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Numerous studies have shown reduced performance in plants that are surrounded by neighbours of the same species^1,2, a phenomenon known as conspecific negative density dependence (CNDD)³. A long-held ecological hypothesis posits that CNDD is more pronounced in tropical than in temperate forests^4,5, which increases community stabilization, species coexistence and the diversity of local tree species^6,7. Previous analyses supporting such a latitudinal gradient in CNDD^8,9have suffered from methodological limitations related to the use of static data^10–12. Here we present a comprehensive assessment of latitudinal CNDD patterns using dynamic mortality data to estimate species-site-specific CNDD across 23 sites. Averaged across species, we found that stabilizing CNDD was present at all except one site, but that average stabilizing CNDD was not stronger toward the tropics. However, in tropical tree communities, rare and intermediate abundant species experienced stronger stabilizing CNDD than did common species. This pattern was absent in temperate forests, which suggests that CNDD influences species abundances more strongly in tropical forests than it does in temperate ones¹³. We also found that interspecific variation in CNDD, which might attenuate its stabilizing effect on species diversity^14,15, was high but not significantly different across latitudes. Although the consequences of these patterns for latitudinal diversity gradients are difficult to evaluate, we speculate that a more effective regulation of population abundances could translate into greater stabilization of tropical tree communities and thus contribute to the high local diversity of tropical forests.

One mechanism proposed to explain high species diversity in tropical systems is strong negative conspecific density dependence (CDD), which reduces recruitment of juveniles in proximity to conspecific adult plants. Although evidence shows that plant-specific soil pathogens can drive negative CDD, trees also form key mutualisms with mycorrhizal fungi, which may counteract these effects. Across 43 large-scale forest plots worldwide, we tested whether ectomycorrhizal tree species exhibit weaker negative CDD than arbuscular mycorrhizal tree species. We further tested for conmycorrhizal density dependence (CMDD) to test for benefit from shared mutualists. We found that the strength of CDD varies systematically with mycorrhizal type, with ectomycorrhizal tree species exhibiting higher sapling densities with increasing adult densities than arbuscular mycorrhizal tree species. Moreover, we found evidence of positive CMDD for tree species of both mycorrhizal types. Collectively, these findings indicate that mycorrhizal interactions likely play a foundational role in global forest diversity patterns and structure.

As the quality and quantity of natural habitats decrease, pressure increases to better understand species–habitat interactions and how animal communities respond to habitat changes. We assessed the relative importance of local habitat heterogeneity and productivity measures as predictors of avian species richness and compared these results to models for species of conservation concern (SCC). We derived three‐dimensional habitat heterogeneity and productivity measures from light detection and ranging data and hyperspectral imagery, and then used a Bayesian multi‐species hierarchical framework to model avian species richness and occupancy. We found both habitat heterogeneity and productivity were important factors for determining avian community richness. Three‐dimensional habitat heterogeneity and productivity metrics accurately predicted species richness at a local scale and were especially important to use within habitat guilds (i.e., alpha diversity). When scaling up to community richness across multiple habitat types (i.e., gamma diversity), two‐dimensional (surface level) productivity and heterogeneity metrics became important additions to the three‐dimensional metrics when estimating total avian richness. We also tested the utility of these metrics for predicting occupancy of SCC and compared community‐level relationships to species‐specific relationships. Species of conservation concern differed from the broader avian community with regard to local habitat heterogeneity and productivity measures. Species of conservation concern had different relationship habitat metrics than the greater avian community. Three‐dimensional measures of habitat heterogeneity and productivity predicted avian richness across the landscape, yet also highlighted the different habitat structure needs of SCC compared with the greater avian community.

Interspecific interactions can provoke temporal and spatial avoidance, ultimately affecting population densities and spatial distribution patterns. The ability (or inability) of species to coexist has consequences for diversity and ultimately ecosystem stability. Urbanization is predicted to change species interactions but its relative impact is not well known. Urbanization gradients offer the opportunity to evaluate the effect of humans on species interactions by comparing community dynamics across levels of disturbance.

As climate change drives increased drought in many forested regions, mechanistic understanding of the factors conferring drought tolerance in trees is increasingly important. The dendrochronological record provides a window through which we can understand how tree size and traits shape growth responses to droughts.