Habitat heterogeneity in karst environments influences the proportion and distribution of random framework

In the absence of exogenous disturbance, natural forests usually display stable and orderly distribution patterns. Recent studies have demonstrated that the random framework (RF), based on the locations of four neighboring trees, remains stable in non-karst forests. However, it is not clear whether tree size, lifeform (tree vs shrub), and habitat heterogeneity affect the proportion and distribution of RF. We established a 200 × 110 m fixed plot in an old-growth karst forest in southwest China. Trees were divided into 20 different-sized groups based on minimum measured diameters (MMDs) of 1–20 cm, and the proportion and distribution of various frameworks (clumped, random, and regular) of the quadrat and trees and shrubs growing on rock and soil were analyzed using the uniform angle index (W). The results indicated that: (1) the proportion of RF was stable at the stand level, and accounted for approximately 58% of trees in all MMD classes; (2) With increasing MMD, the proportions of trees and trees + shrubs growing on soil also increased, while the univariate distribution of W maintained an ideal normal distribution. The distribution patterns shifted from aggregated to random, and then to regular. On rock, the proportion of the RF decreased with increasing MMD; clumped distribution patterns were seen, and the mean value of W was discrete; (3) The proportion of RF decreased with increasing MMD for shrubs growing on soil, but increased for shrubs growing on rock. Shrubs were predominantly clumped; (4) A linear relationship between the relative proportions of frameworks and tree size was observed. Our results suggest that the proportion of RF in karst forests is scarcely affected by tree size, whereas markedly altered by habitat heterogeneity and lifeform. The tradeoff between clumped and regular frameworks alters distribution patterns, and habitat heterogeneity promotes aggregation.