3.4. Driving factors of the ecosystem nitrogen and phosphorus stock
The structural equation model (SEM) revealed that tree biomass (A. nepalensis , R. arboreum , Q. leucotrichophora , and associated species), understory (herbaceous, shrub), litter), and soil (including microbial biomass) was the variable that influenced ecosystem N and P stock (Figure 8). The SEM explained 92.1 % and 92.8 % of the variation in ecosystem N and P stocks, respectively. A. nepalensis biomass N stock, soil TN stock, and litter N stock had a significant (P < 0.05) direct positive influence on ecosystem nitrogen stocks, in which standardized effects were 0.161, 0.619, and 0.601, respectively. Meanwhile, Q. leucotrichophora biomass N stock and associated species biomass N stock had a significant (P < 0.05) direct negative influence on ecosystem N stock, in which standardized effects were -0.142 and -0.112. Meanwhile, R. arboreum biomass N stock, MBN stock, and understory (herbaceous, shrub biomass) N stock had no direct influence on ecosystem N stock. Similarly, A. nepalensis biomass P stock, Q. leucotrichophora biomass P stock, associated species biomass P stock, MBP stock, and soil TP stock had a direct positive influence on ecosystem phosphorus stocks in which standardized effects were 0.497, 0.439, 0.112, 0.223 and 0.394, respectively. Meanwhile, R. arboreum , Q. leucotrichophora, and associated species biomass N and P stock had no direct effects on soil, microbial biomass, and understory (herbaceous, shrub biomass) N and P stock. Furthermore,R. arboreum , and associated species biomass N and P stock had a significant (P < 0.05) direct positive influence on litter biomass N and P stock. The total influence of each driving factor on the ecosystem N stock in ranked in the following order: 0.61 (soil TN stock) > 0.601 (litter biomass N stock) > 0.161 (A. nepalensis biomass N stock) > -0.142 (Q. leucotrichophora biomass N stock) > -0.112 (associated species biomass N stock). The standardized total effect of each driving factor on the ecosystem P stock in ranked in the following order: 0.497 (A. nepalensis biomass P stock) > 0.439 (Q. leucotrichophora biomass P stock) > 0.394 (soil TP stock) > 0.223 (MBP stock) > 0.112 (associated species biomass P stock) > -0.132 (understory biomass P stock). The results of standardized total influence revealed thatA. nepalensis biomass and soil N and P stocks were the most important direct and indirect driving factors of ecosystem N and P stocks (Figure 8).