MATHEMATICAL-MODELS OF THE PHOTOSYNTHETIC RESPONSE OF TREE STANDS TO RISING CO2 CONCENTRATIONS AND TEMPERATURES

Two published models of canopy photosynthesis, MAESTRO and BIOMASS, are simulated to examine the response of tree stands to increasing ambient concentrations of carbon dioxide (C(a)) and temperatures. The models employ the same equations to described leaf gas exchange, but differ considerably in the level of detail employed to represent canopy structure and radiation environment. Daily rates of canopy photosynthesis simulated by the two models agree to within 10% across a range of CO2 concentrations and temperatures. A doubling of C(a) leads to modest increases of simulated daily canopy photosynthesis at low temperatures (10% increase at 10-degrees-C), but larger increases at higher temperatures (60% increase at 30-degrees-C). The temperature and CO2 dependencies of canopy photosynthesis are interpreted in terms of simulated contributions by quantum-saturated and non-saturated foliage. Simulations are presented for periods ranging from a diurnal cycle to several years. Annual canopy photosynthesis simulated by BIOMASS for trees experiencing no water stress is linearly related to simulated annual absorbed photosynthetically active radiation, with light utilization coefficients for carbon of epsilon = 1.66 and 2.07 g MJ-1 derived for C(a) of 350 and 700 mumol mol-1, respectively.