Abstract.A subtropical old-growth forest was studied over a twelve-year period to investigate temporal and spatial fluctuations of biomass and stem fluxes under disturbances. Vegetations were categorized into three types according to disturbances caused by biotic and abiotic factors, including Castanopsis chinensis population, insect direct-influenced population, and insect indirect-influenced population according to disturbance scenarios. The biomass fluxes (growth and mortality) and stem fluxes (stem recruitment and mortality) were used to quantify population fluctuations. Annual average biomass growth rate was stable throughout the study while annual biomass mortality and stem fluxes increased consistently. C. chinensis population predominantly contributed to biomass fluxes of the community. Biomass and stem mortalities of insect direct-influenced population increased significantly during the whole study period (1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004). Results of indirect-influenced population showed that (1) the increase in biomass of the dominant species was well correlated between different intervals. Similar relationships were found in stem fluxes; (2) higher stem mortality occurred within the DBH range of 1 to 10 cm; (3) stem fluxes in the canopy gaps were remarkably higher than those in closed canopy.

Abstract. CO2 fluxes were measured continuously for twelve months (2003) using eddy covariance technique at canopy layer in a dominant subtropical forest in South China. Our results showed that daytime maximum CO2 fluxes of the whole ecosystem varied from −15 to −20 μmol m−2 s−1. The peaks of CO2 fluxes appeared earlier than the peaks of solar radiation. Contribution of CO2 fluxes in a subtropical forest in the dry season was 53% of the annual total from the whole forest ecosystem. Daytime CO2 fluxes were very large in October, November and December, which was therefore an important stage for uptake of CO2 by the forest ecosystem from the atmosphere. Using the estimates of biomass, soil carbon and parameters of leaf photosynthesis from other studies at the same forest, we ran a process-based model, CBM (stands for CSIRO Biosphere Model) for this site, and compared the predicted fluxes of CO2 with measurements. We obtained reasonable agreement. The mean difference between the simulated and measured daytime CO2 fluxes from the year-round (8249 records) was −0.2 μmol m−2 s−1 and implied well within measurement accuracy. Based on estimates of forest ecosystem respiration, NEE was calculated −242 and −276 gCm−2 year−1 for measured and modelled, respectively. In previous study, NPP for this forest stand was 694 gCm−2 year−1 during 2003/04 and litterfall was 424 gCm−2 year−1. We therefore calculated NEE as −270 gCm−2 year−1 and very similar to the values obtained by measured and modelled CO2 fluxes in this study.