Marked tree demographic variation along subtle elevation differences partially explains species' habitat associations in an Amazonian forest

Abstract 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 ( best‐at‐home hypothesis), and they should outperform other species that are found, but not strongly aggregated, in that habitat ( resident‐advantage hypothesis). 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 best‐at‐home or 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. Synthesis . 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.