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).