Submitted to: The Earth's Changing Land Global Change and Terrestrial Ecosystems and Land
Publication Type: Abstract only
Publication Acceptance Date: 5/26/1998
Publication Date: N/A
Citation: Interpretive Summary:
Technical Abstract: In the mechanistic wheat growth model DEMETER, the time course of maximum saturated carboxylation rate (Vcmax) strongly influences the gas and energy exchange. Besides the direct effect on photosynthesis, a generic influence on stomatal aperture appears in the model due to the inclusion of photosynthesis in the calculation of the energy balance. Observed values of Vcmax were used in previous simulation studies. In this way a possible interaction between Vcmax and stomatal conductance (gs) was neglected. In the advanced model version of DEMETER, Vcmax is calculated using the simulated leaf nitrogen content. An experimental study was conducted at the Maricopa, Arizona, Free-Air CO2 Enrichment (FACE) site and in Bernburg (Germany) to determine declining Vcmax on seasonal and daily time scales. Vcmax was estimated from the initial slope of the response of observed assimilation (A) to the leaf internal CO2 concentration (Ci). The results showed: (i) a clearly visible seasonally declining Vcmax, and (ii) no clear evidence but a slight tendency for a diurnally declining Vcmax. The advanced model reflects more accurately the energy and gas exchange on a time basis of minutes to days. The short-term improvements and the more detailed knowledge about the long-term behavior of gs and Vcmax resulted in a more stable behavior of the macroscopic and long-term response; i.e., for the cumulative evapotranspiration.