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Examining the relationship between tree diversity and ecosystem functioning has been a recent focus of forest ecology. Particular emphasis has been given to the impact of tree diversity on productivity and to its potential to mitigate negative global change effects; however, little attention has been paid to tree mortality. This is critical because both tree mortality and productivity underpin forest ecosystem dynamics and therefore forest carbon sequestration. Neglecting tree mortality leaves a large part of the picture undocumented. Here we show that increasingly diverse forest stands have increasingly high mortality probabilities. We found that the most species-rich stands in temperate biomes had mortality probabilities more than sevenfold higher than monospecific stands (∼0.6% year−1 in monospecific stands to 4.0% year−1 in the most species-rich stands) while in boreal stands increases were less pronounced but still significant (∼1.1% year−1 in monospecific stands to 1.8% year−1 in the most species-rich stands). Tree species richness was the third-most-important predictor of mortality in our models in temperate forests and the fifth-most-important predictor in boreal forests. Our results highlight that while the promotion of tree diversity undoubtedly has many positive effects on ecosystem functioning and the services that trees provide to humanity, it remains important to consider all aspects of forest dynamics in order to properly predict the implications of maintaining and promoting tree diversity.

1. Forest productivity may be determined not only by biodiversity but also by environmental factors and stand structure attributes. However, the relative importance of these factors in determining productivity is still controversial for subtropical forests. 2. Based on a large dataset from 600 permanent forest inventory plots across subtropical China, we examined the relationship between biodiversity and forest productivity and tested whether stand structural attributes (stand density in terms of trees per ha, age and tree size) and environmental factors (climate and site conditions) had larger effects on productivity. Furthermore, we quantified the relative importance of environmental factors, stand structure and diversity in determining forest productivity. 3. Diversity, together with stand structure and site conditions, regulated the variability in forest productivity. The relationship between diversity and forest productivity did not vary along environmental gradients. Stand density and age were more important modulators of forest productivity than diversity. 4. Synthesis. Diversity had significant and positive effects on productivity in species rich subtropical forests, but the effects of stand density and age were also important. Our work highlights that while biodiversity conservation is often important, the regulation of stand structure can be even more important to maintain high productivity in subtropical forests.

1. A growing body of empirical evidence has suggested that biodiversity affects the simultaneous performance of multiple ecosystem functions (i.e. ecosystem multifunctionality). Given increasing environmental variability and uncertainty under global change, an emerging question is how biodiversity influences the stability of multiple functions (i.e. multifunctional stability). We currently know little, however, about the determinants and mechanisms of multifunctional stability, which is of practical importance for ensuring the sustainable provision of multiple functions. 2. Here, we examined mechanisms contributing to stability (quantified as the ratio of the mean to the standard deviation) of multiple functions related to ecosystem productivity and carbon sequestration, including plant above-ground and belowground productivity, litter production, gross primary productivity and ecosystem respiration, in a large grassland biodiversity experiment in Inner Mongolia. 3. We found that community-wide species asynchrony was a strong driver to stabilize multiple functions. Community-wide asynchrony mediated the positive effects of species richness and response diversity (describing how species with similar effects on ecosystem function respond differently to environmental change) on multifunctional stability. However, species richness had a negative direct effect on multifunctional stability because, although it increased the averaged temporal mean of multiple functions, it strongly increased the averaged temporal standard deviation of multiple functions. The overall effects of species richness on multifunctional stability were thus negative, whereas those of response diversity were positive. 4. Synthesis. The studied ecosystem functions related to ecosystem productivity and carbon sequestration are important in natural grasslands across the world. We conclude that species asynchrony and response diversity, rather than species richness, are key to the ecosystem multifunctional stability. The loss of response diversity and compensatory mechanisms would likely reduce the long-term sustainability of grasslands in the face of global change.

