Plant–soil feedback (PSF), regulated by both mycorrhizae and soil pathogens, is a primary mechanism maintaining high tree diversity in the tropics. But how warming may affect PSF is not well understood. We conducted warming experiments on seedling survival of a rhizobia-associated species (Ormosia semicastrata) carrying coevolved host-specific soil-borne pathogens and an ectomycorrhizal fungi-associated species (Cyclobalanopsis patelliormis) immune to soil-borne pathogens. We found elevated temperature lowered seedling mortality of O. semicastrata, but had no detectable effect on C. patelliormis seedlings. This indicates that warming weakened the negative PSF on O. semicastrata but did not affect the positive PSF on C. patelliormis. The differential warming effects on seedling mortality of species with different microbial associations imply that global warming could lead to change in tropical tree composition. Our study also predicts that global warming would undermine the role of the JC effect in maintaining tree species diversity in the tropics.

As primary drivers of underlying microbial changes, soil metals have been extensively studied in agroecosystems. However, while their contributions to forest soil microbial processes are crucial for maintaining tree biodiversity, they remain poorly understood. Based on the analysis of 1287 soil samples collected from a 20 ha forest plot, we show that seven metal elements (Al, Ca, Cu, Fe, Mg, Mn, and Zn) shape microbial community structure and function by initially altering influential microbial taxa. Microbial α-diversity and community structure responded differently to these elements at low vs. high C:N ratios, pH, and water content. Moreover, these elements also affected microbial functional guilds (e.g., phosphorus and sulfur metabolism, ectomycorrhizae, plant pathogens, and wood saprotrophs) via sensitive microbial taxa. This study advances our capacity to predict belowground microbial processes by revealing the fundamental importance of metals in forest soils, with important implications for better conserving forest biodiversity under global climate change.

1. The dissimilarity and hierarchy of trait values that characterize niche and fitness differences, respectively, have been increasingly applied to infer mechanisms driving community assembly and to explain species co-occurrence patterns. Here, we predict that limiting similarity should result in the spatial segregation of functionally similar species, while functionally similar species will be more likely to co-occur together either due to environmental filtering or competitive exclusion of inferior competitors (hereafter hierarchical competition). 2. We used a fully mapped 50-ha subtropical forest plot in southern China to explore how pairwise spatial associations were influenced by trait dissimilarity and hierarchy between species in order to gain insight into assembly mechanisms. We assessed pairwise spatial associations using two summary statistics of spatial point patterns at different spatial scales and compared the effects of trait dissimilarity and trait hierarchy of different functional traits on the interspecific spatial associations. These comparisons allow us to disentangle the effects of limiting similarity, environmental filtering and hierarchical competition on species co-occurrence. 3. We found that trait dissimilarity was generally negatively correlated with interspecific spatial associations, meaning that species with similar trait values were more likely to co-occur together and thus supporting environmental filtering or hierarchical competition. We further found that leaf area, wood density and maximum height had stronger trait hierarchy effects on the pairwise spatial associations relative to their corresponding trait dissimilarity effects, which suggests that hierarchical competition played a more (or at least equally) important role in structuring our forest community compared to environmental filtering. 4. This study employed a novel method to disentangle the relative importance of multiple assembly mechanisms in structuring co-occurrence patterns, especially the mechanisms of environmental filtering and hierarchical competition, which lead to indistinguishable co-occurrence patterns. This study also reinforced the importance of trait hierarchy rather than trait dissimilarity in driving neighborhood competition.