Sustainable forest development in the face of global warming: Optimizing Robinia pseudoacacia L. stands to alleviate soil moisture depletion

To foster sustainable forest development amid global warming, it is imperative to enhance the quality of existing forest stands while avoiding excessive depletion of soil moisture. This study established 24 Robinia pseudoacacia L. standard plots (40 x 40 m(2)) in the Caijiachuan watershed, located in the gully region of the Loess Plateau, Shanxi Province, China, to monitor soil moisture content from 2007 to 2021. Additionally, the study examined the influence of 19 ecological factors, including climate, topography, and stand characteristics, on soil moisture. The findings indicated a sustained decline in soil moisture storage across the 0-200 cm soil profile, with significant depletion at 0-140 cm. The soil moisture content at depths of 80-200 cm was critically low, resulting in a "dry soil layer" during years with low precipitation. Climatic factors (average annual temperature and annual precipitation) and stand factors (uniform angle index, stand density, litter thickness, and Simpson diversity index of the shrubs) emerged as dominant factors impacting soil moisture variations. An increase in average annual temperature due to global warming, compounded by suboptimal stand density, primarily drove soil moisture depletion. Using a multivariate optimization equation to model the relationship between dominant factors and soil moisture storage, this study recommends a dynamic stand management strategy. This strategy involves selective thinning or replanting of Robinia pseudoacacia L. trees, targeting a gradual reduction in optimal stand density from 1500 to 1000 plants.ha(-1) over the next two decades. Additionally, transitioning stand spatial arrangements from uniform or clustered to random distributions, maintaining ideal litter thickness (2.43-2.47 cm), and enhancing shrub diversity are advisable. Such measures can alleviate soil moisture deficits in Robinia pseudoacacia L. stands attributable to global warming, thereby promoting the Loess Plateau's sustainable forestry advancement.