or living counterparts did; we extended these results to a wider range of tree genera and ecological conditions than those studied before. Contrary to our expectations, the total number of microhabitat types per tree barely varied with tree genus-though we did find slightly higher accumulation levels for broadleaves than for conifers-nor did it vary with elevation or soil pH, whatever the living status. We observed the same results for the occurrence of individual microhabitat types. However, accumulation levels with diameter and occurrence on dead trees were higher for microhabitats linked with wood decay processes (e.g. dead branches or woodpecker feeding holes) than for other, epixylic, microhabitats such as epiphytes (ivy, mosses and lichens). Promoting large living and dead trees of several tree species may be a relevant, and nearly universal, way to favour microhabitats and enhance the substrates needed to support specific biodiversity. In the future, a better understanding of microhabitat drivers and dynamics at the tree scale may help to better define their role as biodiversity indicators for large-scale monitoring.

Managing forests to preserve biodiversity requires a good knowledge not only of the factors driving its dynamics but also of the structural elements that actually support biodiversity. Tree-related microhabitats (cavities, cracks, conks of fungi) are tree-borne features that are reputed to support specific biodiversity for at least a part of species’ life cycles. While several studies have analysed the drivers of microhabitats number and occurrence at the tree scale, they remain limited to a few tree species located in relatively narrow biogeographical ranges. We used a nationwide database of forest reserves where microhabitats were inventoried on more than 22,000 trees. We analysed the effect of tree diameter and living status (alive or dead) on microhabitat number and occurrence per tree, taking into account biogeoclimatic variables and tree genus.We confirmed that larger trees and dead trees bore more microhabitats than their smaller or living counterparts did; we extended these results to a wider range of tree genera and ecological conditions. Contrary to expectations, the total number of microhabitat types per tree barely varied with tree genus – though we did find slightly higher accumulation levels for broadleaves than for conifers – nor did it vary with elevation or soil pH. We observed the same results for the occurrence of individual microhabitat types. However, accumulation levels with diameter and occurrence on dead trees were higher for microhabitats linked with wood decay processes (e.g. dead branches or woodpecker feeding holes) than for other, epixylic, microhabitats such as epiphytes (ivy, mosses and lichens).Promoting large living and dead trees of several tree species may be an interesting, and nearly universal, way to favour microhabitats and enhance the substrates needed to support specific biodiversity. In addition, a better understanding of microhabitat drivers and dynamics at the tree scale may help to better define their role as biodiversity indicators for large-scale monitoring.

Aim: Primary forests have high conservation value but are rare in Europe due to his-toric land use. Yet many primary forest patches remain unmapped, and it is unclear to what extent they are effectively protected. Our aim was to (1) compile the most com-prehensive European-scale map of currently known primary forests, (2) analyse the spatial determinants characterizing their location and (3) locate areas where so far unmapped primary forests likely occur.Location: Europe.Methods: We aggregated data from a literature review, online questionnaires and 32 datasets of primary forests. We used boosted regression trees to explore which bio-physical, socio- economic and forest- related variables explain the current distribution of primary forests. Finally, we predicted and mapped the relative likelihood of pri-mary forest occurrence at a 1- km resolution across Europe.Results: Data on primary forests were frequently incomplete or inconsistent among countries. Known primary forests covered 1.4 Mha in 32 countries (0.7% of Europe’s forest area). Most of these forests were protected (89%), but only 46% of them strictly. Primary forests mostly occurred in mountain and boreal areas and were un-evenly distributed across countries, biogeographical regions and forest types. Unmapped primary forests likely occur in the least accessible and populated areas, where forests cover a greater share of land, but wood demand historically has been low.Main conclusions: Despite their outstanding conservation value, primary forests are rare and their current distribution is the result of centuries of land use and forest management. The conservation outlook for primary forests is uncertain as many are not strictly protected and most are small and fragmented, making them prone to ex- tinction debt and human disturbance. Predicting where unmapped primary forests likely occur could guide conservation efforts, especially in Eastern Europe where large areas of primary forest still exist but are being lost at an alarming pace.

