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

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 meters. 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, on 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 analyzed 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.