3.1. Spatial variation of forest stand characteristics
The total basal area (TBA), species density, and DBH were significantly different across the stands. In contrast, TBA of different stands was lowest in AER (5.2 ±0.1 m2 ha-1) and highest in AMOO (88.39±4.88 m2 ha-1) (P<0.05). In AMR and AMOO forest stands, the stem sizes distributed among upper diameter classes of 20-50 cm and >50 and resulted in the higher TBA (87.90 m2 ha-1 and 88.39 m2 ha-1, respectively. A large heterogeneity of tree diameter was observed among the stands (Figure 2). Results confirmed the dominance of A. nepalensis that occurred almost exclusively in the DBH category ≥30 cm except in AER stand.  All the studied stands were predominantly composed of three species A. nepalensis , Q. leucotrichophora, and R. arboreum. The most abundant species by the number of stems was Q. leucotrichophora and R. arboreum and comprised 40-80 % of all stems. The proportion of individuals in the small-diameter class (<30 cm DBH) was higher in AER, ALR, APDF, and AMOM stand as compared to AMR and AMOO stands, where the proportion of individuals in the largest diameter class (>30 cm DBH) was higher (Table 2). Stand density was higher for young AER and ALR stands which declined in mature and older (AMR and AMOO) stands. In contrast, the stem diameter of AMOM stand was mostly confined within the diameter ranges of 10- 30 cm (Figure 2).
Species in the medium and large DBH classes were the primary contributors to tree biomass carbon storage in AMOM, AMR, and AMOO stands whereas the small DBH class was the primary contributor to biomass carbon in AER, ALR and APDF stand (Table 2).  Although a majority of biomass carbon storage was contributed by trees of medium and large DBH classes in AMOM, AMR, and AMOO stand, the presence of a higher number of small trees in AER and ALR stand indicate for higher capacity of forest regeneration and thus a great potential for carbon accumulation. The higher contribution of large A. nepalensistrees to the biomass carbon found in the present study. In AER, APDF and ALR stand there were higher small trees than AMOM, AMR, and AMOO stand leading to a proportionally greater contribution of larger A. nepalensis trees to ecosystem carbon pools.
Dynamics of biomass carbon storage
Tree biomass carbon and ecosystem biomass carbon of A. nepalensisstands increased with stand basal area (p<0.01) (Table 3). Based on the already developed allometric equations, the biomass of tree components was estimated (Table 4). Different tree components bole, branch, twig, foliage, stump root, lateral root, and fine root showed the same trend with an increase in stand basal area and followed the order of increase AER>ALR>APDF> AMOM>AMR>AMOO (Figures 3a, b). Total biomass carbon (tree, shrubs, herbs, and litter biomass carbon) and ecosystem biomass carbon increased from 12.54 Mg ha-1to 15.85 Mg ha-1, respectively, in AER stands to 289.85 Mg ha-1to 500 Mg ha-1, respectively in AMOO stand. Across the stands examined, tree bole biomass carbon contributed more to total tree biomass carbon than any other tree components.  The understory vegetation (herbs and shrubs) and litter carbon in AER, ALR, APDF, AMOM, AMR, and  AMOO stands were 3.06, 4.49, 7.14, 6.02, 7.92 and 6.36 Mg ha-1, respectively. The litter biomass carbon increased with an increase in stand basal area, peaking in the AMR stands at 3.56 Mg ha-1 (Figure 3a).
The contribution of A. nepalensis to total biomass carbon was higher in APDF and AMR stands whereas the contribution of Q. leucotrichophora to biomass C was higher in the rest of the stands. The proportion of the bole and branch biomass carbon was the highest in AMR or AMOO and lowest in APDL. Fine root biomass carbon was lowest in APDL and highest in the AMOM while catkin biomass carbon contribution was lowest in all stands (Table 4). The litter and total ecosystem biomass showed a positive correlation with the forest basal area. The understory vegetation (herbs and shrub) did not show any trend with forest basal area. Moreover, the total biomass carbon of trees was always higher than that in the understory vegetation and litter biomass carbon. The highest above ground biomass carbon was found under AMOO (231.5 Mg ha-1) followed by AMOM (183.58 Mg ha-1) and belowground biomass carbon under AMOO (55.2 Mg ha-1) followed by AMOM stand (54.9 Mg ha-1).
Dynamics of soil organic carbon storage
In all the stands, the SOC (%) decreased significantly with an increase in soil depths, while soil bulk density showed the reverse trends (Fig.4). The SOC % at 0-10 cm was higher than that of other soil depths. Average total soil C stock in AER (0-10 cm), APDF (0-30 cm), ALR (0-100 cm), AMOM (0-100 cm), AMR (0-100 cm), and AMOO (0-100 cm) was 3.31, 31.21, 75.47, 157.04, 159.43 and 210.13 Mg ha-1, respectively (Figure 4).
