Neighborhood structure, more than soil nutrients, influences net tree interactions among different functional types in a temperate forest

• Considering functional trait differences offers new insights into testing the stress gradient hypothesis. • Heterospecific neighbors enhance facilitation for the survival of the focal tree. • SLA significantly affects whether the focal tree benefits from a diverse neighborhood structure. • Neighborhood structure plays a more crucial role than soil nutrients in determining interaction intensity. Tree-tree interactions are fundamental in shaping forest community dynamics, driven by factors such as nutrient availability and species composition. While recent research underscores the role of functional traits in influencing tree sensitivity to neighborhood interactions, a clear consensus is lacking on how these traits interact with biotic and abiotic factors. Based on a multi-level modeling approach, this study examines how specific leaf area (SLA), potential maximum tree height (Hmax), and wood density (WD) mediate the combined effects of soil nutrients and neighborhood biotic factors on net tree interactions among 4,210 individual stems for 13 species in a 30-ha temperate forest in northeastern China. The findings indicate that while functional traits independently account for a small portion of the variation in net tree interaction intensity (NInt C ), they significantly mediate the effects of neighborhood crowding, neighborhood structure, and soil nutrients on NInt C . Specifically, only the response of growth NInt C of species with low-Hmax to available phosphorus (AP) was consistent with the stress gradient hypothesis. Conspecific neighborhood crowding consistently increases competitive NInt C , whereas heterospecific neighborhood crowding significantly facilitates survival NInt C . Moreover, the positive impact of heterospecific crowding on survival NInt C is significantly greater for high-Hmax species compared to low-Hmax species. Neighborhoods with higher tree species diversity, greater size heterogeneity, and more uniform distribution increase the facilitative NInt C of high-SLA or high-WD species but intensify the competitive NInt C of low-SLA or low-WD species. Our results also indicate that the relative importance of neighborhood structure exceeds that of soil nutrients in explaining variation in NInt C . When environmental conditions are difficult to change, adjusting the forest stand structure may be a more efficient way to regulate the intensity of interactions between trees and improve forest productivity.