4.3. Ecosystem carbon storage change
Ecosystem carbon storage increased significantly with the stand basal area and was mainly affected by tree species composition (Hu et al., 2015).  Nitrogen-fixing tree species can accumulate more carbon than non-nitrogen-fixing tree species due to their fast-growing capacity (Chaer et al., 2011; Hoogmoed et al., 2014). The biomass carbon storage of reached 286.61 Mg ha-1 in AMOO stand. Similar or higher biomass carbon stock has been reported from other oak forests in central Himalaya (Sharma et al., 2011).  Variation in ecosystem carbon storage determined by the soil organic carbon and biomass carbon variations in stands. Moreover, the contribution of biomass carbon storage (Tree, shrubs, herbs, and litter) to ecosystem carbon storage decreasing with the stand basal area. Soil is the second-largest carbon storage after tree biomass. Soil organic carbon storage was 3.31, 31.21, 75.47, 157.04, 159.43, and 210.13 Mg ha-1 at the AER, APDF, ALR, AMOM, AMR, and AMOO stand respectively, representing 20.89 %, 17.0 %, 34.9 %, 40.2 %, 39.5 % and 42.0 % of total ecosystem carbon. In addition, nitrogen-fixing tree species have a significant impact on forest soil and increase the carbon content of the soil. The colonization of pioneer A. nepalensis tree has been improving tree seedling establishment by acting as ‘succession facilitators’ improving soil nutrient status through nitrogen-rich litter production and providing favorable microclimatic condition too late succession tree e.g. Q. leucotrichophora, R. arboreum in the different forest (Callaway, and Walker 1997; Walker, and Reddell., 2007; Frouz et al., 2015). In our study area, the nitrogen-fixing A. nepalensis was the dominant species in term of standing biomass in early succession stand (AER, APDF, and ALR), as well as in late succession stands (AMOO, AMR, and AMOO), and accounted for a large part of the biomass storage. One conceivable reason for its success could be its potential capacity for atmospheric nitrogen fixation, which could significantly increase the rate of accumulation of biomass (Temperton et al., 2003; Knoth et al., 2014; Uri et al., 2017; Brookshire et al., 2019).