Drought tolerance as a driver of tropical forest assembly: Separating spatial signatures for competition from habitat associations
Spatial patterns in trait variation reflect underlying community assembly processes, allowing us to test hypotheses about their trait and environmental drivers by identifying the strongest correlates of characteristic spatial patterns. For 43 evergreen tree species (>1cm dbh) in the seasonally dry Xishuangbanna Tropical Botanical Garden forest plot in Yunnan, China, we compared the ability of drought tolerance traits, physiological traits, and commonly measured functional traits to predict the spatial patterns expected from the assembly processes of habitat associations, niche overlap-based competition, and hierarchical competition. To identify spatial signatures for competition, we used a wavelet method to decompose spatial correlation into independent values across scales to distinguish neighborhood-scale (0- 20m) patterns from larger-scale habitat associations.
Species’ drought tolerance and habitat variables related to soil water supply were strong drivers of habitat associations, and drought tolerance showed a significant spatial signal for influencing competition. Overall, the strongest traits associated with habitat across the plot, as quantified using multivariate habitat models, were leaf density, leaf turgor loss point (πtlp; also known as the leaf wilting point), and stem hydraulic conductivity traits KS and KL (r2 range for the best fit models = 0.27-0.36). At neighborhood scales, spatial aggregation among species was positively correlated with similarity in πtlp, consistent with predictions based on hierarchical competition. Although the correlation between πtlp and interspecific spatial associations association was weak (r2 < 0.01), this indicated a persistent signature of the influence of drought tolerance on neighborhood interactions and community assembly. Quantifying the full impact of traits on competitive interactions in forest communities may require incorporating plasticity among individuals within species, and thus focusing on specific life stages, and moving beyond individual traits to approaches for estimating the influence of multiple traits on whole-plant performance and resource demand.