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Tree species diversity plays a central role for forest productivity, but factors driving positive biodiversity-productivity relationships remain poorly understood. In a biodiversity experiment manipulating tree diversity and mycorrhizal associations, we examined the roles of above- and belowground processes in modulating wood productivity in young temperate tree communities, as well as potential underlying mechanisms. We found that tree species richness increased forest productivity indirectly by enhancing structural complexity within communities. After six years, structurally complex communities were twice as productive as structurally simple stands. This pattern was consistent across stands with different mycorrhizal associations. Our results also demonstrate that taxonomic diversity and functional variation in shade tolerance, but not phenotypic plasticity, are key drivers of structural complexity in mixtures, which in turn lead to overyielding. Consideration of stand structural complexity appears to be a crucial element in predicting carbon sequestration in the early successional stages of mixed-species forests.

Many recent studies have analysed plant species responses to environmental change, but interactive effects of global change drivers and how they are modulated by biotic interactions are still poorly understood. In a mesocosm experiment, we studied the interactive effects of nitrogen (N) fertilization and drought events on plant growth and how these effects are shaped by competitive interactions, using a segetal plant community typical of the lowlands of central Europe (composed of Lilium bulbiferum (segetal species) and Secale cereale (crop species)). We expected that N fertilization increases the drought sensitivity of Lilium (negative interaction effect), and that these effects are shaped by interspecific competition with Secale. Secale and Lilium showed opposing responses to N fertilization (second year of the experiment): Whilst Secale aboveground and belowground biomass almost doubled with N fertilization, Lilium aboveground and belowground biomass showed no response or decreased, respectively, providing Secale with a competitive advantage. Lilium aboveground tissue dieback (as a proxy for growth vigour) was 22% in N and 35% in drought treatments (control: 6%), but reached 91% when combining these treatments. Increasing Lilium tissue dieback was strongly related to decreasing belowground (root) biomass, caused by both negative direct effects of combined treatments (N fertilization + drought), and negative indirect effects acting via treatment-induced increase in Secale biomass. Our results demonstrate that competitive interactions can shape the effects of global change drivers on plant growth. This knowledge in turn could be important for plant species conservation, particularly in the face of ongoing shifts in environmental conditions.

International audience; Forests are ecosystems critical to understanding the global carbon budget, due to their carbon sequestration potential in both aboveground and belowground compartments, especially in species-rich forests. Soil carbon sequestration is strongly linked to soil microbial communities, and this link is mediated by the tree community, likely due to modifications of microenvironmental conditions (i.e., biotic conditions, soil properties, and microclimate). We studied soil carbon concentration and the soil microbial biomass of 180 local neighborhoods along a gradient of tree species richness ranging from 1 to 16 tree species per plot in a Chinese subtropical forest experiment (BEF-China). Tree productivity and different tree functional traits were measured at the neighborhood level. We tested the effects of tree productivity, functional trait identity, and dissimilarity on soil carbon concentrations, and their mediation by the soil microbial biomass and microenvironmental conditions. Our analyses showed a strong positive correlation between soil microbial biomass and soil carbon concentrations. In addition, soil carbon concentration increased with tree productivity and tree root diameter, while it decreased with litterfall C:N content. Moreover, tree productivity and tree functional traits (e.g., fungal root association and litterfall C:N ratio) modulated microenvironmental conditions with substantial consequences for soil microbial biomass. We also showed that soil history and topography should be considered in future experiments and tree plantations, as soil carbon concentrations were higher at sites where historical (i.e., at the beginning of the experiment) carbon concentrations were high, themselves being strongly affected by the topography. Altogether, these results implied that the quantification of the different soil carbon pools is critical for understanding microbial community–soil carbon stock relationships and their dependence on tree diversity and microenvironmental conditions.

Human-induced biodiversity loss negatively affects ecosystem function, but the interactive effects of biodiversity change across trophic levels remain insufficiently understood. We sampled arboreal spiders and lepidopteran larvae across seasons in 2 years in a subtropical tree diversity experiment, and then disentangled the links between tree diversity and arthropod predator diversity by deconstructing the pathways among multiple components of diversity (taxonomic, phylogenetic and functional) with structural equation models. We found that herbivores were major mediators of plant species richness effects on abundance, species richness, functional and phylogenetic diversity of predators, while phylogenetic, functional and structural diversity of trees were also important mediators of this process. However, the strength and direction differed between functional, structural and phylogenetic diversity effects, indicating different underlying mechanisms for predator community assembly. Abundance and multiple diversity components of predators were consistently affected by tree functional diversity, indicating that the variation in structure and environment caused by plant functional composition might play key roles in predator community assembly. Our study highlights the importance of an integrated approach based on multiple biodiversity components in understanding the consequences of biodiversity loss in multitrophic communities.

A more efficient use of limited canopy space and, thus, a higher canopy space occupation (CSO) in forests can result in an increased absorption of photosynthetically active radiation, which in turn can promote productivity. Although there is some evidence for a positive relationship between tree diversity (TD) and CSO, the generality of this hypothesis is still under debate. Here, we propose a conceptual framework that accounts for both the spatial complexity of canopy space and size-dependent interspecific tree interactions and tested it using mobile laser scanning data across larger spatial scales. We assessed the CSO at high resolution with two diversity indices, tree species richness (TSR) and the effective number of species (ENS) along a TSR gradient ranging from monocultures to 8-species mixtures in a mature and structurally complex mixed-species temperate forest. We found that the direction and strength of the TD-CSO relationship largely depended on the way how canopy space is defined and which tree size classes are considered to calculate TSR. Using an broad deliniation of canopy space no significant relationship between TD and CSO was evident. In contrast, when considering only the upper canopy space, a significant effect of TSR on CSO emerged. Importantly, the direction of this relationship was critically dependent on the tree size threshold underlying the TSR determination. For all trees with a diameter at breast height > 7 cm, we observed a significant negative relationship, while the opposite was the case when considering only large-sized trees. Our novel conceptual framework demonstrates that accurate estimation of canopy space complexity and tree size dependence is key to better understanding the processes underlying CSO. However, further information on diversity-canopy space occupation relationships from studies in different forests and forest types is needed.