International audience; 1. National and international forest biodiversity assessments largely rely on indirect indicators, based on elements of forest structure that are used as surrogates for species diversity. These proxies are reputedly easier and cheaper to assess than biodiversity. Tree microhabitats - tree-borne singularities such as cavities, conks of fungi or bark characteristics - have gained attention as potential forest biodiversity indicators. However, as with most biodiversity indicators, there is a lack of scientific evidence documenting their quantitative link with the biodiversity they are supposed to assess. 2. We explored the link between microhabitat indices and the richness and abundance of three taxonomic groups: bats, birds, and saproxylic beetles. Using a nation-wide multi-taxon sampling design in France, we compared 213 plots located inside and outside strict forest reserves. We hypothesized that the positive effect setting aside forest reserves has on biodiversity conservation is indirectly due to an increase in the proportion of large structural elements (e.g. living trees, standing and lying deadwood). These, in turn, are likely to favour the quantity and diversity of microhabitats. We analysed the relationship between the abundance and species richness of different groups and guilds (e.g. red-listed species, forest specialists, cavity dwellers) and microhabitat density and diversity. We then used confirmatory structural equation models to assess the direct and indirect effects of management abandonment, large structural elements and microhabitats on the biodiversity of the target species. 3. For several groups of birds and bats, the indirect effect of management abandonment and large structural elements on biodiversity was mediated by microhabitats. However, the magnitude of the link between microhabitat indices and biodiversity was moderate. In particular, saproxylic beetles' biodiversity was poorly explained by microhabitats, large structural elements or management abandonment. 4. Synthesis and applications: Tree microhabitats may serve as indicators for bats and birds, but they are not a universal biodiversity indicator. Rather, compared to large structural elements, they most likely have a complementary role to biodiversity. In terms of forest management and conservation, preserving diversity of microhabitats at the local scale benefits several groups of both bats and birds.

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.

Le projet Gestion forestière, Naturalité et Biodiversité a pour but d'étudier le lien entre biodiversité, exploitation forestière et naturalité en comparant des parcelles exploitées à des parcelles non-exploitées (Réserves Biologiques Intégrales ou parties intégrales de Réserves Naturelles). En partenariat avec l'ONF, RNF et l'INRA, nous travaillons depuis 2008 sur des réserves intégrales soustraites à l'exploitation depuis au moins 20 ans. L'étude repose sur l'échantillonnage de 7 groupes taxonomiques : plantes vasculaires, mousses, champignons, chauve-souris, oiseaux, coléoptères carabiques et saproxyliques.

International audience

National audience; La gestion durable des forêts repose largement sur la biodiversité dont le fonctionnement reste cependant encore très insuffisamment connu. Il en découle la nécessité d’améliorer, de manière générale, le socle de connaissances relatif à la biodiversité et, plus particulièrement, d’analyser les interactions entre changement climatique, productivité forestière et biodiversité en forêt. Par ailleurs, les stratégies et décisions des acteurs influent sur ces interactions et constituent également des objets d’étude à privilégier. C’est pourquoi le programme de recherche « Biodiversité, gestion forestière et politiques publiques » a lancé en 2013 un appel à projets de recherche avec deux entrées :- l’une par les sciences de la nature axée sur le changement climatique, les mesures de gestion et la dynamique de la biodiversité dans les écosystèmes ;- l’autre par les sciences humaines et sociales sur les stratégies et décisions des acteurs, la gouvernance et les politiques publiques correspondant à la première entrée.Le présent colloque a pour objectifs de :- présenter les résultats des cinq projets de recherche qui ont été sélectionnés à cette occasion sur chacune des deux entrées en les accompagnant en tant que de besoin de regards complémentaires et en illustrant l’un des projets par une visite sur le terrain ;- initier une réflexion sur le progrès des connaissances dans ce domaine, en s’inspirant du devenir de projets antérieurs et en identifiant les principales lacunes à combler.Il se déroulera en trois sessions, chacune introduite par des exposés d’une quinzaine de minutes et conclue par une table ronde permettant un débat avec les participants et les invités.

La protection d’espaces forestiers en libre évolution, définie formellement pour la première fois dans le parc national des Cévennes, s’inscrit dans la continuité d’une longue histoire de préservation des espaces naturels. Ces forêts couvrent en 2022 environ 41 000 ha en métropole (0,24 % des forêts), principalement grâce aux réserves biologiques intégrales, et contribuent significativement à la préservation d’espèces et écosystèmes forestiers remarquables parfois sur de grandes surfaces d’un seul tenant. Elles sont toutefois actuellement en proportion encore insuffisantes pour assurer la conservation de l’ensemble des espèces forestières. Ces forêts constituent également des terrains d’observation privilégiés pour des recherches scientifiques à long terme. À la recherche du meilleur compromis entre enjeux de protection et faisabilité, la protection d’espaces forestiers en libre évolution nécessite de déployer différents leviers d’acceptabilité sociale. L’expérience ainsi acquise trouvera toute son utilité pour la mise en œuvre de la stratégie nationale pour les aires protégées.