Age-Class-Based Thinning Affects Soil Fertility and Understory Diversity in Cunninghamia lanceolata Lamb. Plantations

Cunninghamia lanceolata Lamb. occupies a significant role in artificial forests globally, making its sustainable management crucial for terrestrial forest ecology. We experimentally determined soil physicochemical properties and the shrub and herb diversity of different age classes of Cunninghamia lanceolata plantations in Southwest China in 2023. The Mantel tests, RDA, and PLS-SEM were used to analyze the effects of stand factors on soil fertility and shrub and herb diversity. Shrub and herb diversity, as well as soil physicochemical properties, vary significantly across age classes in Cunninghamia lanceolata plantations. The maximum values of organic carbon, total nitrogen, total phosphorus, and available silicon were observed in the mature forest (36.62 g/kg, 1.90 g/kg, 0.53 g/kg, and 84.33 mg/kg, respectively), while the minimum values were found in the middle-aged forest (17.77 g/kg, 0.81 g/kg, 0.34 g/kg, and 53.70 mg/kg). TPH was the most influential stand factor. TBH was strongly correlated with RDA1 (r = 0.821, p < 0.001); soil organic carbon, total nitrogen, total phosphorus, and available silicon were negatively correlated with stand density. In this study, we propose a detailed age class-based thinning plan with strong implementability: cultivating large-diameter timber, maintaining soil fertility and understory plant diversity, and being friendly to forest farm management personnel. This approach could enhance biodiversity and ecosystem stability in Cunninghamia lanceolata plantations and serves as a reference for the sustainable management and operation of the Cunninghamia lanceolata forest ecosystem.