Riverine forests play a significant role in ecosystem processes, including energy flow and carbon dynamics (Allan et al., 2021). These forests are also known to serve as important bio-corridors for the movement of a variety of faunal groups during various seasons (Rawat, 2017). They are a productive ecosystem and are hotspots for epiphytic orchids and agricultural output (Marwah et al., 2021). Riverine forests hold discrete ecological traits because of their interaction with the aquatic ecosystem. Thus, their borders can be identified by variations in soil conditions, vegetation, and other elements that represent this aquatic–terrestrial interaction (Naiman et al., 2000). The riverine forests all over the Himalayan region are susceptible to natural (floods and landslides), anthropogenic, and climatic fluctuations (Bookhagen and Burbank, 2010). Despite the importance of these forests, overexploitation, unrestricted grazing, fuel wood collection, agricultural development, boundary disputes, developmental projects, and linear infrastructures pose a serious threat to these ecosystems (Merawi, 2016; Rajpara et al., 2022).

The riverine forests in the Himalayan region are represented by a number of early and mid-seral communities such as Acacia catechu–Dalbergia sissoo along foothills, Drypetes roxburghii–Toona ciliata, Duabanga grandiflora–Terminalia myriocarpa (eastern Himalaya), Macaranga pustulata, Alnus nepalensis, Debregeasia hypoleuca, Hippophae salicifolia, Salix species, and Populus ciliata, etc., at higher elevations (Champion and Seth, 1968; Rawat, 2017). Of these, most dominant forest formations, especially between 900 and 2,200 m above sea level, are formed by Macaranga pustulata and Alnus nepalensis. These species form early successional stages (Dhar et al., 1997), colonizing freshly eroded river banks and landslide areas. In Central and Eastern Himalaya, the following communities of riverine forests become more prevalent: Macaranga forests (below 1,000 m), Alnus forests between 1,000 and 2000 m, and Quercus-Machilus forests between 1,500 and 2,200 m asl. The alder forest frequently forms pure stands in landslide-prone areas, although it also occurs mixed with other late successional species in the Himalayan region (Joshi and Garkoti, 2021). It is primarily found close to moist habitats and can fix atmospheric nitrogen through symbiotic interaction with gram-positive Frankia, allowing it to survive exceptionally hard climatic conditions and hasten the natural succession (Crisafulli et al., 2015; Joshi and Garkoti, 2021). Due to their crucial role in socioeconomic advantages, ecological functioning, and soil conservation, the local communities depend on these forests for their sustenance needs (Bisht et al., 2022a).

The dry matter dynamics is a key indicator from the energy fixation and flux of biological materials point of view (Anderson, 1971). According to the Intergovernmental Panel on Climate Change, carbon stored in a forest ecosystem is divided into three main pools, i.e., biomass, litter, and soil. The carbon pools in a forest ecosystem are influenced by topography, forest structure, tree species composition, species diversity, age structure, edaphic conditions, and human-induced disturbances (Arasa-Gisbert et al., 2018; FSI, 2021). The current total carbon pool in the forested zone of the Indian Himalayan Region is about 103.9 Mg ha⁻¹, of which 23% is in the sub-tropical forests (FSI, 2021). Among the edaphic factors, the nutrient content in the soil may affect parameters such as tree growth and basal area and thus consequently influence the structure of plant communities (Becknell and Powers, 2014) and carbon storage. To formulate viable strategies to increase the soil carbon, it is critical to include soil restoration, forest regeneration, and nutrient management (Sagar and Singh, 2005; Lal, 2015). The majority of organic matter elaborated by plants through photosynthesis is returned to the soil as litter. The litterfall results in seasonal or constant input of organic matter and nutrients to the forest floor and represents one of the most important pathways for the transfer of energy, affecting the growth and development of the vegetation and soil pool (Mfilinge et al., 2002).

There is a paucity of literature on the structure and functioning of these forests from the region. Therefore, the assessment of riverine forests, including their conservation status and potential for carbon flux, standing biomass, productivity, and carbon accumulation, is critical for long-term conservation planning and national accounting of the United Nations convention on climate change. Since forests sequester carbon from the atmosphere as part of the growth process, any increase in the biomass constitutes a sink that reduces the build-up of atmospheric carbon dioxide. This emphasizes the importance of forest ecosystems in the global carbon cycle and the necessity to accurately evaluate the amount of carbon stored in forest ecosystems (Pan et al., 2011). Interestingly, this is the first study on the dynamics of dry matter and carbon flux in riverine forests from the Gori valley, Western Himalaya. There is no prior literature on forest biomass and accumulation of carbon for this region. This stimulated the estimation of productivity and sequestration to understand the functioning of the riverine forest ecosystem. The information obtained from this study will significantly contribute to the dry matter dynamics and carbon cycle across the globe. Furthermore, it will benefit forest officials and ecologists for the conservation and restoration of riverine habitats. Thus, the present study was conducted to address the following questions: (1) How does the altitude affect phytosociological parameters in the region? (2) What is the pattern of biomass and net primary productivity in the riverine forests? (3) What is the magnitude of litterfall? (4) How do the carbon stock and sequestration potential vary in the riverine forests of Kumaun Himalaya, India?

