Background and Aims <p>The mountain treelines are reported to be moving upwards in response to climate warming. Little is known how this treeline redistribution is going to affect the key ecological processes such as litter decomposition. This study aims to disentangle the drivers of litter decomposition in treelines with focus on fine-scale microclimatic factors along with soil nutrient properties, litter quality, elevation and ecotone structure.</p> Methods <p>We conducted a litter decomposition experiment at a Himalayan treeline ecotone using standardized green and rooibos teabags to quantify decomposition rate (<i>k</i>) and stabilization factor (<i>S</i>). We recorded microclimatic factors using data loggers and assessed nutrient properties. We employed structural equation modelling to quantify the role of microclimatic and other factors on litter decomposition.</p> Results <p>Litter mass loss was significantly influenced by microclimatic factors and declined with elevation. Soil temperature was the strongest predictor of decomposition, followed by snow duration and soil pH. Green tea decomposed faster than rooibos tea (54.9% vs. 23.3% average mass loss), highlighting the role of litter quality. While the <i>k</i> showed weak correlation with microclimate, the <i>S</i> was positively correlated with snow duration, nutrient-poor and colder conditions at higher elevations, indicating enhanced carbon stabilization under extreme alpine environments.</p> Conclusion <p>Our findings suggest that litter decomposition in the treeline ecotone is regulated by the interactive effects of multiple factors with soil temperature and snow duration being the dominant drivers. Looking through the microclimate lens, our study underscores the need to incorporate abiotic variability in predictive carbon models under changing climate.</p>