The use of pesticides within agricultural ecosystems has led to wide concern regarding negative effects on the environment. One possible alternative is the use of predators of pest species that naturally occur within agricultural ecosystems. However, the mechanistic basis for how species can be manipulated in order to maximize pest control remains unclear. We carried out a meta-analysis of 51 studies that manipulated predator species richness in reference to suppression of herbivore prey to determine which components of predator diversity affect pest control. Overall, functional diversity (FD) based on predator’s habitat domain, diet breadth and hunting strategy was ranked as the most important variable. Our analysis showed that increases in FD in polycultures led to greater prey suppression compared to both the mean of the component predator species, and the most effective predator species, in monocultures. Further analysis of individual traits indicated these effects are likely to be driven by broad niche differentiation and greater resource exploitation in functionally diverse predator communities. A decoupled measure of phylogenetic diversity, whereby the overlap in variation with FD was removed, was not found to be an important driver of prey suppression. Our results suggest that increasing FD in predatory invertebrates will help maximize pest control ecosystem services in agricultural ecosystems, with the potential to increase suppression above that of the most effective predator species.

Viewing facilitation through the lens of the niche concept is one way to unify conceptual and empirical advances about the role of facilitation in community ecology. We clarify conceptually and through examples from marine and terrestrial environments how facilitation can expand species’ niches and consider how these interactions can be scaled up to understand the importance of facilitation in setting a species’ geographic range. We then integrate the niche‐broadening influence of facilitation into current conceptual areas in ecology, including climate change, diversity maintenance and the relationship between diversity and ecosystem functioning. Because facilitation can influence the range of physical conditions under which a species can persist, it has the potential to mitigate the effects of climate change on species distributions. Whereas facilitation has mostly been considered as a diversity‐promoting interaction by ameliorating abiotic stresses, if facilitated species’ niches expand and become less distinct as a result of habitat amelioration, the forces that maintain diversity and promote coexistence in regions or habitats dominated by the facilitator could be reduced (i.e. the sign of the effects of facilitation on populations could be species‐specific). Finally, shifting or broadening ecological niches could alter the relationship between diversity and ecosystem functioning. A niche‐based perspective on the effects of facilitation can foster a greater mechanistic understanding of the role played by facilitation in regulating species coexistence, range shifts and ecosystem functioning in a changing world.

An enormous recent research effort focused on how plant biodiversity (notably species richness) influences ecosystem functioning, usually through experiments in which diversity is varied through random draws of species from a species pool. Such experiments are increasingly used to predict how species losses influence ecosystem functioning in ‘real’ ecosystems. However, this assumes that comparisons of experimental communities with low vs high species richness are analogous to comparisons of natural communities from which species either have or have not been lost. I explore the validity of this assumption, and highlight difficulties in using such experiments to draw conclusions about the ecosystem consequences of biodiversity loss in natural systems. Notably, these experiments do not mimic what happens in real ecosystems either when local extinctions occur or when species losses are offset by gains of new species. Despite limitations, this single experimental approach for studying how biodiversity loss affects ecosystems has often been advocated and implemented at the expense of other approaches; this limits understanding of how natural ecosystems respond to biodiversity loss. I conclude that a broader spectrum of approaches, and more explicit consideration of how species losses and gains operate in concert to influence ecosystems, will help progress this field.

Bacteria are essential for many ecosystem services but our understanding of factors controlling their functioning is incomplete. While biodiversity has been identified as an important driver of ecosystem processes in macrobiotic communities, we know much less about bacterial communities. Due to the high diversity of bacterial communities, high functional redundancy is commonly proposed as explanation for a lack of clear effects of diversity. The generality of this claim has, however, been questioned. We present the results of an outdoor dilution-to-extinction experiment with four lake bacterial communities. The consequences of changes in bacterial diversity in terms of effective number of species, phylogenetic diversity, and functional diversity were studied for (1) bacterial abundance, (2) temporal stability of abundance, (3) nitrogen concentration, and (4) multifunctionality. We observed a richness gradient ranging from 15 to 280 operational taxonomic units (OTUs). Individual relationships between diversity and functioning ranged from negative to positive depending on lake, diversity dimension, and aspect of functioning. Only between phylogenetic diversity and abundance did we find a statistically consistent positive relationship across lakes. A literature review of 24 peer-reviewed studies that used dilution-to-extinction to manipulate bacterial diversity corroborated our findings: about 25% found positive relationships. Combined, these results suggest that bacteria-driven community functioning is relatively resistant to reductions in diversity.

