Analysis of Key Factors for Natural Regeneration of Cypress Forests in the Karst Area of the Lijiang River

The natural regeneration of planted forests in karst landscapes is severely constrained by extreme substrate heterogeneity and fragile edaphic conditions. However, the relative importance and interaction pathways of environmental versus stand structural drivers remain poorly quantified. In this study, 54 plots (10 m × 10 m) were surveyed across Cupressus funebris plantations in the karst landscape of the Lijiang River Basin, southern China. To identify the key factors and causal pathways governing regeneration, redundancy analysis (RDA), variation partitioning, partial least squares structural equation modeling (PLS-SEM), and threshold analyses were applied. Regeneration exhibited pronounced spatial heterogeneity, with 42.6% of plots showing complete recruitment failure and a characteristic inverted J-shaped size class distribution. The analysis identified soil rock fragment content (as a negative constraint) and canopy gap area (as a positive driver) as the two dominant predictors. PLS-SEM revealed that environmental factors influence regeneration primarily through an indirect pathway mediated by stand structure (R2=0.683) rather than through direct effects. Threshold analyses identified quantitative benchmarks for key drivers, including a gap area breakpoint of approximately 10 m2 and a presence–absence effect of soil rock fragments. These findings contribute to a more sophisticated mechanistic understanding of forest regeneration in karst ecosystems and provide an empirical foundation for silvicultural management that aims to encourage natural regeneration and ecological restoration of degraded karst plantations.