RESPONSE OF TEMPERATE AND TROPICAL GRASSLANDS TO CO2 AND CLIMATE-CHANGE

Under a recent SCOPE collaborative project, longterm data from eleven tropical and temperate grassland sites were used (a) to validate the C ENTURY model of plant-soil ecosystems and (b) to model climate change and CO2 effects for thirty-one temperate and tropical grassland sites, representing seven ecoregions of the world. Model calibration and tes ting showed that soil carbon and nitrogen dynamics can be well simulat ed for the grassland biome worldwide, over a wide range of climate and soil types. The interannual response of above ground biomass and plan t residue to variation in climate resulted in a good correspondence be tween simulated and observed dynamics on a monthly basis. These result s are useful for analysis and description of grassland carbon dynamics , and as a reference point for testing predictions of net primary prod uction (NPP) and biomass dynamics from levels of more physiologically based models. Prediction of plant and soil organic matter C and N dyna mics requires knowledge of climate, soil texture, N inputs and fire an d grazing patterns. CENTURY simulations of climate change and CO2 effe cts showed increased NPP for climate change alone, except in cold dese rt steppe regions, and CO2 increased production everywhere. Climate ch anges, predominantly a warming of these ecosystems, caused soil carbon to decrease overall, especially in cold desert and temperate steppes. Increased production due to elevated CO2 tended to ameliorate soil ca rbon losses and tropical savannas were actually soil carbon sinks. Cli mate change alone projected a carbon loss of 3-4 Pg after 50 years, an d 1-2 Pg for the combined climate change and CO2 simulated effects. We analysed the dynamic response of some of the major CENTURY output par ameters (e.g. NPP, soil organic matter, N mineralization and decomposi tion) for their sensitivity to climate change and increasing CO2 for o ne of the two general circulation models (GFHI scenario). This analysi s was limited to a subset of five well-known study sites, representing five of the seven ecoregions.