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Plant diversity is an important driver of diversity at other trophic levels, suggesting that cascading extinctions could reduce overall biodiversity. Most evidence for positive effects of plant diversity comes from grasslands. Despite the fact that forests are hotspots of biodiversity, the importance of tree diversity, in particular its relative importance compared to other management related factors, in affecting forest‐associated taxa is not well known. To address this, we used data from 183 plots, located in different forest types, from Mediterranean to Boreal, and established along a climatic gradient across six European countries (FunDivEUROPE project). We tested the influence of tree diversity, tree functional composition (i.e. functional trait values), forest structure, climate and soil on the diversity and abundance/activity of nine taxa (bats, birds, spiders, microorganisms, earthworms, ungulates, foliar fungal pathogens, defoliating insects and understorey plants) and on their overall diversity and abundance/activity (multidiversity,multiabundance/activity). Tree diversity was a key driver of taxon‐level and overall forest‐associated biodiversity, along with tree functional composition, forest structure, climate and soil. Both tree species richness and functional diversity (variation in functional trait values) were important. The effects of tree diversity on the abundance/activity of forest‐associated taxa were less consistent. Nonetheless, spiders, ungulates and foliar fungal pathogens were all more abundant/active in diverse forests. Tree functional composition and structure were also important drivers of abundance/activity: conifer stands had lower overall multidiversity (although the effect was driven by defoliating insects), while stands with potentially tall trees had lower overall multiabundance/activity. We found more synergies than trade‐offs between diversity and abundance/activity of different taxa, suggesting that forest management can promote high diversity across taxa. Our results clearly show the high value of mixed forest stands for multiple forest‐associated taxa and indicate that multiple dimensions of tree diversity (taxonomic and functional) are important.

Tree diversity is increasingly acknowledged as an important driver of insect herbivory. However, there is still a debate about the direction of associational effects that can range from associational resistance (i.e., less damage in mixed stands than in monocultures) to the opposite, associational susceptibility. Discrepancies among published studies may be due to the overlooked effect of spatially dependent processes such as tree location within forests. We addressed this issue by measuring crown defoliation and leaf damage made by different guilds of insect herbivores on oaks growing among conspecific versus heterospecific neighbors at forest edges versus interior, in two closed sites in SW France forests. Overall, oaks were significantly less defoliated among heterospecific neighbors (i.e., associational resistance), at both forest edge and interior. At the leaf level, guild diversity and leaf miner herbivory significantly increased with tree diversity regardless of oak location within stands. Other guilds showed no clear response to tree diversity or oak location. We showed that herbivore response to tree diversity varied among insect feeding guilds but not between forest edges and interior, with inconsistent patterns between sites. Importantly, we show that oaks were more defoliated in pure oak plots than in mixed plots at both edge and forest interior and that, on average, defoliation decreased with increasing tree diversity from one to seven species. We conclude that edge conditions could be interacting with tree diversity to regulate insect defoliation, but future investigations are needed to integrate them into the management of temperate forests, notably by better understanding the role of the landscape context.

European forests host a diversity of tree species that are increasingly threatened by fungal pathogens, which may have cascading consequences for forest ecosystems and their functioning. Previous experimental studies suggest that foliar and root pathogen abundance and disease severity decrease with increasing tree species diversity, but evidences from natural forests are rare. Here, we tested whether foliar fungal disease incidence was negatively affected by tree species diversity in different forest types across Europe. We measured the foliar fungal disease incidence on 16 different tree species in 209 plots in six European countries, representing a forest-type gradient from the Mediterranean to boreal forests. Forest plots of single species (monoculture plots) and those with different combinations of two to five tree species (mixed species plots) were compared. Specifically, we analyzed the influence of tree species richness, functional type (conifer vs. broadleaved) and phylogenetic diversity on overall fungal disease incidence. The effect of tree species richness on disease incidence varied with latitude and functional type. Disease incidence tended to increase with tree diversity, in particular in northern latitudes. Disease incidence decreased with tree species richness in conifers, but not in broadleaved trees. However, for specific damage symptoms, no tree species richness effects were observed. Although the patterns were weak, susceptibility of forests to disease appears to depend on the forest site and tree type., This project has received funding from the European Union’s Seventh Programme (FP7/2007–2013) under grant agreement No. 26517