The dry matter dynamics and the carbon flux of the forest are influenced by the structural and functional characteristics of the forest stand. The tree density (r = 0.73) and total basal area (r = 0.71) increased with increasing altitude in the riverine forests. In the present study, the density and total basal area in riverine forests were reported to be higher in contrast to previous studies (Rawal and Pangtey, 1994; Dhar et al., 1997). The richness and diversity across the sites were observed to be lesser than the values reported by Dhar et al. (1997) in Kumaun Himalaya. A higher tree density in Alnus forest was recorded, while slightly lesser tree diversity and richness were recorded when compared with the values recorded by Gairola et al. (2011) in Garhwal Himalaya. In addition, lower tree density, richness, and diversity in broadleaf forest were recorded compared to the values observed by Gairola et al. (2011).

The physico-chemical characteristics of soil vary within space and time due to variation in topography, climate, weathering processes, vegetation cover, microbiological activity, season, and other biotic and abiotic factors (Bargali et al., 2019; Manral et al., 2020). Therefore, soil characteristics vary over relatively small distances throughout the Himalayan region, and species composition plays an important role in the formation of soil organic matter and influences soil-forming processes (Bargali et al., 2018). All the forest communities showed distinct soil physico-chemical characteristics in the present study. It was observed that soil organic carbon was significantly correlated (r = 0.80, p < 0.05) with the total basal area of the forest. The higher concentration of soil nutrients in Alnus forest was due to its nitrogen-fixing ability and nutrient-rich litter, which enhances the physico-chemical and biological characteristics of the soil (Awasthi et al., 2022). Alder, once established in degraded land, improves the physicochemical properties of soil (Perakis and Pett-Ridge, 2019; Joshi and Garkoti, 2021). Moreover, alder develops clustered roots that release phosphorus and carboxylates and other important elements available in the soil (Lambers et al., 2019). However, the soil in broadleaf forests is often deep and fertile and has a thick layer of humus.

Litterfall serves as a conduit between the tree canopy and the soil below, contributing nutrients gathered from the biomass and affecting the productivity of the forest (Pitman, 2013). Seasonality plays a key role in the fluctuation of litterfall. Both anthropogenic and natural factors, including climate change, may have an impact on litterfall output and seasonal progression. In the present study, the lower values of litterfall were due to lower tree density (r = 0.69), lower total basal area (r = 0.72), and other site conditions (Table 6). The amount of litterfall in any ecosystem shows a linear relationship with the canopy cover, tree size, tree density, and basal area of the forest (Navarroa et al., 2013). With an increase in annual litterfall input, the soil organic carbon (r = 0.96) and soil carbon stock (r = 0.69) have significantly increased in the forest sites. The litter of Alnus is nitrogen rich, which promotes soil carbon (Bissonnette et al., 2014; Perakis and Pett-Ridge, 2019) and thus markedly benefits soil fertility. The annual litterfall in the present study was estimated to be higher than the values reported by Sharma and Ambasht (1987) in eastern Himalaya, due to higher values of tree density and total basal area. According to Chen et al. (2014), litterfall peaks in the monsoon and summer seasons for temperate broadleaf forests. The peak litterfall production during the summer season reported in this study is an indication of the tree’s physiological response to increased temperature.

Forest structure and composition had a substantial effect on biomass, carbon stock, and soil organic carbon. The tree biomass of the forest depends upon vegetation structure, species composition, and stand age (Bisht et al., 2022b). In terms of forest communities, Quercus-Machilus forest had the highest aboveground biomass, indicating a greater number of individuals with higher girth class. Moreover, Macaranga forest, with lower density and total basal area, had the lower aboveground biomass in the forest sites. The total biomass in Alnus and Quercus-Machilus forest was recorded to be higher compared to the values recorded by Singh and Singh (1987), Rikhari et al. (1997), Gairola et al. (2011), and Joshi and Garkoti (2021) in Uttarakhand Himalaya, which might be due to variation in species composition and girth class size. The value of tree biomass falls within the range reported by Sharma et al. (1998) in Alnus forest. Moreover, the tree biomass in the present study was recorded to be higher than the riverine Shorea robusta and riverine A. catechu forest of Sharma et al. (2010) and Sajad et al. (2021). In the Himalayan region, very few studies have been conducted on dry matter dynamics of riverine forests.

