INTRODUCTION
Forest ecosystems play an important role in global biogeochemical cycles and mitigate the negative impact of climate change (Hui et al., 2017). Furthermore, forests act as a key carbon pool (C) and store more carbon per unit area than any terrestrial ecosystem (Dixon et al., 1994; Pugh et al., 2019). Figures suggest that half of the carbon in the terrestrial ecosystem is stored in forests (Pan et al., 2011; Popkin, 2019). Forest stand growth can change the forest structure, biomass, and soil nutrient content (Shanin et al., 2014; Goebes et al., 2019). The distribution of carbon stocks in different elements of ecosystems is one of the important features of succession (Robinson et al., 2015; Badalamenti et al., 2019). Natural vegetation in freshly degraded land/slip and the naturally regenerated forest is an effective way to deter soil degradation, tree establishment, improve the ecological environment, and drives succession (Walker, & del Moral2009; Berrahmouni et al., 2015; Pandit et al., 2018). Early succession species have strongly favored by vegetation restoration efforts due to their capability of tolerating extremely harsh environmental conditions (Walker, & del Moral2009). After forest degradation, early successional species invade the habitat and radically alter vegetation and soil physicochemical properties (Lebrija-Trejos et al., 2010). Early-successional forest ecosystems that grow after forest degradation or stand- replacement are very important, particularly in terms of carbon storage by pioneer trees after landscape recovery (Swanson et al., 2010; Preem et al., 2012; Lorenc-PluciƄska et al., 2013; Becker et al., 2015). The ability of Alnus tree to fixing nitrogen from the air through symbiotic with nitrogen-fixing bacteria, collectively calledFrankia , generates to the concept that this tree could be effective agents to tolerating extremely harsh environmental conditions and accelerate the natural succession. Alnus is found mostly in degraded habitats with nutrient impoverished conditions in the soil (Sharma et al., 1998; Resh et al., 2002; Binkley., 2003; Perakis and Pett-Ridge 2019). Additionally, alder also produces cluster roots which secrete carboxylates and mobilizes available phosphorus and other mineral elements in the soil (Lambers et al., 2019).
Nitrogen-fixing Nepalese alder (Alnus nepalensis D. Don.) being an early successional species arises as a pioneer in post-disruption (landslides/ landslips) in most Himalayan forests below 2500 m (Sharma et al., 1998; Rana et al., 2018; Joshi and Garkoti 2020). In the central Himalaya, A. nepalensis often forms pure patches and sometimes grows in association with white oak (Quercus leucotrichophora A. Camus) as well as other broad-leaved species throughout different stages of forest succession after disturbance (Singh, 2014; Frouz et al., 2015; Joshi and Garkoti 2020). Owing to the association of Frankia in roots, the Alnus species fix nitrogen and have a stronger impact on soil physicochemical characteristics (Binkleyet al 1992; Myrold et al., 1994; Resh et al., 2002; Binkley., 2003; Khan et al., 2007; Bissonnette et al., 2014; Perakis and Pett-Ridge 2019; Krishna et al. 2019). Therefore, it is widely used in agroforestry, forest management, and restoration systems and also has long been traditionally used as an intercropping tree species (Semwal et al., 2013; Sakalli et al., 2013; Sakalli et al., 2017; Rana et al., 2018). Despite a large series of studies addressing the advantages ofAlnus species, knowledge on biomass C storage variations in alder species or alder mixed forest ecosystems in the central Himalayas is still limited. The proposed work envisages the role of A. neplanesis in the recovery of degraded ecosystems in central Himalaya. In the present study, a comprehensive analysis of the carbon storage in soil and vegetation along a chronosequence in A. nepalansisforest stands in central Himalaya is presented. The study was conducted to determine: (1) assess the above-and below-ground carbon storage of different A. nepalansis forest stands in central Himalaya (2) estimate carbon stock dynamics in the whole ecosystem (plant, litter, and soil) in of different A. nepalansis forest stands and (3) evaluate the role of A. nepalensis in influencing total ecosystem C pools as well as provide helpful suggestions on how to manage forests to mitigate impacts of climate change.