Fons van der Plas [et al.].- Received 8 September 2015, Accepted 19 February 2016, Published 24 March 2016, There is considerable evidence that biodiversity promotes multiple ecosystem functions (multifunctionality), thus ensuring the delivery of ecosystem services important for human well-being. However, the mechanisms underlying this relationship are poorly understood, especially in natural ecosystems. We develop a novel approach to partition biodiversity effects on multifunctionality into three mechanisms and apply this to European forest data. We show that throughout Europe, tree diversity is positively related with multifunctionality when moderate levels of functioning are required, but negatively when very high function levels are desired. For two well-known mechanisms, ‘complementarity’ and ‘selection’, we detect only minor effects on multifunctionality. Instead a third, so far overlooked mechanism, the ‘jack-of-all-trades’ effect, caused by the averaging of individual species effects on function, drives observed patterns. Simulations demonstrate that jack-of-all-trades effects occur whenever species effects on different functions are not perfectly correlated, meaning they may contribute to diversity–multifunctionality relationships in many of the world’s ecosystems.

International audience; Whether increasing forest diversity should result in less insect damage (Associational Resistance, AR) or more damage (Associational Susceptibility, AS) is still debated. Moreover little is known about diversity - resistance relationships in mature forests due to methodological constraints. To address this question we used the FundivEurope exploratory platform. In this network of 209 mature forest plots, sampled along two orthogonal gradients of increasing tree species richness (from one to five species mixtures) and latitude (spanning from the Mediterranean to the boreal forest biomes), we assessed insect defoliation in tree crown. Focusing on eleven broadleaved species, we could for the first time demonstrate a global pattern of reduced defoliation (AR) in mature forests across Europe. We replicated the comparison of insect herbivory in pure vs. mixed plots of oak trees, at the edge or within forest patches. Here we found again significantly less damage on oaks surrounded by heterospecific neighbors, and showed that the magnitude of AR was larger at forest edge than in forest interiors. Last we tested the AR hypothesis with an invasive alien species, the Asian chestnut gall wasp, in Italy. Based on the same methodological approach, our study showed a significant decrease in gall damage with increasing tree species richness in mixed chestnut stands. Our work therefore provides new experimental evidence supporting the Associational Resistance hypothesis with three original contributions: i) AR does apply to mature forests, ii) irrespective of the identity of insect herbivores, iii) including alien pest species. These findings may have important implications for pest management in forest with the maintenance or improvement of tree species diversity being a promising tool for preventing insect damage.

