A comparison of CO₂ fluxes via eddy covariance measurements with model predictions in a dominant subtropical forest ecosystem

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.