Liaodong oak (Quercus liaotungensis) dominated forests contribute to the few natural forest ecosystems in the semiarid Loess Plateau region in China. This study investigated the influence of soil nutrients and stand structure on aboveground net primary productivity (ANPP) in the oak communities. We aimed to understand the major factors and their possible pathways that determine the ecosystem productivity. The biomass and ANPP were estimated using plot surveys and litterfall collection in fixed plots. Linear regressions and structural equation models were used to evaluate the relationships among soil nutrients, stand structure (including vertical and horizontal structure) and ANPP. The results showed that the effect sizes of soil nutrient on ANPP and leaf production were near 60% and 70%, respectively. More than 70% variance of woody production was explained by stand structure, while litterfall production was largely dependent on stand vertical structure with effect size of 64%. The effect of soil nutrients on stand structural indices was detected only on the vertical structure. The results suggested that soil nutrients could affect forest productivity both directly and indirectly in coordination with stand vertical structure, and that the effect of stand structure was limited in these communities.

Fine roots are the most active and functional component of root systems and play a significant role in the acquisition of soil resources. Density is an important structural factor in forest plantations but information on changes in fine roots along a density gradient is limited. In this study, plantations of black locust (Robinia pseudoacacia L.) and Chinese pine (Pinus tabuliformis Carr.) with four density classes were analyzed for the influence of soil and leaf traits on fine root growth. Fine root biomass increased with stand density. High fine root biomass was achieved through increases in the fine root production and turnover rate in the high-density black locust plantations and through an increase in fine root production in the pine plantations. In the high-density Chinese pine stand, there was a high fine root turnover which, coupled with high fine root production, contributed to a high fine root biomass. Overall, fine root production and turnover rate were closely related to soil volumetric water content in both kinds of plantations, while fine root biomass, especially the component of necromass, was related to soil nutrient status, which refers to phosphorous content in black locust plantations and nitrogen content in Chinese pine plantations. There was a close linkage between leaf area index and fine root dynamics in the black locust plantations but not in the pine plantations.