Qui du phénomène de résistance (RA) ou de susceptibilité (SA) par association est prépondérant en milieu forestier ? En d’autres termes, la défoliation par les insectes herbivores est-elle moins (RA) ou plus (SA) importante sur des arbres hôtes situés en mélange comparés à des monocultures ? A cause des contraintes méthodologiques, les connaissances sur la relation diversité - résistance en forêts matures restent faibles. Pour répondre à cette question, nous avons utilisé le dispositif exploratoire du projet FunDivEUROPE. Sur un réseau de 209 parcelles forestières sélectionnées le long de deux gradients orthogonaux de richesse spécifique (d’une à cinq essences en mélange) et de latitude (de la forêt méditerranéenne à la boréale), nous avons évalué la défoliation des houppiers des arbres. A l’aide d’un échantillon constitué de onze essences différentes, nous avons démontré pour la première fois un patron global de diminution de l’herbivorie (RA) en forêts matures à travers l’Europe. Nous avons également comparé l’herbivorie des insectes dans des placettes constituées de chênes purs ou mélangés à d’autres espèces, placées en lisière ou à l’intérieur de petits bois présents dans les vallées et coteaux de Gascogne. Nous avons observé moins de dégâts foliaires sur les chênes entourés par des voisins hétérospécifiques, et nous avons montré que la magnitude de cette résistance (RA) était plus importante en lisière qu’à l’intérieur des bois. Enfin, nous avons testé cette hypothèse de RA sur une espèce d’herbivore invasif en Italie, le cynips du châtaignier. Basée sur la même approche méthodologique, notre étude a montré de plus faibles dégâts de galles sur les châtaigniers lorsque la richesse spécifique de la parcelle était élevée. Notre travail fournit donc de nouvelles preuves supportant l’hypothèse de RA à travers trois contributions originales : i) la RA existe en forêt mature, ii) quelle que soit l’espèce d’herbivore, iii) y compris sur des espèces exotiques. Ces résultats devraient avoir d’importantes implications pour la gestion des insectes herbivores en forêt puisque le maintien et l’amélioration de la diversité des essences représentent un outil prometteur pour prévenir les dégâts d’insectes., Whether increasing forest diversity should result in less insect damage (Associational Resistance, AR) or more damage (Associational Susceptibility, AS) is still debated. Moreover little is known about the diversity - resistance relationships in mature forests due to methodological constraints. To answer this question we used the FundivEUROPE exploratory platform. In this network of 209 mature forest plots sampled along two orthogonal gradients of tree species richness (from one to five species mixtures) and latitude (from the Mediterranean to the boreal forest biomes), we assessed insect defoliation in tree crown. Focusing on eleven broadleaved species, we could for the first time demonstrate a global pattern of reduced defoliation (AR) in mature forests across Europe. We replicated the comparison of insect herbivory in pure vs. mixed plots of oak trees, at the edge or within forest patches of the valleys and hills of Gascony. Here we found significantly less damage on oaks surrounded by heterospecific neighbors, and showed that the magnitude of AR was larger at forest edge than in forest interiors. Last we tested the AR hypothesis with an invasive alien species, the Asian chestnut gall wasp, in Italy. Based on the same methodological approach, our study showed lower gall damage on chestnuts when tree species richness was higher in mixed mature stands. Our work therefore provide new evidences to support the AR hypothesis with three original contributions: i) AR does apply to mature forests, ii) irrespective of the identity of insect herbivores, iii) including exotic species. These findings may have important implications for pest management in forest since the maintenance or improvement of tree species diversity represent a promising tool to prevent insect damage.

