Carbon dioxide and energy exchanges in a temperate grassland ecosystem

A micrometeorological study was conducted from May to October in 1987 to evaluate the surface exchanges of carbon dioxide and energy at a temperate grassland site in northeastern Kansas. The fluxes of CO{dollar}sb2{dollar}(F{dollar}sb{lcub}rm c{rcub}{dollar}), latent heat (LE), sensible heat (H) and momentum were measured using the eddy correlation technique. Pertinent microclimatic, plant and soil variables were also monitored through the season. The prairie was dominated by three C{dollar}sb4{dollar} grass species: Andropogon gerardii, Sorghastrum nutans, Panicum virgatum.; The energy partitioning was controlled mainly by net radiation (R{dollar}sb{lcub}rm n{rcub}{dollar}) and by soil water availability. On sunny days with no moisture stress, about 58 to 83% of R{dollar}sb{lcub}rm n{rcub}{dollar} was consumed in evapotranspiration during midday. The midday canopy surface conductance averaged about 15 mm s{dollar}sp{lcub}-1{rcub}{dollar}. Under moisture stress conditions the magnitude of midday LE/R{dollar}sb{lcub}rm n{rcub}{dollar} ranged from 0.35 to 0.45 whereas the canopy surface conductance was less than 3 mm s{dollar}sp{lcub}-1{rcub}{dollar}.; Considerable daily and seasonal variability of Co{dollar}sb2{dollar} flux was observed. The midday F{dollar}sb{lcub}rm c{rcub}{dollar} reached a seasonal peak of 1.3 mg m{dollar}sp{lcub}-2{rcub}{dollar} (ground area) s{dollar}sp{lcub}-1{rcub}{dollar} in early July and was near zero during a dry period in late July. The dependence of the daytime F{dollar}sb{lcub}rm c{rcub}{dollar} on pertinent controlling variables (e.g., light, temperature, vapor pressure deficit, soil water availability, and canopy surface conductance) was investigated. The magnitude of nocturnal F{dollar}sb{lcub}rm c{rcub}{dollar}(soil plus plant respiration) averaged 0.4 mg m{dollar}sp{lcub}-2{rcub}{dollar}s{dollar}sp{lcub}-1{rcub}{dollar} under well-watered conditions and was about 0.2 mg m{dollar}sp{lcub}-2{rcub}{dollar}s{dollar}sp{lcub}-1{rcub}{dollar} during the dry period.; A leaf stomatal conductance model was developed for the major grass species in this ecosystem, and was scaled up to a canopy level using a canopy radiative transfer model. Comparisons with field measurements indicated that the model provided reasonable estimates of canopy stomatal conductance for well-watered conditions. The modelled half-hourly values of LE agreed well(generally within 5%) with those measured, under these conditions.; A biochemical leaf photosynthesis model was adapted to the major grass species studied here, and was scaled up to a canopy level. Comparisons with field measurements showed that the model realistically simulated the magnitudes and the diurnal variations of canopy photosynthesis under well-watered conditions. Although the modelled canopy photosynthesis overestimated the measured values by 0.2-0.3 mg m{dollar}sp{lcub}-2{rcub}{dollar}s{dollar}sp{lcub}-1{rcub}{dollar} under moisture stress conditions, the model did simulate the diurnal patterns adequately.