Seasonal change in CO₂ and H₂O exchange between grassland and atmosphere

The seasonal change in CO2 flux over an artificial grassland was analyzed from the ecological and meteorological point of view. This grassland contains C3 and C4 plants; the three dominant species belonging to the Gramineae; Festuca elatior (C3) dominated in early spring, and Imperata cylindrica (C4) and Andropogon virginicus (C4) grew during early summer and became dominant in mid-summer. CO2 flux was measured by the gradient method, and the routinely observed data for the surface-heat budget were used to analyze the CO2 and H2O exchange between the grassland and atmosphere. From August to October in 1993, CO2 flux was reduced to around half under the same solar-radiation conditions, while H2O flux decreased 20% during the same period. The monthly values of water use efficiency, i.e., ratio of CO2 flux to H2O flux decreased from 5.8 to 3.3 mg CO2/g H2O from August to October, the Bowen ratio increased from 0.20 to 0.30, and the ratio of the bulk latent heat transfer coefficient CE to the sensible heat transfer coefficient CH was maintained around 0.40-0.50. The increase in the Bowen ratio was explained by the decrease in air temperature from 22.3 °C in August to 16.6 °C in October without considering biological effects such as stomatal closure on the individual leaves. The nearly constant CE/CH ratios suggested that the contribution ratio of canopy resistance to aerodynamic resistance did not change markedly, although the meteorological conditions changed seasonally. The decrease in the water use efficiency, however, suggested that the photosynthetic rate decreased for individual leaves from August to October under the same radiation conditions. Diurnal variations of CO2 exchange were simulated by the multi-layer canopy model taking into account the differences in the stomatal conductance and photosynthetic pathway between C3 and C4 plants. The results suggested that C4 plants played a major role in the CO2 exchange in August, the contribution of C4 plants decreased in September, and daily variations of CO2 exchange were mainly due to C3 plants in October. The results also suggested that the decrease in the net canopy CO2 exchange from August to October was induced partly by the decrease of net photosynthesis on the individual leaves in both C4 and C3 plants, which could be due to aging of the leaves