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It is widely recognized that large‐scale topographic variation affects the distribution of tree diversity, yet the effects of topography at smaller scales are less appreciated but can be no less consequential. We evaluated how small‐scale topographic variation affects tree demography and diversity in a hyperdiverse Amazonian forest where species distributions respond strongly to elevation differences as small as 22 m. For topographically structured species distributions to arise, species should grow and survive (perform) better in the topographic habitat they are associated with, and they should outperform other species that are found, but not strongly aggregated, in that habitat. Here, we tested these demographic hypotheses using data on the growth and mortality of 79,911 trees (352 species) among three topographic habitats (valleys, slopes and ridges) in the 25‐ha Amacayacu Forest Dynamics Plot.Despite the small variation in elevation, there was significant community‐level variation in growth and mortality among topographic habitats: trees growing in valleys, where soil moisture is higher, had significantly higher growth and mortality rates than those growing on slopes and ridges. However, tree growth rates did not depend on, and mortality rates varied inconsistently with, species' habitat association. Our results partially support the resident‐advantage hypotheses for valley‐associated species, which grew best in their home habitat (valleys) than elsewhere and had lower mortality there compared to slope‐associated or generalist species (foreigners). For slope‐ and ridge‐associated species, our results did not support these hypotheses at the community level. Species‐specific analyses revealed that 73 out of the 352 species analysed at the community level supported either hypothesis. Our findings show that even small differences in elevation can lead to biologically meaningful variation in resource access that translates into significant differences in tree growth and survival. However, resource access could not fully explain the patterns of topographically driven demographic variation we observed. While certain species may still exhibit home and resident advantages in specific habitats, even when community‐level averages partially reflect this pattern, alternative hypotheses are likely driving the patterns observed at the community level. 

The future trajectory of global forests is closely intertwined with tree demography, and a major fundamental goal in ecology is to understand the key mechanisms governing spatio‐temporal patterns in tree population dynamics. While previous research has made substantial progress in identifying the mechanisms individually, their relative importance among forests remains unclear mainly due to practical limitations. One approach to overcome these limitations is to group mechanisms according to their shared effects on the variability of tree vital rates and quantify patterns therein. We developed a conceptual and statistical framework (variance partitioning of Bayesian multilevel models) that attributes the variability in tree growth, mortality, and recruitment to variation in species, space, and time, and their interactions – categories we refer to asorganising principles(OPs). We applied the framework to data from 21 forest plots covering more than 2.9 million trees of approximately 6500 species. We found that differences among species, thespeciesOP, proved a major source of variability in tree vital rates, explaining 28–33% of demographic variance alone, and 14–17% in interaction withspace, totalling 40–43%. Our results support the hypothesis that the range of vital rates is similar across global forests. However, the average variability among species declined with species richness, indicating that diverse forests featured smaller interspecific differences in vital rates. Moreover, decomposing the variance in vital rates into the proposed OPs showed the importance of unexplained variability, which includes individual variation, in tree demography. A focus on how demographic variance is organized in forests can facilitate the construction of more targeted models with clearer expectations of which covariates might drive a vital rate. This study therefore highlights the most promising avenues for future research, both in terms of understanding the relative contributions of groups of mechanisms to forest demography and diversity, and for improving projections of forest ecosystems. 

Abstract 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.