Development and Evaluation of Simplified Models for Simulating Canopy Photosynthesis and Transpiration

Several soil-plant-atmosphere models have been developed to simulate the energy transport and CO2 exchange of vegetation (Waggoner, Furnival & Reifsnyder 1969; Stewart & Lemon 1969; Sinclair, Allen & Stewart 1971; Murphy & Knoerr 1970, 1972; Goudriaan & Waggoner 1972). These models were designed to simulate, at a detailed and complex level, the physical processes involved. Submodels of physiological processes were included in these models so that predictions of micrometeorological conditions could be used to simulate the exchange of CO2 and water vapour by foliage. Previous models were so complex that they required considerable input data and computer time. We were therefore interested in developing and evaluating simpler models which require less input data and less computation. Such models should simulate, if possible, the canopy environment sufficiently well to predict photosynthesis and transpiration rates comparable to those of the complex models. With these objectives in mind, there are at least two reasons why all the micrometeorological processes may not need to be considered. First, the most significant resistance to CO2 and water vapour diffusion is generally at the foliage surface and bulk exchange in the atmosphere is comparatively much less important. Second, the microenvironment within many canopies usually does not deviate greatly from ambient conditions, except for radiation, and may be relatively unimportant in affecting CO2 and water vapour exchange. Two simpler models are here compared with a complex micrometeorological model to see to what extent complex models are necessary to obtain satisfactory estimates of CO2 and water vapour exchange. The micrometeorological model, SPAM, developed by Stewart & Lemon (1969) and modified by Sinclair et al. (1971), is used as the standard for evaluating the less complex models because it has been shown to produce adequate simulations of microenvironment and energy flux densities (Lemon, Stewart & Shawcroft 1971). A simplified version of SPAM was tested, which assumes the air temperature, water vapour, and CO2 within the canopy are similar to those above the canopy, i.e. the aerodynamic transfer coefficients are infinite. This model therefore tests directly the hypothesis that canopy microenvironment, except for radiation distribution, is unimportant. The second less-complex model assumes that a canopy can be mathematically condensed to a single plane and treated as a 'big leaf'. While the 'big leaf' model assumes all leaves are exposed to the same microenvironment, the leaf environment is different from ambient conditions above the canopy. The big leaf model for transpiration is essentially the Monteith (1965) evapotranspiration equation derived from the approach developed by Penman (1953). However, the evaluation of the variables in the Monteith