Change in ecosystem carbon stock
The ecosystem carbon stock increased significantly with stand basal area from 15.85 Mg ha-1 in the AER with TBA 5.2 m2ha-1 to 500.09 Mg ha-1 in the AMOO stand with TBA 88.3 m2 ha-1 Tree biomass carbon stock, soil organic carbon stock, and ecosystem carbon stock showed a positive correlation with stand basal area (P<0.01) and A. nepalensis basal area (P<0.01) (Figure 5a). The biomass carbon along with soil organic carbon increased with an increasing basal area of the stand (Figure 5b). The relationship between stand basal area and ecosystem carbon storage was y=4.96254x+27.534 (R2=0.8959) was developed. In the case of A. nepalensis basal area to ecosystem carbon storage was, y=6.8711x+91937 (R2= 0.9032), and slope indicate that for the increase in A.nepalensis basal area, ecosystem carbon storage increased. Similarly, stand basal area and A.nepalensis basal area showed a positive increment with tree biomass carbon storage and SOC storage (Figure 5). Across the study sites, the percentage of biomass carbon contributed by A. nepalensis to the total ecosystem carbon storage ranged from 7.07 % (in ALR stand) - 63.20 % in AER stand). Similarly, Q. leucotrichophora contributed 5.07- 26.95 %.R. arboreum 1.43-19.36 % and associated species 8.31-21.58 % to the total ecosystem C stock. The shrubs, herbs, and litter contributed a small portion to the total ecosystem C stock of shrubs, and herbs C stock did not vary with stand basal area whereas litter biomass carbon showed a linear increase with a basal area of stands (Figure 6a). The vegetation biomass carbon storage contributed 77.08 %, 82.20 %, 63.92 %, 58.95 %, 59.62 % and 57.31 % of the total ecosystem carbon stock to the AER, APDF, ALR, AMOM, AMR, and AMOO stands respectively and the contribution of soil organic carbon stock to ecosystem carbon storage increased with the total basal area and ranged 20.89 % for APDF to 42.02 % in AMOO stand, indicating an increase in the soil carbon stock with the increase in the basal area of forest and A. nepalensisbasal area (Figure 6b).  Tree and soil were the two largest contributors to the total ecosystem carbon in all the stands. Ecosystem carbon storage in AER and APDF was low compared to other stands and ecosystem carbon stock significantly increased with the increase in A. nepalnsis basal area.
DISCUSSION
Stand characteristics and biomass carbon storage in the tree, understory and forest floor
The basal area recorded in study stands (5.2-88.39 m2ha-1) is similar or higher than that reported for other oak forests in central Himalaya (Rawat & Singh 1988; Verma and Garkoti 2019). It has been observed that the stand basal area has a significant impact on stand productivity and carbon storage. The results of this study clearly illustrated that the stand-basal area reveals a strong correlation with the stand carbon stock in A.nepalensisstands (Figure 5). Our results indicate that there were significant differences in the biomass carbon storage in tree layer components across the chronosequence which indicate that; during the forest stand development, trees absorb atmospheric CO2 and store it in plant structural parts. Biomass carbon availability mainly depends on dominant tree species. The structure of plant species can affect the storage of biomass carbon in the forest ecosystem (Conti and Díaz 2013). Our study underlines the importance of fast-growing and nitrogen-fixingA. nepalensis tree as a driver of the C cycle in the studies central Himalaya forest and includes detailed evidence of medium-and larger diameter classes trees were the primary contributor to above and below-ground biomass carbon storage. The carbon storage of the 3 most dominant species (A. nepalensis , Q. leucotrichophora, andR. arboreum ) contribute for 63.50 % to 83.17% of the total biomass carbon storage and the A.neplalensis alone, accounted 10.77 to 69.19 % to the total biomass carbon storage, demonstrating that the majority of the biomass carbon storage is influenced by theA. nepalensis in the studies central Himalaya. The larger trees (DBH > 30 cm) contribute more to the basal area and thus in biomass carbon storage rather than total stem density. The larger size and fast-growing nature of A. nepalensis helped a greater amount of carbon storage. Variation in the density of large-diameter A. nepalensis to be impotent for storing more carbon stocks in forests. Results from our study agree with those of previous studies suggesting that forest stand structure playing an impotent role affecting biomass carbon storage in the forest. Large trees are the main contributors to forest biomass and ecosystem functions (Lutz et al., 2012; Lutz et al., 2018).
Other studies have also demonstrated that in matured forests large trees contain a large proportion of above-ground biomass (Slik et al., 2013; Bradford and Murphy 2019). These results cause us to predict thatA. nepalensis will function as strong organizers of forest stand structure. This is an important discovery, as this tree is a more important and main source of biomass carbon storage. As a result, biomass carbon storage expanded as the basal area expanded and the related biomass carbon storage increased. The stand-basal area has a positive impact on the carbon storage of the forest ecosystem. Further, we anticipate that the being pioneer tree species itself, A. nepalensis  showed good regeneration in AER stands and showing facilitative effects of this species on their neighbor’s late successionQ. leucotrichophora and R. arboreum species. Plant community structure played a significant role in the storage of biomass carbon. In this study, ecosystem carbon storage was influenced by