Spatial structure indirectly regulates C, N, P, and K allocation of Pinus kesiya var. langbianensis in natural forests

Clarifying the role of individual-tree neighborhood structure in shaping the allocation patterns of carbon (C), nitrogen (N), phosphorus (P), and potassium (K) in natural forests is fundamental to understanding tree resource-allocation strategies and to providing a theoretical basis for stand structure optimization and precision forest management. However, existing studies have largely focused on leaf nutrient traits and their environmental responses, while overlooking the regulatory role of spatial structure in organ-scale nutrient allocation. In this study, Pinus kesiya var. langbianensis was selected as the study species. Along gradients of tree-size status and competition intensity, 70 sample trees were destructively harvested, and the concentrations of C, N, P, and K in bark, branches, leaves, and wood were determined. We employed a unified framework that integrates tree-size status with spatial structure to quantify differences among organs in terms of element concentration, stoichiometric characteristics, storage capacity, and allocation proportions, and to assess how spatial structure influences the patterns of nutrient allocation across organ scales during tree differentiation. The results showed that (1) Significant inter-organ differences were detected in C, N, P, and K concentrations, stoichiometric relationships, and allocation proportions, indicating pronounced functional differentiation and elemental heterogeneity. (2) Tree-size status was the dominant driver of organ-scale nutrient traits and allocation patterns, whereas spatial structure exerted weaker but significant regulatory effects through neighborhood relationships and local competition, with bark, branches, and leaves showing the strongest responses. (3) Compositional data analysis (CoDA) further revealed that shifts in inter-organ element allocation were not independent changes in individual organs, but rather a coordinated redistribution process among bark, branches, leaves, and wood; (4) Partial least squares structural equation modeling (PLS-SEM) suggested that neighborhood spatial structure was associated with organ-scale elemental allocation both directly and through its covariation with tree-size status. Overall, organ-scale multi-element allocation in Pinus kesiya var. langbianensis is a dynamic process shaped by organ functional differentiation, dominated by tree-size status, and further regulated by spatial structure. Spatial structural effects are primarily manifested in the relationship between neighborhood scale and local competition intensity, providing a theoretical basis for improving nutrient use efficiency and management precision through the optimization of stand structure.