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Abstract Forest canopy complexity (i.e., the three‐dimensional structure of the canopy) is often associated with increased species diversity as well as high primary productivity across natural forests. However, canopy complexity, tree diversity, and productivity are often confounded in natural forests, and the mechanisms of these relationships remain unclear. Here, we used two large tree diversity experiments in North America to assess three hypotheses: (1) increasing tree diversity leads to increased canopy complexity, (2) canopy complexity is positively related to tree productivity, and (3) the relationship between tree diversity and tree productivity is indirect and driven by the positive effects of canopy complexity. We found that increasing tree diversity from monocultures to mixtures of 12 species increases canopy complexity and productivity by up to 71% and 73%, respectively. Moreover, structural equation modeling indicates that the effects of tree diversity on productivity are indirect and mediated primarily by changes in internal canopy complexity. Ultimately, we suggest that increasing canopy complexity can be a major mechanism by which tree diversity enhances productivity in young forests. 

Summary Decades of studies have demonstrated links between biodiversity and ecosystem functioning, yet the generality of the relationships and the underlying mechanisms remain unclear, especially for forest ecosystems.Using 11 tree‐diversity experiments, we tested tree species richness–community productivity relationships and the role of arbuscular (AM) or ectomycorrhizal (ECM) fungal‐associated tree species in these relationships.Tree species richness had a positive effect on community productivity across experiments, modified by the diversity of tree mycorrhizal associations. In communities with both AM and ECM trees, species richness showed positive effects on community productivity, which could have resulted from complementarity between AM and ECM trees. Moreover, both AM and ECM trees were more productive in mixed communities with both AM and ECM trees than in communities assembled by their own mycorrhizal type of trees. In communities containing only ECM trees, species richness had a significant positive effect on productivity, whereas species richness did not show any significant effects on productivity in communities containing only AM trees.Our study provides novel explanations for variations in diversity–productivity relationships by suggesting that tree–mycorrhiza interactions can shape productivity in mixed‐species forest ecosystems. 

Abstract Plant diversity effects on community productivity often increase over time. Whether the strengthening of diversity effects is caused by temporal shifts in species-level overyielding (i.e., higher species-level productivity in diverse communities compared with monocultures) remains unclear. Here, using data from 65 grassland and forest biodiversity experiments, we show that the temporal strength of diversity effects at the community scale is underpinned by temporal changes in the species that yield. These temporal trends of species-level overyielding are shaped by plant ecological strategies, which can be quantitatively delimited by functional traits. In grasslands, the temporal strengthening of biodiversity effects on community productivity was associated with increasing biomass overyielding of resource-conservative species increasing over time, and with overyielding of species characterized by fast resource acquisition either decreasing or increasing. In forests, temporal trends in species overyielding differ when considering above- versus belowground resource acquisition strategies. Overyielding in stem growth decreased for species with high light capture capacity but increased for those with high soil resource acquisition capacity. Our results imply that a diversity of species with different, and potentially complementary, ecological strategies is beneficial for maintaining community productivity over time in both grassland and forest ecosystems. 

Abstract Human actions are decreasing the diversity and complexity of forests, and a mechanistic understanding of how these changes affect predators is needed to maintain ecosystem services, including pest regulation. Using a large‐scale tree diversity experiment, we investigate how spiders respond to trees growing in plots of single or mixed species combinations (4 or 12) by repeatedly sampling 540 trees spanning 15 species. In 2019 (6 years post‐establishment), spider responses to tree diversity varied by tree species. By 2021, diversity had a more consistently positive effect, with trees in 4‐ or 12‐species plots supporting 23% or 50% more spiders, respectively, compared to conspecifics in monocultures. Spiders showed stronger tree species preferences in late summer, and the positive impact of plot diversity doubled. In early summer, the positive diversity effect was tied to higher canopy cover in diverse plots, leading to higher spider densities. This indirect path strengthened in late summer, with an additional direct effect of plot diversity on spiders. Prey availability was higher in diverse plots but was not tied to spider density. Overall, diverse plots supported more predators, partly by increasing available habitat. Adopting planting strategies focused on species mixtures may better maintain higher trophic levels and ecosystem functions. 

Tree plantings have the potential to increase species diversity and sequester carbon, yet planting failure and early mortality pose significant barriers to their success. Biodiversity‐ecosystem function theory suggests that diverse tree plantings could improve survival outcomes through either the portfolio or facilitation effect, yet there remain few tests of this hypothesis. Here, we use a large‐scale tree‐diversity experiment (BiodiversiTREE), with monitoring of nearly 8,000 individual trees to test whether (1) tree species diversity increases survival rates, (2) tree diversity stabilizes the risk of planting failure, and/or (3) diversity effects are important relative to other common drivers of seedling mortality (e.g. herbivory and soil moisture). We found that only species identity significantly impacted the likelihood of survival, not plant functional diversity nor plot species richness nor phylogenetic diversity. There were minor effects of elevation and soil moisture on survival, but both explained a very small amount of variation in the data (r²_marg ≤ 0.011). Higher tree diversity did, however, strongly reduce variation in survival across plots, with nearly 2‐fold higher coefficients of variation in monocultures (30.4%, 28.4–32.6% 95% bootstrapped confidence interval) compared to 4‐ (16.3%, 13.8–18.7%) and 12‐species plots (12.8%, 10.8–14.7%). Ultimately, our results suggest that employing diverse species can lower the risk of planting failure (i.e. the portfolio effect), but that species selection still plays a large role in early establishment.