SIMPLE CARBON ASSIMILATION RESPONSE FUNCTIONS FROM ATMOSPHERIC CO2, AND DAILY TEMPERATURE AND SHORTWAVE RADIATION

A global 'CO2 fertilizer effect' multiplier is often used in crop or e cosystem models because of its simplicity. However, this approach does not take into account the interaction between CO2, temperature and li ght on assimilation. This omission can lead to significant under- or o verestimation of the magnitude of beneficial effects from elevated CO2 , depending on environmental conditions. We use a mechanistic model of the biochemistry of photosynthesis to represent the response of net a ssimilation to different levels of CO2, temperature and radiation, on the daily time scale. Instantaneous assimilation rates for an idealize d canopy model are integrated through diurnal cycles of environmental variables derived from historical climate data at three locations in N orth America. The calculated CO2 fertilizer effect is greatest at high light and warm temperatures. The results are summarized by assimilati on response surfaces specified by the CO2 concentration, the canopy le af area index, and by daily values of temperature and radiation availa ble from climatic records. These summary functions are suitable for in corporation into crop or ecosystem models for predicting carbon assimi lation or biomass production on a daily time step. An example applicat ion of the function reveals that for a relatively cool, high latitude location, the beneficial effects from a CO2 doubling would be negligib le during the early spring, even assuming a + 4 degrees C global warmi ng scenario. In contrast, the beneficial effects from increasing CO2 a t a relatively warm, lower latitude location are greatest in the sprin g, but decline in late summer because of excessively warm temperatures with a + 4 degrees C global warming.