Stable reverse J-shaped diameter distribution occurs in an old-growth karst forest

The reverse J-shaped diameter distribution is considered an inherent attribute of natural forests, crucial for forest resource utilization and community stability. However, in karst regions, intense habitat heterogeneity might alter species composition, spatial distribution, growth, biomass allocation, and mortality processes, yet its impact on diameter structure remains unclear. A fixed plot of 200 m × 110 m was established in the Nanpan River Basin, Southwest China, within an old-growth oak forest (> 300 years old), and the influence of site substrates (i.e., rock and soil), topographic factors, sample area, and orientation on diameter distribution was analyzed. Trees on both rock and soil exhibited a reverse-J shape, quantifiable through the Weibull function. The substrates had a similar density, approximately 2100 plants/ha. However, the average and range of diameter of trees on rock were smaller than those on soil, suggesting that rock constrains tree growth. The diameter distribution of trees across microtopography also displayed a reverse-J shape. Yet, higher elevations and sunny slopes showed a greater curvature of diameter classes compared to lower elevations and shady slopes, indicating habitat preferences in karst trees. Sample area and orientation had minimal effects on diameter class curve that reached stability when the plot size was 6000 m². These results suggest that the reverse J-shaped diameter distribution prevails at small scales in karst old-growth forests, encompassing multiple curvatures and spanning forest ecosystems.