Whether increasing forest diversity should result in less insect damage (Associational Resistance, AR) or more damage (Associational Susceptibility, AS) is still debated. Moreover little is known about the diversity - resistance relationships in mature forests due to methodological constraints. To answer this question we used the FundivEUROPE exploratory platform. In this network of 209 mature forest plots sampled along two orthogonal gradients of tree species richness (from one to five species mixtures) and latitude (from the Mediterranean to the boreal forest biomes), we assessed insect defoliation in tree crown. Focusing on eleven broadleaved species, we could for the first time demonstrate a global pattern of reduced defoliation (AR) in mature forests across Europe. We replicated the comparison of insect herbivory in pure vs. mixed plots of oak trees, at the edge or within forest patches of the valleys and hills of Gascony. Here we found significantly less damage on oaks surrounded by heterospecific neighbors, and showed that the magnitude of AR was larger at forest edge than in forest interiors. Last we tested the AR hypothesis with an invasive alien species, the Asian chestnut gall wasp, in Italy. Based on the same methodological approach, our study showed lower gall damage on chestnuts when tree species richness was higher in mixed mature stands. Our work therefore provide new evidences to support the AR hypothesis with three original contributions: i) AR does apply to mature forests, ii) irrespective of the identity of insect herbivores, iii) including exotic species. These findings may have important implications for pest management in forest since the maintenance or improvement of tree species diversity represent a promising tool to prevent insect damage.; Qui du phénomène de résistance (RA) ou de susceptibilité (SA) par association est prépondérant en milieu forestier ? En d’autres termes, la défoliation par les insectes herbivores est-elle moins (RA) ou plus (SA) importante sur des arbres hôtes situés en mélange comparés à des monocultures ? A cause des contraintes méthodologiques, les connaissances sur la relation diversité - résistance en forêts matures restent faibles. Pour répondre à cette question, nous avons utilisé le dispositif exploratoire du projet FunDivEUROPE. Sur un réseau de 209 parcelles forestières sélectionnées le long de deux gradients orthogonaux de richesse spécifique (d’une à cinq essences en mélange) et de latitude (de la forêt méditerranéenne à la boréale), nous avons évalué la défoliation des houppiers des arbres. A l’aide d’un échantillon constitué de onze essences différentes, nous avons démontré pour la première fois un patron global de diminution de l’herbivorie (RA) en forêts matures à travers l’Europe. Nous avons également comparé l’herbivorie des insectes dans des placettes constituées de chênes purs ou mélangés à d’autres espèces, placées en lisière ou à l’intérieur de petits bois présents dans les vallées et coteaux de Gascogne. Nous avons observé moins de dégâts foliaires sur les chênes entourés par des voisins hétérospécifiques, et nous avons montré que la magnitude de cette résistance (RA) était plus importante en lisière qu’à l’intérieur des bois. Enfin, nous avons testé cette hypothèse de RA sur une espèce d’herbivore invasif en Italie, le cynips du châtaignier. Basée sur la même approche méthodologique, notre étude a montré de plus faibles dégâts de galles sur les châtaigniers lorsque la richesse spécifique de la parcelle était élevée. Notre travail fournit donc de nouvelles preuves supportant l’hypothèse de RA à travers trois contributions originales : i) la RA existe en forêt mature, ii) quelle que soit l’espèce d’herbivore, iii) y compris sur des espèces exotiques. Ces résultats devraient avoir d’importantes implications pour la gestion des insectes herbivores en forêt puisque le maintien et l’amélioration de la diversité des essences représentent un outil prometteur pour prévenir les dégâts d’insectes.

International audience

National audience

National audience; Étudier le fonctionnement des écosystèmes requiert l’utilisation d’un échantillon suffisamment représentatif pour mettre en évidence des patrons généraux. Travailler à l’échelle d’un territoire de plusieurs milliers d’hectares devient alors intéressant mais coûteux en temps de préparation des placettes expérimentales in situ. Le système d’information géographique (SIG), de plus en plus utilisé en appui aux travaux menés en écologie, peut alors devenir un outil indispensable pour le travail en amont des relevés sur le terrain. A partir d’un cas d’étude où l’objectif expérimental était de sélectionner des placettes forestières selon quatre modalités écologiques, cet article présente la complémentarité entre l’utilisation d’un SIG et la prospection sur le terrain afin de minimiser les coûts de préparation de l’échantillon de placettes forestières souhaitées.

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National audience

International audience

Plant diversity is an important driver of diversity at other trophic levels, suggesting that cascading extinctions could reduce overall biodiversity. Most evidence for positive effects of plant diversity comes from grasslands. Despite the fact that forests are hotspots of biodiversity, the importance of tree diversity, in particular its relative importance compared to other management related factors, in affecting forest‐associated taxa is not well known. To address this, we used data from 183 plots, located in different forest types, from Mediterranean to Boreal, and established along a climatic gradient across six European countries (FunDivEUROPE project). We tested the influence of tree diversity, tree functional composition (i.e. functional trait values), forest structure, climate and soil on the diversity and abundance/activity of nine taxa (bats, birds, spiders, microorganisms, earthworms, ungulates, foliar fungal pathogens, defoliating insects and understorey plants) and on their overall diversity and abundance/activity (multidiversity, multiabundance/activity). Tree diversity was a key driver of taxon‐level and overall forest‐associated biodiversity, along with tree functional composition, forest structure, climate and soil. Both tree species richness and functional diversity (variation in functional trait values) were important. The effects of tree diversity on the abundance/activity of forest‐associated taxa were less consistent. Nonetheless, spiders, ungulates and foliar fungal pathogens were all more abundant/active in diverse forests. Tree functional composition and structure were also important drivers of abundance/activity: conifer stands had lower overall multidiversity (although the effect was driven by defoliating insects), while stands with potentially tall trees had lower overall multiabundance/activity. We found more synergies than tradeoffs between diversity and abundance/activity of different taxa, suggesting that forest management can promote high diversity across taxa. Our results clearly show the high value of mixed forest stands for multiple forest‐associated taxa and indicate that multiple dimensions of tree diversity (taxonomic and functional) are important.

