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Abstract

Local tree species distributions in tropical forests correlate strongly with soil
water availability. However, it is unclear how species distributions are shaped by
demographic responses to soil water availability. Specifically, it remains unknown
how growth affects species distributions along water availability gradients relative
to mortality.
2. We quantified spatial variation in dry season soil water potential (SWP) in the
moist tropical forest on Barro Colorado Island, Panama, and used a hierarchical
Bayesian approach to evaluate relationships between demographic responses of
naturally regenerating seedlings to SWP (RGRs and first‐year mortality) and species
distributions along the SWP gradient for 62 species. We also tested whether
species that were more abundant at the wet or dry end of the gradient performed
better (a) at their “home end” of the gradient (“best at home” hypothesis) and (b)
“at home” compared to co‐occurring species (“home advantage” hypothesis).
3. Four and five species responded significantly to SWP in terms of growth or mortality
respectively. Growth (but not mortality) responses were positively related
to species distributions along the SWP gradient; species with a more positive
(negative) growth response to SWP were more abundant at higher (lower) SWP,
that is, at wetter (drier) sites. In addition, wet distributed species grew faster on
the wet end of the SWP gradient than on the dry end (“best at home”) and grew
faster on the wet end than dry distributed species (“home advantage”). Mortality
rates declined with seedling size for all species. Thus, seedling growth responses
to SWP indirectly shaped local species distributions by influencing seedling size
and thereby mortality risk.
4. Synthesis. By demonstrating how growth responses to spatial variation in soil
water availability affect species distributions, we identified a demographic process
underlying niche differentiation on hydrological gradients in tropical forests.
Recognizing the role of these growth responses in shaping species distributions
should improve the understanding of tropical forest composition and diversity
along rainfall gradients and with climate change.