Abstract: Accurate quantification of forest carbon (C) density and distribution is important for modeling C cycling in forest ecosystems. In this study, forest inventory and allometric approaches were used to measure and analyze C density and allocation in soft broad-leaved mixed forest of different stand age classes (young, halfmature, nearmature, mature and overmature forest) in Heilongjiang Province, Northeast China. The main results were: (1) The C sink function of soft broadleaved mixed forest was significant in Heilongjiang Province. The differences in C density were significant among the different stand age classes in terms of the forest ecosystem, vegetation, detritus, and soil C pool. However, there were no significant differences in distribution proportions of C densities among different stand age classes in the terms of the vegetation, detritus, and soil C pool. (2) The C densities of the ecosystem C pools increased with the stand age class. There were significant differences in the C densities of the overstory vegetation and the overstory components (except branches) among stand age classes. There were no significant differences in the distribution proportions of C densities in the overstory components among different stand age classes. The change of understory C densities was not significantly different among stand age classes. The C density of litter was significantly different among stand age classes, and the C density increased with stand age class in the detritus C pool. No significant differences were found for woody debris C density among stand age classes. (3) For the same stand age class, woodland characteristic differences in different areas have influence on C densities and the distribution proportions of ecosystem C pools in soft broad-leaved mixed forest in Heilongjiang Province. (4) Basal area could accurately reflect the dynamic variations of forest C density with the forest age.

Key words: Xihu Lake, catchment development, sediments, diatoms, eutrophication