The importance of biodiversity in supporting ecosystem functioning is generally well accepted. However, most evidence comes from small-scale studies, and scaling-up patterns of biodiversity–ecosystem functioning (B-EF) remains challenging, in part because the importance of environmental factors in shaping B-EF relations is poorly understood. Using a forest research platform in which 26 ecosystem functions were measured along gradients of tree species richness in six regions across Europe, we investigated the extent and the potential drivers of context dependency of B-EF relations. Despite considerable variation in species richness effects across the continent, we found a tendency for stronger B-EF relations in drier climates as well as in areas with longer growing seasons and more functionally diverse tree species. The importance of water availability in driving context dependency suggests that as water limitation increases under climate change, biodiversity may become even more important to support high levels of functioning in European forests.

One of the current advances in functional biodiversity research is the move away from short-lived test systems towards the exploration of diversity-ecosystem functioning relationships in structurally more complex ecosystems. In forests, assumptions about the functional significance of tree species diversity have only recently produced a new generation of research on ecosystem processes and services. Novel experimental designs have now replaced traditional forestry trials, but these comparatively young experimental plots suffer from specific difficulties that are mainly related to the tree size and longevity. Tree species diversity experiments therefore need to be complemented with comparative observational studies in existing forests. Here we present the design and implementation of a new network of forest plots along tree species diversity gradients in six major European forest types: the FunDivEUROPE Exploratory Platform. Based on a review of the deficiencies of existing observational approaches and of unresolved research questions and hypotheses, we discuss the fundamental criteria that shaped the design of our platform. Key features include the extent of the species diversity gradient with mixtures up to five species, strict avoidance of a dilution gradient, special attention to community evenness and minimal covariation with other environmental factors. The new European research platform permits the most comprehensive assessment of tree species diversity effects on forest ecosystem functioning to date since it offers a common set of research plots to groups of researchers from very different disciplines and uses the same methodological approach in contrasting forest types along an extensive environmental gradient. (C) 2013 Elsevier GmbH. All rights reserved.

Many experiments have shown that local biodiversity loss impairs the ability of ecosystems to maintain multiple ecosystem functions at high levels (multifunctionality). In contrast, the role of biodiversity in driving ecosystem multifunctionality at landscape scales remains unresolved. We used a comprehensive pan-European dataset, including 16 ecosystem functions measured in 209 forest plots across six European countries, and performed simulations to investigate how local plot-scale richness of tree species (α-diversity) and their turnover between plots (β-diversity) are related to landscape-scale multifunctionality. After accounting for variation in environmental conditions, we found that relationships between α-diversity and landscape-scale multifunctionality varied from positive to negative depending on the multifunctionality metric used. In contrast, when significant, relationships between β-diversity and landscape-scale multifunctionality were always positive, because a high spatial turnover in species composition was closely related to a high spatial turnover in functions that were supported at high levels. Our findings have major implications for forest management and indicate that biotic homogenization can have previously unrecognized and negative consequences for large-scale ecosystem multifunctionality.

There is considerable evidence that biodiversity promotes multiple ecosystem functions (multifunctionality), thus ensuring the delivery of ecosystem services important for human well-being. However, the mechanisms underlying this relationship are poorly understood, especially in natural ecosystems. We develop a novel approach to partition biodiversity effects on multifunctionality into three mechanisms and apply this to European forest data. We show that throughout Europe, tree diversity is positively related with multifunctionality when moderate levels of functioning are required, but negatively when very high function levels are desired. For two well-known mechanisms, ‘complementarity’ and ‘selection’, we detect only minor effects on multifunctionality. Instead a third, so far overlooked mechanism, the ‘jack-of-all-trades’ effect, caused by the averaging of individual species effects on function, drives observed patterns. Simulations demonstrate that jack-of-all-trades effects occur whenever species effects on different functions are not perfectly correlated, meaning they may contribute to diversity–multifunctionality relationships in many of the world’s ecosystems.

