Tree size and age induced stem carbon content variations cause an uncertainty in forest carbon stock estimation

Stem carbon (C) content is widely used to present tree C content to estimate forest C stocks. However, size- and age-dependent changes in tree stem C content are still unclear. Based on 576 tree size (expressed by diameter at breast height (DBH) and biomass), age and C content data, our results showed that C content varied significantly among organs, and the mean value of C content for bark, branch, leaf, reproductive organ, root and stem was 48.4 %, 49.2 %, 49.6 %, 50.1 %, 48.8 %, 49.7 %, respectively. C content of stem was significantly correlated with that of leaf, branch and root, while showed no relationship with that of bark and reproductive organ. With the increasing tree size and age, stem C content showed increasing trends. Using stem C content as tree C content could produce an error of -2.49%−5.87% in the estimations of forest C stock. Thus, it is necessary to consider tree organ C content of stand in estimating forest C stock.