Seasonal change in CO2 and H2O exchange between grassland and atmosphere

The seasonal change in CO 2 flux over an artificial grassland was analyzed from the ecological and meteorological point of view. This grassland contains C 3 and C 4 plants; the three dominant species belonging to the Gramineae; Festuca elatior (C 3 ) dominated in early spring, and Imperata cylindrica (C 4 ) and Andropogon virginicus (C 4 ) grew during early summer and became dominant in mid-summer. CO 2 flux was measured by the gradient method, and the routinely observed data for the surface-heat budget were used to analyze the CO 2 and H 2 O exchange between the grassland and atmosphere. From August to October in 1993, CO 2 flux was reduced to around half under the same solar-radiation conditions, while H 2 O flux decreased 20% during the same period. The monthly values of water use efficiency, i.e., ratio of CO 2 flux to H 2 O flux decreased from 5.8 to 3.3 mg CO 2 /g H 2 O from August to October, the Bowen ratio increased from 0.20 to 0.30, and the ratio of the bulk latent heat transfer coefficient C E to the sensible heat transfer coefficient C H 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 C E / C H 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 CO 2 exchange were simulated by the multi-layer canopy model taking into account the differences in the stomatal conductance and photosynthetic pathway between C 3 and C 4 plants. The results suggested that C 4 plants played a major role in the CO 2 exchange in August, the contribution of C 4 plants decreased in September, and daily variations of CO 2 exchange were mainly due to C 3 plants in October. The results also suggested that the decrease in the net canopy CO 2 exchange from August to October was induced partly by the decrease of net photosynthesis on the individual leaves in both C 4 and C 3 plants, which could be due to aging of the leaves.