Seasonal variations of ecosystem apparent quantum yield ( α) and maximum photosynthesis rate ( P max) of different forest ecosystems in China

Continuous measurements of carbon dioxide using the eddy covariance (EC) technique were made in 2003 at three ChinaFLUX forest sites, including a temperate mixed forest (CBS), a subtropical evergreen coniferous plantation (QYZ) and a subtropical evergreen broad-leaved forest (DHS) along the North–South Transect of Eastern China (NSTEC). The three forest sites are influenced by Asian monsoon climate to varying degrees. The environmental controls on the seasonal variations of ecosystem apparent quantum yield ( α), maximum photosynthesis rate ( P max) and ecosystem respiration ( R e) were investigated in the three forest ecosystems. At the CBS site, temperature was found to be the dominant factor controlling the seasonal patterns of ecosystem α and P max, while the phenology, which was mainly embodied by the development of LAI, also displayed important influence. The seasonal patterns of α and P max at QYZ were mainly ascribed to vapor pressure deficit (VPD). Ecosystem α decreased with increasing VPD and P max decreased below and above the optimum VPD as a result of the Asian subtropical monsoon climate which resulted in the extremely high temperature and low moisture. At the DHS site, ecosystem α only appeared weak negative correlation with temperature, while there was no obvious dominant factor controlling the seasonal pattern of ecosystem P max. The ecosystem respiration of the three forest ecosystems could be estimated well by temperature with an exponential equation. During the growing season, the ecosystem α, P max and R e at CBS were much stronger than that of QYZ and DHS. The R e at CBS was more sensitivity to temperature than that of QYZ and DHS, so its response to changing climate deserved more attention in the evaluation of the carbon budget of such ecosystem.