Modeling the effects of elevated CO2 on plants: extrapolating leaf response to a canopy
The response of canopies to short-duration exposure to elevated CO2 was examined by using a detailed submodel of single-leaf gas exchange combined with a model of canopy structure and light penetration. The leaf model included a mechanistic gas exchange model and leaf energy balance equations, and the canopy model included a detailed description of spatial variability in environmental conditions within the canopy. The structure of the canopy model was designed to facilitate implementation of different leaf aggregation schemes. To compare six aggregation methods of increasing simplicity, daily carbon gain, and water use were simulated for Quercus coccifera under current ambient and future doubled CO2. Analyses of simulated canopy responses confirmed the importance of including (1) leaf energy balance and (2) distinguishing between sunlit and shaded leaves. A multi-layer canopy model with Gaussian integration for sunlit leaves and a single leaf class for shaded leaves in each layer gave excellent results. A multi-layer model with one shaded and one sunlit leaf class gave a reasonable approximation, and the single-layer model with one sunlit and one shaded leaf class resulted in errors of up to 15%. Vertical gradients in leaf nitrogen content and leaf and stem area index had greater effects on canopy assimilation and transpiration than did gradients of stem or leaf inclination or leaf width. However, predictions of the relative response of CO2 assimilation and transpiration to doubled CO2 are rather robust and were not greatly affected by simplifications of the canopy model. © 1992.
Reynolds, JF; Chen, JL; Harley, PC; Hilbert, DW; Dougherty, RL; Tenhunen, JD
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