Effects of climate and atmospheric CO2 partial pressure on the global distribution of C4 grasses: present, past, and future.

摘要

C4 photosynthetic physiologies exhibit fundamentally different responses to temperature and atmospheric CO2 partial pressures (pCO2) compared with the evolutionarily more primitive C3 type. All else being equal, C4 plants tend to be favoured over C3plants in warm humid climates and, conversely, C3 plants tend to be favoured over C4 plants in cool climates. Empirical observations supported by a photosynthesis model predicted the existence of a climatological crossover temperature above which C4 species have a carbon gain advantage and below which C3 species are favoured. Model calculations and analysis of current plant distribution suggested that this pCO2-dependent crossover temperature is approximated by a mean temperature of 22deg C for the warmest month at the current pCO2 (35 Pa). In addition to favourable temperatures, C4 plants require sufficient precipitation during the warm growing season. C4 plants which are predominantly graminoids of short stature can be competitively excluded by trees (nearly all C3 plants) - regardless of the photosynthetic superiority of the C4 pathway - in regions otherwise favourable for C4. To construct global maps of the distribution of C4 grasses for current, past and future climate scenarios, use was made of climatological data sets which provide estimates of the mean monthly temperature to classify the globe into areas which should favour C4 photosynthesis during at least 1 month of the year. This area was further screened by excluding areas where precipitation is <25 mm per month during the warm season and by selecting areas classified as grasslands (i.e., excluding areas dominated by woody vegetation) according to a global vegetation map. Using this approach, grasslands of the world were designated as C3,C4, and mixed under current climate and pCO2. Published floristic studies were used to test the accuracy of these predictions in many regions of the world, and agreement with observations was generally good. This protocol was then used to examine changesin the global abundance of C4 grasses in the past and the future using plausible estimates for the climates and pCO2. When pCO2 is lowered to pre-industrial levels. C4 grasses expanded their range into large areas now classified as C3 grasslands, especially in North America and Eurasia. During the last glacial maximum (～18 ka BP) when the climate was cooler and pCO2 was about 20 Pa, the analysis predicted substantial expansion of C4 vegetation particularly in Asia, despite cooler temperatures. Continued use of fossil fuels is expected to result in double the current pCO2 by sometime in the next century, with some associated climate warming. The analysis predicted a substantial reduction in the area of C4 grasses under these conditions. These reductions from the past and into the future were based on greater stimulation of C3 photosynthetic efficiency by higher pCO2 than inhibition by higher temperatures. The predictions are testable through large-scale controlled growth studies and analysis of stableisotopes and other data from regions where large changes are predicted to have occurred.