Effects of Planting Density and Site Index on Stand and Soil Nutrients in Chinese Fir Plantations

This study investigated the effects of planting density and site index on stand attributes and soil nutrients in mature Chinese fir [ Cunninghamia lanceolata (Lamb.) Hook.] plantations across Fujian and Sichuan Provinces, elucidating the pathways through which these factors influence standing volume (SV). The results showed that (1) planting density significantly affected stand variables, with average diameter at breast height (ADBH) decreasing and SV initially increasing and then declining with higher density. The number of mortality plants (NMP) and actual stand density (ASD) both increased significantly with higher density. Average tree height (ATH) and dominant height (DH) responses varied by region, with ATH decreasing in Sichuan and DH decreasing in Fujian with higher density. (2) Planting density affected soil nutrients differently in the two provinces, with soil total potassium (TK) increasing in Fujian and phosphorus decreasing in Sichuan. (3) Site index was positively correlated with ATH and ADBH but negatively correlated with ASD and NMP. Its relationship with soil nutrients was province-specific: in Fujian, site index was negatively correlated with total phosphorus (TP) and positively correlated with TK and soil pH, while in Sichuan it was only positively correlated with TK. (4) Structural equation modeling revealed different regulatory pathways: in Fujian, planting density influenced SV through both ASD and soil nutrients, while in Sichuan it affected only through ASD. This study highlights the region-specific interactions between planting density, site index, stand structure, and soil nutrients, providing a foundation for optimized plantation management.