The net primary productivity is the key indicator of concern from the forest management and sustainability point of view. The productivity of the forest ecosystem depends upon the population dynamics, forest structure, species composition, growth traits, and rate of increment in given environmental and site conditions. The NPP reported for Alnus forest falls within the range reported by Singh and Singh (1987) and Rikhari et al. (1997). Furthermore, a key indicator of the potential age of the dominating species and the harshness of the environment, which measures the accumulation of primary persistent material, is the biomass accumulation ratio (Whittaker, 1975). The BAR in Quercus-Machilus forest was recorded at 41.3 ± 8.4, which was higher than the value (28.3 ± 1.9) reported by Singh et al. (1994) in Kumaun Himalaya. For Alnus plantation, the value falls within the range reported by Sharma et al. (1998). As the altitude rises, the mean value of the BAR increased due to larger amount of biomass production in the riverine forests.

The process of absorbing and exporting carbon varies considerably in forest ecosystems, depending on forest density, soil, and carbon stock. The carbon flux varies with the size, age, and species composition of trees in the forest ecosystem (Covey et al., 2012). The total carbon stock of the Alnus and Quercus-Machilus forest was higher than the values observed previously (Rikhari et al., 1997; Gairola et al., 2011). The carbon stock in the present study was higher than that of the riverine S. robusta and A. catechu forest recorded by Sharma et al. (2010). The significant relation between total basal area and carbon stock (r = 0.85) indicates that an increase in the stand basal area led to an increase in the carbon stock of the forest ecosystem. Carbon sequestration potential showed strong correlation with the tree density of the forest sites (r = 0.81, p < 0.01). According to Gera (2012), the variations in the sequestration potential can be attributed to the mean annual increment, which varies with forest site, age, density, and plantation. The present study reveals that soil is the largest pool of forest carbon, contributing 48.2–50.6%, followed by aboveground carbon (38.7–39.5%) and belowground carbon (10.1–11.6%, Figure 3).

It was observed that as altitude increased, there was a significant increase in both tree density and total basal area within riverine forests. The highest values for these parameters were documented in the Quercus-Machilus forest, while the lowest values were observed in the Macaranga forest. Similarly, tree biomass and productivity exhibited their peak values in the Quercus-Machilus forest (692.1 Mg ha⁻¹) and their lowest values in the Macaranga forest (66.91 Mg ha⁻¹). The annual litterfall demonstrated a seasonal variation, with higher levels during the summer (3.40 ± 0.01 Mg ha⁻¹) and lower levels in the winter (0.74 ± 0.01 Mg ha⁻¹), ranging from 4.01 Mg ha⁻¹ in the Macaranga forest to 6.72 Mg ha⁻¹ in the Quercus-Machilus forest. This investigation unveiled that the Quercus-Machilus forest not only stores a substantial amount of carbon (280.7 ± 26.0 Mg ha⁻¹ yr.⁻¹) but also sequesters carbon effectively (7.1 ± 0.9 Mg C ha⁻¹ yr.⁻¹) compared to the Macaranga and Alnus forests. In conclusion, the research indicates that a greater litterfall and higher forest density in riverine habitats exert a more pronounced influence on soil organic carbon content. Additionally, it was observed that Alnus forests significantly enhanced the nitrogen content in the forest ecosystem, positively affecting soil fertility.

This study contributes to our comprehension of carbon storage and fluxes within riverine forests in the Western Himalaya. Carbon offset programs such as REDD+ offer abundant opportunities and resources for the conservation of these forests, allowing for the retention of carbon reserves while simultaneously delivering supplementary ecosystem services to local communities. Consequently, it is advisable to contemplate the utilization of riverine forests for the restoration of deteriorated sites in the Western Himalaya, and the implementation of alder-based restoration initiatives should be examined as a potential means to enhance carbon storage within ecosystems. This should be viewed as an integral element of climate change mitigation strategies and forest ecosystem management.

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding authors.

SB collected data, performed analysis, and prepared the manuscript. YR and SSB conceptualized. KB and GR finally reviewed the paper. All authors contributed to the article and approved the submitted version.

NMHS is duly acknowledged for providing funding for the project, Mainstreaming landscape approach for biodiversity conservation, improved livelihoods and ecosystem health in Kailash Sacred Landscape part of India, received by SB and GR.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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