Seasonal Changes of CO2 and H2O Exchanges over a Temperate Grassland.

Abstract

Field observations and data analyses of CO2 and H2O exchanges have been conducted with respect to several environmental variables, between the atmosphere and mixed grassland consisting of C3 and C4 plants. The observations were conducted over the period from April to November 1996, which took the growing season into consideration. A total of 43 species grew in the grassland, and a number of predominant species among C3 and C4 plants exhibited a seasonal variation. The CO2 flux over the grassland was measured by an aerodynamic method, while latent heat flux was estimated as the residual of heat budget over grassland, using routine meteorological field data.From the sequence of prudent analysis, the seasonal variations of both the fluxes of CO2 (FCO2) and H2O (λE), and canopy conductance gc were classified into three stages. During the first stage, from April to May, FCO2, λE, and gc changed in proportion with an increase of leaf area index (LAI) over grassland. In the second stage, from July to September, daily assimilated FCO2 and daily averaged λE achieved maximum values (53.7g CO2m-2 day-1, and 166Wm-2) during the growing season when high values of LAI were maintained (2.4m2m-2 0.7). At the beginning of the growing season (April) and the end (November), Ω values were less (Ω<0.6) than during the other months (May-October), indicating that evapotranspiration was relatively more sensitive to gc during these months.