Scale-Dependent Stand Spatial Structure and Crown Competition in a Subtropical Mixed Forest Based on Individual-Tree Spatial Data

Background and Objectives: Aggregating mapped trees into larger cells necessarily reduces some among-cell variation; ecological scale responses must therefore be distinguished from arithmetic averaging. We evaluated stand structure and crown competition across spatial grains and tested whether the observed spatial contrast exceeded a fixed-pattern random-label benchmark. Materials and Methods: A mapped inventory from Nanshan Nature Reserve, Hunan Province, China, contained 114,524 cleaned stems (DBH ≥ 1.0 cm) from 297 recorded species over approximately 24 ha. Four-nearest-neighbor structural indices, Hegyi competition (CI), crown-width distance competition (CCI), and circular crown overlap (COI) were calculated for every stem and summarized in 5–50 m grids. The analyses included 199 random-label permutations, pair- and mark-correlation functions, 50 m spatial-block bootstrap resampling, and a complete reanalysis restricted to DBH ≥ 5 cm. Results: At 20 m, the observed among-cell coefficients of variation for Hegyi, COI, and mingling were 2.31, 8.73, and 3.24 times their random-label medians, respectively, whereas crown occupancy was below its null envelope. Hegyi exceeded the 95% null envelope at fine and intermediate grains but not at 40 or 50 m, where few cells remained. Pair correlation exceeded one from 0 to 20 m under both DBH thresholds, and conspecific association was strongest at short distances. As expected from the DBH-based crown model, CCI was strongly associated with Hegyi at tree level (ρs=0.953), whereas COI was less strongly associated (ρs=0.236). At 20 m, spatial-block bootstrap intervals were 0.818–0.878 for Hegyi–CCI and 0.294–0.484 for Hegyi–COI. Reanalysis of 38,128 stems with DBH ≥ 5 cm preserved rank patterns but changed local Hegyi hotspot membership (Jaccard similarity 0.222); COI hotspots were more stable (0.658). Conclusions: Fine and intermediate grains retained spatially concentrated competition and mixing signals beyond arithmetic averaging, but no single grain was universally optimal. The results support 10–20 m units for local screening and 30–50 m units for broader summaries, with explicit attention to stem-size thresholds and coarse-grain uncertainty.