1. Forest ecosystem functioning generally benefits from higher tree species richness, but variation within richness levels is typically large. This is mostly due to the contrasting performances of communities with different compositions. Evidence‐based understanding of composition effects on forest productivity, as well as on multiple other functions will enable forest managers to focus on the selection of species that maximize functioning, rather than with diversity per se. 2. We used a dataset of thirty ecosystem functions measured in stands with different species richness and composition in six European forest types. First, we quantified whether the compositions that maximize annual aboveground wood production (productivity) generally also fulfill the multiple other ecosystem functions (multifunctionality). Then, we quantified the species identity effects and strength of interspecific interactions to identify the ‘best’ and ‘worst’ species composition for multifunctionality. Finally, we evaluated the real‐world frequency of occurrence of best and worst mixtures, using harmonized data from multiple national forest inventories. 3. The most productive tree species combinations also tended to express relatively high multifunctionality, although we found a relatively wide range of compositions with high or low average multifunctionality for the same level of productivity. Monocultures were distributed among the highest as well as the lowest performing compositions. The variation in functioning between compositions was generally driven by differences in the performance of the component species and, to a lesser extent, by particular interspecific interactions. Finally, we found that the most frequent species compositions in inventory data were monospecific stands and that the most common compositions showed below‐average multifunctionality and productivity. 4Synthesis and applications. Species identity and composition effects are essential to the development of high‐performing production systems, for instance in forestry and agriculture. They therefore deserve great attention in the analysis and design of functional biodiversity studies if the aim is to inform ecosystem management. A management focus on tree productivity does not necessarily trade‐off against other ecosystem functions; high productivity and multifunctionality can be combined with an informed selection of tree species and species combinations.

Many experiments have shown that local biodiversity loss impairs the ability of ecosystems to maintain multiple ecosystem functions at high levels (multifunctionality). In contrast, the role of biodiversity in driving ecosystem multifunctionality at landscape scales remains unresolved. We used a comprehensive pan-European dataset, including 16 ecosystem functions measured in 209 forest plots across six European countries, and performed simulations to investigate how local plot-scale richness of tree species (α-diversity) and their turnover between plots (β-diversity) are related to landscape-scale multifunctionality. After accounting for variation in environmental conditions, we found that relationships between α-diversity and landscape-scale multifunctionality varied from positive to negative depending on the multifunctionality metric used. In contrast, when significant, relationships between β-diversity and landscape-scale multifunctionality were always positive, because a high spatial turnover in species composition was closely related to a high spatial turnover in functions that were supported at high levels. Our findings have major implications for forest management and indicate that biotic homogenization can have previously unrecognized and negative consequences for large-scale ecosystem multifunctionality.

Humans require multiple services from ecosystems, but it is largely unknown whether trade-offs between ecosystem functions prevent the realisation of high ecosystem multifunctionality across spatial scales. Here, we combined a comprehensive dataset (28 ecosystem functions measured on 209 forest plots) with a forest inventory dataset (105,316 plots) to extrapolate and map relationships between various ecosystem multifunctionality measures across Europe. These multifunctionality measures reflected different management objectives, related to timber production, climate regulation and biodiversity conservation/recreation. We found that trade-offs among them were rare across Europe, at both local and continental scales. This suggests a high potential for ‘win-win’ forest management strategies, where overall multifunctionality is maximised. However, across sites, multifunctionality was on average 45.8-49.8 below maximum levels and not necessarily highest in protected areas. Therefore, using one of the most comprehensive assessments so far, our study suggests a high but largely unrealised potential for management to promote multifunctional forests.