Species Associations in a Heterogeneous Sri Lankan Dipterocarp Forest

TLDR: This article used point pattern analysis to examine the spatial distribution of 46 common tree species (diameter at breast height 110 cm) in a fully mapped -m tropical forest plot in Sin- 500 # 500 haraja, Sri Lanka.

Abstract: We used point pattern analysis to examine the spatial distribution of 46 common tree species (diameter at breast height 110 cm) in a fully mapped -m tropical forest plot in Sin- 500 # 500 haraja, Sri Lanka. We aimed to disentangle the effect of species in- teractions (second-order effects) and environment (first-order ef- fects) on the species' spatial distributions. To characterize first-order associations (segregation, overlap), we developed a classification scheme based on Ripley's K and nearest-neighbor statistics. We sub- sequently used heterogeneous Poisson null models, accounting for possible environmental heterogeneity, to reveal significant uni- and bivariate second-order interactions (regularity, aggregation and re- pulsion, attraction). First-order effects were strong; overall, 53% of all species pairs occupied largely disjoint areas (segregation), 40% showed partial overlap, and 6% overlapped. Only 5% of all species pairs showed significant second-order effects, but about half of the species showed significant intraspecific effects. Significant plant-plant interactions occurred mostly within 2-4 m and disappeared within 15-20 m of the focal plant. While lack of significant species inter- actions suggests support for the unified neutral theory, species' ob- served spatial segregation does not support the assumptions of the neutral theory. The strong observed tendency of species to segregate may have supplementary effects on other processes promoting species coexistence.