The influence of seasonal water availability on global C3 versus C4 grassland biomass and its implications for climate change research

Climate-change induced alterations in the global distribution of cool season (C-3) and warm season (C-4) grasses would impact the global carbon cycle and have differing, local effects on range and agricultural production. We hypothesize that a major influence on C-3/C-4 distribution may be the seasonal timing of water availability with respect to the different C-3 and C-4 growing seasons. An algorithm expressing this hypothesis (the SAW hypothesis for Seasonal Availability of Water), estimates C-3 versus C-4 grass biomass from climate data. Sensitivity analysis indicated that temperatures used to delineate the start and end of the C-3 and C-4 grass growing seasons were more important than photosynthetic responses to temperature. To evaluate the SAW hypothesis, this algorithm was applied globally on a 1degrees x 1degrees latitude-longitude grid. When compared with vegetation survey data at 141 locations in North America, Argentina, Australia, and South Africa, SAW algorithm predictions yielded art R-2 of 0.71. Error resulted primarily from comparing large grid cells to plot data, interannual variability of climate, and from gridding measured climate to data-sparse locations with a single lapse rate of air temperature with elevation. Application of the SAW algorithm to a climate change scenario suggested that changes in temperature and precipitation patterns could offset C-3 photosynthetic advantages offered by elevated atmospheric CO2 concentrations. These results underscored the importance of accurately representing the timing and spatial distribution as well as the magnitude of temperature and precipitation in scenarios of future climate. (C) 2003 Elsevier Science B.V. All rights reserved.