Seasonal variations of the exchange of CO2 and H2O between a grassland and the atmosphere:: An experimental study

Seasonal variations in the CO2 and H2O fluxes were investigated experimentally over a grassland in central Japan during the growing periods of 1993 and 1994. The CO2 flux over the grassland was measured by an aerodynamic method. The surface heat budget was routinely measured and the data were used to analyze the CO2 and H2O exchange between the grassland and the atmosphere. Conditions were relatively cool and wet due to the prolonged rainy season from July to August 1993, being hot during the 1994 summer. The annual maximum value of sensible heat flux density was observed prior to the growing period on March 1993 and on April 1994, accounting for 40-50% of the net radiation. The annual maximum of latent heat flux density was observed in August, accounting for about 85% of the net radiation in both years. The latent heat flux observed in August 1994 was quite high, being 135 W m(-2) (monthly mean, daily basis), while that in 1993 was 96 W m(-2). The grassland absorbed CO2 during the period from the beginning of May to October in 1994, and maximum of daily net ecosystem CO2 exchange during the growing period amounted to values mon than 40 g CO2 m(-2) day(-1), in both 1993 and 1994. The dependence of daily net ecosystem CO2 exchange on leaf area index (LAI) and incoming photosynthetic photon flux density Q(p) was investigated. When the Q(p) was higher than 45 mol m(-2) day(-1), the daily CO2 exchange strongly increased with an increase in LAI. The daily canopy surface conductance was calculated based on the Penman-Monteith equation, and it showed a linear increase with LAI when the LAI had small values (typically less than one). With an increase in LAI from one to four, the maximum level of surface conductance became dominated by water vapor pressure deficits. (C) 1998 Elsevier Science B.V.