Despite evidence of a positive effect of functional diversity on ecosystem productivity, the importance of functionally distinct species (i.e. species that display an original combination of traits) is poorly understood. To investigate howdistinct species affect ecosystem productivity,we used a forest-gap model to simulate realistic temperate forest successions along an environmental gradient and measured ecosystem productivity at the end of the successional trajectories.We performed 10 560 simulations with different sets and numbers of species, bearing either distinct or indistinct functional traits, and compared them to random assemblages, to mimic the consequences of a regional loss of species. Long-term ecosystem productivity dropped when distinct species were lost first from the regional pool of species, under the harshest environmental conditions. On the contrary, productivity was more dependent on ordinary species in milder environments. Our findings showthat species functional distinctiveness, integrating multiple trait dimensions, can capture species-specific effects on ecosystem productivity. In a context of an environmentally changing world, they highlight the need to investigate the role of distinct species in sustaining ecosystem processes, particularly in extreme environmental conditions.

• Insect herbivore damage and abundance are often reduced in diverse plant stands. However, few studies have explored whether this phenomenon is a result of plant diversity effects on host plant traits. • We explored indirect effects of tree species diversity on herbivory via changes in leaf traits in a long-term forest diversity experiment in Finland. We measured 16 leaf traits and leaf damage by four insect guilds (chewers, gall formers, leaf miners and rollers) on silver birch (Betula pendula) trees growing in one-, two-, three- and five-species mixtures. • A decline in the frequency of birch in mixed stands resulted in reduced leaf area. This, in turn, mediated the reduction in chewing damage in mixed stands. In contrast, associational resistance of birch to leaf miners was not trait-mediated but driven directly by concurrent declines in birch frequency as tree species richness increased. • Our results show that leaf trait variation across the diversity gradient might promote associational resistance, but these patterns are driven by an increase in the relative abundance of heterospecifics rather than by tree species richness per se. Therefore, accounting for concurrent changes in stand structure and key foliar traits is important for the interpretation of plant diversity effects and predictions of associational patterns.

1. Overyielding in mixed-species forests has been demonstrated in a vast body of literature, and the focus of functional biodiversity research is now shifting towards a mechanistic understanding of these observations. 2. We explored diversity-productivity relationships at two sites of a large-scale tree diversity experiment, with harsh (Ged) and benign (Zed) environmental conditions for plantation establishment. Additive partitioning methodologies were adopted to detect phenomenological patterns in the productivity data, and the trait structure of mixed communities was used to advance insights into compositional effects. 3. After 6 years of plantation development, biomass productivity was significantly higher in mixtures compared to the monocultures of component species. We observed that processes operated through direct tree—tree interactions, as the diversity signal disappeared where trees in mixed stands were surrounded by conspecific neighbours only. This result is particularly relevant for mixed-species plantation systems, as trees are commonly planted in monospecific patches to simplify management. Partitioning unveiled strong selection effects at both plantation sites. However, at the harsh Ged-site this was caused by competitive dominance of species with fast young growth, whereas at the benign Zed-site, species with slow young growth improved their performances but not at the expense of others (i.e. trait-dependent complementarity). Species tolerance to shading is an influential trait for predicting biodiversity effects, with community-weighted means in shade tolerance mediating dominance effects (Ged) and functional diversity in shade tolerance mediating (trait-dependent) complementarity effects (Zed). 4. Synthesis. This study highlights that biodiversity effects in young tree plantations could be explained by the functional composition of mixed communities, with a key role for species levels of shade tolerance. As contrasting results between plantation sites were observed, future research should target the context-dependency of diversity-productivity relationships.