Topoclimatic factors create favourable conditions for carbon-dense forests in the Central Himalayas

The Central Himalayas, characterized by one of the most pronounced elevation gradients globally, harbor forest stands of high carbon density. With estimated forest aboveground biomass (AGB) densities of up to 1000 t ha^-1, these forests are among the most carbon-rich ecosystems within the Himalayas and high mountain ranges globally. However, existing global and regional models of forest carbon distribution fail to accurately capture the remarkable carbon density observed in these Himalayan forest stands. Our objective was to quantify how fine-scale topoclimatic conditions influence the spatial variability of AGB, with the aim of identifying the environmental factors that contribute to the high carbon density observed in high mountain forests of Nepal. Our analysis focused on quantifying the contribution of terrain-driven variation in climatic energy and water availability in creating favourable site conditions for carbon-dense forests. We found that extreme forest carbon density is associated with distinct topographic settings related to slope, aspect and curvature that provide a combination of adequate levels of both climatic energy and water availability, while forest carbon was reduced in topographic positions associated with high likelihood of disturbance such as avalanches and mass movements. Our findings shed light on the intricate relationship between topoclimatic factors and conditions for carbon storage in high-elevation forests, providing valuable insights for conservation and management strategies in mountainous regions.