The accurate characterization of aboveground biomass (AGB) dynamics and their driving forces in sub-tropical forests is important to evaluate AGB's contribution to global carbon stocks. In order to estimate the effects of local variables on temporal and spatial patterns of AGB we investigated the variation of AGB in a 20-ha sub-tropical permanent plot. We found that the AGB was 153.7 Mg ha^-1 in 2005, and 149.3 Mg ha^-1 in 2010. During the five-year period, AGB changed due to tree mortality, growth (diameter at breast height, DBH) and recruitment. Medium-sized trees (DBH: 30–50 cm) were the largest contributor to total AGB. Both the tree species diversity index and the number of individuals within medium (DBH: 30–50 cm) and large (DBH: >50 cm) DBH ranges had significant positive effects on AGB. In addition, the abiotic factors such as elevation, slope, and convexity had negative significant relationships with AGB. The results indicate the importance of including forest community characteristics and environmental variables in forest carbon studies. This study also provides important data that can be used to develop and validate carbon cycling models for old-growth sub-tropical forests.