Herbaceous biomass dynamics and the interacting roles of nitrogen and plant diversity across elevational and habitat degradation gradients

Mountain ecosystems are under increasing pressure from forest degradation, which can alter key ecological indicators, including herbaceous biomass production. This study considered how herbaceous above-ground biomass (AGB) is influenced by degradation intensity, nitrogen availability, elevation, and species richness in temperate and subtropical forests of the Hindu Kush Himalayas. Data from 200 systematically placed plots were analyzed using bivariate and correlation methods, followed by structural equation modeling (SEM) to assess both direct and indirect pathways affecting herbaceous biomass. The results showed that available nitrogen was the strongest predictor of biomass (β = 0.77, p < 0.001), followed by species richness (β = 0.18, p < 0.05). Degradation reduced biomass indirectly by decreasing nitrogen (β = -0.72, p < 0.001) and species richness (β = -0.59, p < 0.001). Moderate degradation also negatively influenced subtropical habitat (β = -0.82, p < 0.001) and nitrogen (β = -0.43, p < 0.01). Higher nitrogen levels were significantly associated with less degradation (β = -0.47, p < 0.001), suggesting a protective effect of nitrogen-rich soils. Elevation had no direct effect on biomass but reduced subtropical forests (β = -0.82, p < 0.001) and increased moist temperate forests (β = 0.35, p < 0.01). The model identified nitrogen as the main driver of productivity, while degradation disrupted soil fertility and biodiversity, especially in vulnerable low-elevation forests.