The surface exchange of energy, water, and trace gases at the soil-plant-atmo-sphere interface occurs over a wide range of spatial scales from a single leaf or plant up to an entire canopy, ecosystem or region. Usually, the exchange is also interacting on these scales and is difficult to describe even with complex models. The comparison of flux measurements performed simultaneously at different scales can help to understand the interactions of interest.
In the present study, the exchange of CO2 and H2O of a cereal field was investigated simultaneously on three different scales. The net ecosystem flux on the field/ecosystem scale was measured by eddy correlation and aerodynamic profile methods. A porometric leaf chamber system was used to determine the gas exchange rates of the various leaf levels of the canopy. The gap between this two scales was filled by in-canopy profile measurements and the application of the inverse Lagrangian method after Raupach (1989). It evaluates the vertical source-sink-distribution and, by integration, the vertical flux profile within the canopy. Measurements reported here were performed in June and July 1995 on a triticale cereal field in middle Europe (Bellheim, Southern Germany, 49.18 N, 8.28 E, 127 m a.s.l.). The canopy was in the state of senescence showing a continuous decrease in active leaf area.
From all three methods applied, the net canopy exchange flux could be derived. For that purpose, the porometry measurements were scaled up to the canopy level using a relatively simple model. For water vapour, a reasonable agreement was found between all three methods indicating the good quality of the measurement systems and a negligible contribution of the soil evaporation to the total H2O flux. For carbon dioxide, the upscaled porometry flux differed systematically from the other methods due to the substantial contribution of soil respiration. The results of the inverse Lagrangian modeling and the porometry measurements were compared in detail for the source-sink-distribution within the canopy. Both methods showed the continuous drying out of the lower leaves and a corresponding restriction of the daytime H2O source to the higher leaf levels. The CO2 assimilation was limited to the uppermost two leaf levels throughout the whole measurement period and occurred simultaneously with daytime respiration from the soil and the lower leaf levels. This conditions led to characteristic C-shape profiles of the CO2 concentration in the canopy. During the night, the stomatal exchange was negligibly small and the soil represented the only CO2 source. This led to monotonically decreasing concentration profiles.