Modeling Forest Age and NPP and Developing Site Index Tables for the Manas River Basin, Xinjiang

The dynamic changes in Net Primary Productivity (NPP) with forest age are key to assessing forest carbon sink capacity. This study aims to construct an age-NPP relationship model for the main tree species in the Manas River Basin of Xinjiang and compile a site index table to reveal the evolution of forest productivity and its site dependence in the region. The findings will provide scientific basis for precise carbon sink assessments and sustainable forest management in the area. Based on the forest resource survey data of Category I and Category II, five dominant tree species— Spruce , Poplar , Elaeagnus angustifolia , Populus euphratica , and Mixed wood —were fitted with five functional models (SEM, SDP, L, M, and Γ) to establish the forest age-NPP relationship and select the optimal model. At the same time, growth processes of dominant trees’ height were fitted using guiding curve methods such as the Schumacher formula, Richards formula, Korf formula, Gompertz formula, and Weibull formula to determine the baseline age and index distance, and a site index table was compiled. Finally, a point-check method was used for accuracy verification. The results showed that: (1) SEM and Γ functions were the best models for simulating the forest age-NPP relationship. The NPP of dominant tree species follows a "initial growth-middle-aged peak-senescence decline" dynamic pattern, but the peak age varies significantly, reflecting the regional characteristic that drought stress delays the forest development process. (2) Site index tables for each tree species were compiled, and the point-check rate was ≥ 94% for all species. The site index levels revealed a significant gradient of site quality within the basin. The site index (SI) for Spruce ranged from 16 to 24 m, indicating sensitivity to environmental gradients; the SI for Poplar ranged from 15 to 19 m, indicating stronger adaptability. The site index was significantly positively correlated with the NPP peak, suggesting that site quality is a key factor controlling the forest carbon sink potential. Based on this, carbon sink capacity assessments can be made under different site conditions within the basin. The forest age-NPP model and site index table developed in this study can accurately depict the temporal and spatial variation of forest productivity in the Manas River Basin. The results not only deepen the theoretical understanding of forest carbon cycling in arid areas but also provide direct quantitative tools and decision-making support for developing differentiated forest management strategies, such as harvesting plans based on site quality, ecological restoration, and enhancing regional forest carbon sink functions.