Optimization of stand structure in Robinia pseudoacacia Linn. based on soil and water conservation improvement function

The analysis of complex relationship between stand structure and soil and water conservation function (SWCF) of forests and the precise adjustment of stand structure based on an improvement in function are the key technical problems of forestry ecological engineering construction in the Loess Plateau. This study was conducted in the Caijiachuan watershed of the Loess Plateau in western Shanxi, China. Sixty standard plots (20 × 20 m2), including 20 stand densities (900 ∼ 2,800 plants·hm−2), of Robinia pseudoacacia L. were established in the watershed. The 10 factors of stand structure and four functional indices of the SWCF, including 29 sub-functional indices, were investigated and monitored each year from June to September in 2016 to 2019. The results indicated that there are differences in the SWCF under different stand structures, and the differences are concentrated in the water conservation function (FWC) and soil conservation function (FSC) of functional indices of the SWCF. In particular, they are reflected in the soil moisture and hydrological characteristics of the litter of sub-functional indices. Stand structure factors could effectively explain the differences of functional indices of the SWCF of a R. pseudoacacia forest, and a rate of 81.4% of functional indices could be explained by stand structure factors. Based on the configuration of the optimal stand structure (included a stand density of 1,800 plants·hm−2, mean diameter at breast height of 15.19 cm, mean tree height of 10.49 m, leaf area index of 2.86, canopy density of 0.85, uniform angle index of 0.50, story differentiation of 0.59, diameter dominance of 0.45, and biomass dominance of 0.46), the density of existing stands was concentrated in the range of 1,500 ∼ 1,900 plants·hm−2 and was more effective at optimizing the SWCF. The optimization strategy of the stand structure of the R. pseudoacacia forest in the study area should focus on increasing the differentiation of forest layer, enrich the vertical structure of the stand and maintain a random distribution pattern of trees. The overall growth of the stand remained uniform, avoiding the excessive dominance of individual trees and unreasonable stand density. These results will provide a more scientific and complete reference to optimize the stand structure of the R. pseudoacacia forest in the Loess Plateau.