Trait-mediated neighborhood complementarity promotes tree growth in a temperate forest

Quantifying the effect of neighborhood functional composition on the performance of neighboring trees can help elucidate the processes of forest ecosystem formation. Traits may mediate niche and fitness differences of neighbors, as predicted by the niche-complementarity (absolute trait distance ATD, non-directional) and competitive-hierarchy (hierarchical trait distance HTD, directional) hypotheses. However, there is little understanding of the importance of these hypotheses in shaping neighborhood dynamics of forest tree communities, especially how trait-based niche complementarity and competitive hierarchy affect intra- and interspecific interactions, which regulate the growth of neighboring trees To address this knowledge gap, we conducted a field study in a temperate forest in China, focusing on Pinus koraiensis and Ulmus laciniata, two dominant tree species with distinct functional traits and resource-use strategies. Pinus koraiensis adopts a conservative strategy, characterized by functional traits such as slow resource-use efficiency and strong stress tolerance. In contrast, Ulmus laciniata employs an acquisitive strategy, typified by traits associated with high resource uptake and high environmental sensitivity. Our results showed that growth rates of both species increased with ATD, but not HTD. Specifically, interspecific complementarity (ATD_inter) positively influenced focal tree growth, whereas intraspecific complementarity (ATD_intra) did so only for the acquisitive species. Both ATD_inter and ATD_intra interacted with abiotic variables to affect the growth of acquisitive species, whereas the strength of the neighborhood complementarity effects did not vary with abiotic environment for the conservative species. Our findings suggest that the performance of individual trees may be regulated by neighborhood interactions that depend on intra- and interspecific trait differences, resource-use strategies, neighborhood density, and environmental conditions. This has implications for local or individual-based forest management, as well as for trait-based ecology from individual performance to forest ecosystem.