Limiting similarity shapes the functional and phylogenetic structure of root neighborhoods in a subtropical forest.

Environmental filtering and limiting similarity mechanisms can simultaneously structure community assemblages. However, how they shape the functional and phylogenetic structure of root neighborhoods remains unclear, hindering the understanding of below-ground community assembly processes and diversity maintenance. In a 50-ha plot in a subtropical forest, China, we randomly sampled > 2700 root clusters from 625 soil samples. Focusing on ten root functional traits measured on 76 woody species, we examined the functional and phylogenetic structure of root neighborhoods and linked their distributions with environmental cues. Functional overdispersion was pervasive among individual root traits (50% of the traits) and accentuated when different traits were combined. Functional clustering (20% of the traits) seemed to be associated with a soil nutrient gradient with thick roots dominating fertile areas whereas thin roots dominated infertile soils. Nevertheless, such traits were also sorted along other environmental cues, showing multidimensional adaptive trait syndromes. Species relatedness was also an important factor defining root neighborhoods, resulting in significant phylogenetic overdispersion. These results suggest that limiting similarity may drive niche differentiation of coexisting species to reduce competition, and that alternative root strategies could be crucial in promoting root neighborhood resources use and species coexistence.