Growth responses to soil water potential indirectly shape local species distributions of tropical forest seedlings

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. 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). 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. 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.

Performance of tropical forest seedlings under shade and drought: an interspecific trade-off in demographic responses

“…weaker performance, with increasing shade or drought (Supplementary Table S1.1). As reported earlier from these seedling data 34 , relative growth rates decreased and survival increased significantly with height for the large majority of species (86% and 76% of species, respectively, see Supplementary Table S1.1). Explained variance (R 2 ) was 0.24 in the growth model and 0.12 in the survival model.

Figure 1Relationship between observed and fitted relative growth rate (RGR, upper panels) and survival rate (lower panels) and shade ( a,d ), spatial drought ( b,e ) and inter-annual drought ( c,f ) of the abundant treelet Faramea occidentalis .

…”

Floristic similarity between adult and saplings strata of Caatinga hyperxerophilous woody vegetation – a Brazilian seasonally tropical dry forest

“…In addition to this fact, given the great importance of water availability in tropical dry forests (TDF), tree species are expected to have a non random pattern of spatial distribution along water availability gradients, as well as differential responses of individual species [9], [10]. It is also important to highlight the importance of vegetative reproduction strategies for the perpetuation of treeshrub communities in dry tropical forests.…”

“…However, factors with uniform spatial distribution on a local scale (e.g. precipitation regimes, typology and edaphic attributes or even the implications of anthropogenic disturbances) can contribute to communities with similar and homogeneous characteristics from a composition and structure standpoint [9], [10]. These communities can be made up of by a small number of dominant species, that are responsible for the key-processes that structure and control the main mechanisms of existence and persistence of these ecosystems [1], [11], [12], [13].…”

Dry season soil water potential maps of a 50 hectare tropical forest plot on Barro Colorado Island, Panama

“…Consequently, water availability strongly influences plant performance, species distributions, functional composition and ecosystem functioning across biomes 2–6 . On local scales, spatial variation in soil moisture differentially affects performance among species 7–9 , promoting niche differentiation in plant communities and fostering coexistence 10 . Understanding how local soil moisture variation affects plants will become increasingly important, given the predicted shifts in rainfall patterns caused by climate change and their expected effects on plant performance, community composition and species distributions 2,4,11 .…”

“…In tropical forests, local variation in soil moisture causes tree species to perform differently among habitats 12–14 , which promotes habitat associations and may contribute to the maintenance of high species diversity in these forests 8,15,16 . However, soil moisture also affects species performance and distributions at smaller scales than habitats, highlighting the importance of measuring fine-scale spatial variation in soil moisture 9 . Most studies that link species performance to soil moisture have measured soil water content 17–20 .…”