4.1. Tree and understory components C, N, and P concentrations and stoichiometry
Vegetation components show variations in C, N, and P concentrations and stoichiometry ratios due to the differentiation in their function and structures. The possible explanation for this is that various plant communities and their structural components have different environmental adaptability and nutrient requirements, and conservation strategies (Wright al., 2004; Laliberté et al., 2014). In the present study, leaf N concentrations in tree species were significantly greater than the shrubs and herbaceous species, while leaf P for shrub and herbaceous species was greater than the tree species. These findings are consistent with the reports of Wright et al. (2004), who found that herbaceous plants have considerably higher leaf P concentrations than tree species. Pan et al. (2011) revealed that N and P concentrations in tree leaves were considerably greater compared to litter, possibly due to resorption strategies.
Variations in C: N: P stoichiometry (C:N, C:P, and N:P ratios) were noticed among the different vegetation components (Niklas, & Cobb 2006; Ågren 2004; Ågren 2008). Our findings revealed that there were significantly different C:N:P ratios among the different components, which tested the second hypothesis. N and P concentrations in leaves, twigs, and litter were higher than other vegetation components and thus showed lower C:P and C:N ratios, which is in agreement with earlier findings (Minden & Kleyer, 2014). Leaf N concentration in tree species was 2 to 4 times greater than other plant components. Tree bole is responsible for support and storage; therefore, it contains more C than other structural components. Because leaves are responsible for photosynthesis, they require a higher amount of N and P to support various biochemical reactions. Present C:N ratios for vegetation components ranged from 8 to 130, and C:P ratios ranged from 138 to 2830 (Fan et al.,2010; Yong et al., 2018). The C:N ratio in bole, branch, and twig was greater than in leaves and roots. Being a nitrogen-fixing tree,A. nepalensis showed higher N concentrations in different plant components compared to other plant species. The C:N ratio was lowest inA. nepalensis leaf compared to other structural parts and leaves of different plant species. Furthermore, the N:P ratio varies by plant species, growth stage, and study area (Koerselman & Meuleman, 1996; Güsewell, 2004). According to Güsewell (2004), the leaf N/P ratio can be used to evaluate whether the ecosystem is N-limited (N/P ratio < 10) or P-limited (N/P ratio > 20). In the present study, the leaf N/P ratio in A. nepalensis and associated species were 44.74 and 78.3, respectively, indicating that their development was P-limited. The leaf N/P ratios in Q. leucotrichophora and R. arboreum were 19.39 and 16.10, respectively, implying that P limited their growth. The leaf N/P ratios in herbaceous and shrub plants were 6.17 and 9.67, respectively, suggesting that N was limiting their growth. The C: N : P ratios in the bole, branches, and roots were markedly greater than in the leaves, twig, and litter (Table 2), indicating that plants allocate higher nutrients to the leaves to ensure growth (Sardans and Peñuelas 2013). This result aligns with previous conclusions that different components have different demands for nutrients N and P. More dynamic components (e.g., leaves, fine roots, and twigs) have a higher nutrient content to meet the requirements of plant growth (Sterner & Elser, 2002).