There are four components of carbon (C) pools in a natural forest ecosystem: vegetation, woody debris, litter, and soil. Quantifying these C pools and their contributions to forest ecosystems is important in understanding C cycling in forests. Here, we investigated these four C pools in nine Fagus forests along an altitudinal gradient in southwest China. We found that the C pools of Fagus forest ecosystems ranged from 190.7 to 503.9 Mg C ha^–1, mainly attributed to vegetation C (accounting for 33.7–73.9%) and soil C (accounting for 24.6–65.4%). No more than 4% of ecosystem C pools were stored in woody debris (0.25–3.4%) and litter (0.2–0.7%). Ecosystem C storage increased significantly with elevation, where the vegetation and woody debris C pools increased concomitantly with increasing altitude, while those of litter and soil exhibited no significant variations. Except for climatic factors (mainly temperature), the forest stand age was found to be a key driver of such altitudinal patterns, especially for vegetation C storage. The present study provides reliable data for developing and evaluating C cycling models for subtropical forest ecosystems, and emphasizes the importance of considering stand age in related studies.