EFFECTS OF CLIMATE AND ATMOSPHERIC CO2 PARTIAL-PRESSURE ON THE GLOBALDISTRIBUTION OF C-4 GRASSES - PRESENT, PAST, AND FUTURE

C-4 photosynthetic physiologies exhibit fundamentally different respon ses to temperature and atmospheric CO2 partial pressures (pCO(2)) comp ared to the evolutionarily more primitive C-3 type. All else being equ al, C-4 plants tend to be favored over C-3 plants in warm humid climat es and, conversely, C-3 plants tend to be favored over C-4 plants in c ool climates. Empirical observations supported by a photosynthesis mod el predict the existence of a climatological crossover temperature abo ve which C-4 species have a carbon gain advantage and below which C-3 species are favored. Model calculations and analysis of current plant distribution suggest that this pCO(2)-dependent crossover temperature is approximated by a mean temperature of 22 degrees C for the warmest month at the current pCO(2) (35 Pa). In addition to favorable temperat ures, C-4 plants require sufficient precipitation during the warm grow ing season. C-4 plants which are predominantly graminoids of short sta ture can be competitively excluded by trees (nearly all C-3 plants) - regardless of the photosynthetic superiority of the C-4 pathway - in r egions otherwise favorable for C-4. To construct global maps of the di stribution of C-4 grasses for current, past and future climate scenari os, we make use of climatological data sets which provide estimates of the mean monthly temperature to classify the globe into areas which s hould favor C-4 photosynthesis during at least 1 month of the year. Th is area is further screened by excluding areas where precipitation is <25 mm per month during the warm season and by selecting areas classif ied as grasslands (i.e., excluding areas dominated by woody vegetation ) according to a global vegetation map. Using this approach, grassland s of the world are designated as C-3, C-4, and mixed under current cli mate and (pCO(2) Published floristic studies were used to test the acc uracy of these predictions in many legions of the world, and agreement with observations was generally good. We then make use of this protoc ol to examine changes in the global abundance of C-4 grasses in the pa st and the future using plausible estimates for the climates and pCO(2 ). When pCO(2) is lowered to pre-industrial levels, C-4 grasses expand ed their range into large areas now classified as C-3 grasslands, espe cially in North America and Eurasia. During the last glacial maximum ( similar to 18 ka BP) when the climate was cooler and pCO(2) was about 20 Pa, our analysis predicts substantial expansion of C-4 vegetation - particularly in Asia, despite cooler temperatures. Continued use of f ossil fuels is expected to result in double the current pCO(2) by some time in the next century, with some associated climate warming. Our an alysis predicts a substantial reduction in the area of C-4 grasses und er these conditions. These reductions from the past and into the futur e are based on greater stimulation of C-3 photosynthetic efficiency by higher pCO(2) than inhibition by higher temperatures. The predictions are testable through large-scale controlled growth studies and analys is of stable isotopes and other data from regions where large changes are predicted to have occurred.