title: Effects of climate and atmospheric CO2partial pressure on the global distribution of C4grasses: present, past, and future.
abstract: C4photosynthetic physiologies exhibit fundamentally different responses to temperature and atmospheric CO2partial pressures (pCO2) compared to the evolutionarily more primitive C3type. All else being equal, C4plants tend to be favored over C3plants in warm humid climates and, conversely, C3plants tend to be favored over C4plants in cool climates. Empirical observations supported by a photosynthesis model predict the existence of a climatological crossover temperature above which C4species have a carbon gain advantage and below which C3species are favored. Model calculations and analysis of current plant distribution suggest that this pCO2-dependent crossover temperature is approximated by a mean temperature of 22°C for the warmest month at the current pCO2(35 Pa). In addition to favorable temperatures, C4plants require sufficient precipitation during the warm growing season. C4plants which are predominantly graminoids of short stature can be competitively excluded by trees (nearly all C3plants) - regardless of the photosynthetic superiority of the C4pathway - in regions otherwise favorable for C4. To construct global maps of the distribution of C4grasses for current, past and future climate scenarios, we make use of climatological data sets which provide estimates of the mean monthly temperature to classify the globe into areas which should favor C4photosynthesis during at least 1 month of the year. This area is 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 are 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. We then make use of this protocol to examine changes in the global abundance of C4grasses in the past and the future using plausible estimates for the climates and pCO2. When pCO2is lowered to pre-industrial levels, C4grasses expanded their range into large areas now classified as C3grasslands, especially in North America and Eurasia. During the last glacial maximum (∼18 ka BP) when the climate was cooler and pCO2was about 20 Pa, our analysis predicts substantial expansion of C4vegetation - particularly in Asia, despite cooler temperatures. Continued use of fossil fuels is expected to result in double the current pCO2by sometime in the next century, with some associated climate warming. Our analysis predicts a substantial reduction in the area of C4grasses under these conditions. These reductions from the past and into the future are based on greater stimulation of C3photosynthetic efficiency by higher pCO2than inhibition by higher temperatures. The predictions are testable through large-scale controlled growth studies and analysis of stable isotopes and other data from regions where large changes are predicted to have occurred.
authors.affiliation: Code 923, Biospheric Sciences Branch, NASA's Goddard Space Flight Center, Greenbelt, MD 20771, USA e-mail: jcollatz@biome.gsfc.nasa.gov; fax: 301-286-0239, , , , , , US.
authors.forename: G James
authors.initials: GJ
authors.lastname: Collatz
authors.affiliation: Department of Plant Biology, Carnegie Institution of Washington, Stanford, CA 94305, USA, , , , , , US.
authors.forename: Joseph A
authors.initials: JA
authors.lastname: Berry
authors.affiliation: Department of Botany, Duke University, Durham, NC 27708, USA, , , , , , US.
authors.forename: James S
authors.initials: JS
authors.lastname: Clark
keywords: C4; CO2; Climate change; Grassland; Key words Photosynthesis