Establishment of apparent quantum yield and maximum ecosystem assimilation on Tibetan Plateau alpine meadow ecosystem

The alpine meadow is widely distributed on the Tibetan Plateau with an area of about 1.2×106km2. Damxung County, located in the hinterland of the Tibetan Plateau, is the place covered with this typical vegetation. An open-path eddy covariance system was set up in Damxung rangeland station to measure the carbon flux of alpine meadow from July to October, 2003. The continuous carbon flux data were used to analyze the relationship between net ecosystem carbon dioxide exchange (NEE) and photosynthetically active radiation (PAR), as well as the seasonal patterns of apparent quantum yield (α) and maximum ecosystem assimilation (Pmax). Results showed that the daytime NEE fitted fairly well with the PAR in a rectangular hyperbola function, with αdeclining in the order of peak growth period (0.0244 μmolCO2 ·μmol-1PAR) > early growth period > seed maturing period > withering period (0.0098 μmolCO2 ·μmol-1PAR). The Pmax did not change greatly during the first three periods, with an average of 0.433 mgCO2