Effects of Stand Density on Growth, Soil Water Content and Nutrients in Black Locust Plantations in the Semiarid Loess Hilly Region

Stand density is an important index of forest structure, which strongly affects local environments and functions in the forest. Many black locust (Robinia pseudoacacia) plantations with low quality in the Loess hilly region are assumed to be caused by inappropriate stand density. In this study, the growth status, spatio-temporal variations in soil water and nutrient conditions were investigated in the nearly middle-aged plantations with three density classes. The proportion of stunted trees increased with the increase in density class. The stands of <2500 stems ha−1 not only had the distribution peak of diameter at breast height (DBH) being at a larger diameter class, but also showed relatively rapid growths in diameter and biomass per tree. However, stand density did not show a significant effect on the growth rate of both mean tree height and biomass density. The maximum biomass density and relatively high soil NH4+-N content appeared in the density class of 2500–3500 stems ha−1. The temporal stability of soil water content (SWC) on a seasonal scale increased with the deepening of the soil layer, and spatio-temporal variations in the SWC remained relatively stable in the deep layer (200–300 cm). While the infiltration depth after rainfall was rainfall-amount-dependent, the depth of effective replenishment reduced with the density class increasing. The average SWC and its temporal stability in 0–300 cm of soil layer are the best in a stand density of less than 2500 stems ha−1. No significant differences were observed among the stand density classes in the contents of total nitrogen, total phosphorus, NO3−-N, and available phosphorus in soils of these nearly middle-aged plantations that have experienced similar management history since the afforestation of abandoned cropland. Overall, the stand condition of density class I is superior at present. But thinning of stand density may be needed to maintain the best stand conditions in the future, with the stand age increasing. The results contribute to further understanding of the relations between density gradient and multiple variables in the plantations, which offer a reference for the forest management and sustainable development of ecosystems in the semiarid region of the Loess Plateau.