VEGETATION ECOSYSTEM MODELING AND ANALYSIS PROJECT - COMPARING BIOGEOGRAPHY AND BIOGEOCHEMISTRY MODELS IN A CONTINENTAL-SCALE STUDY OF TERRESTRIAL ECOSYSTEM RESPONSES TO CLIMATE-CHANGE AND CO2 DOUBLING

We compare the simulations of three biogeography models (BIOME2, Dynam ic Global Phytogeography Model (DOLY), and Mapped Atmosphere-Plant Soi l System (MAPSS)) and three biogeochemistry models (BIOME-BGC (BioGeoc hemistry Cycles), CENTURY, and Terrestrial Ecosystem Model (TEM)) for the conterminous United States under contemporary conditions of atmosp heric CO2 and climate. We also compare the simulations of these models under doubled CO2 and a range of climate scenarios. For contemporary conditions, the biogeography models successfully simulate the geograph ic distribution of major vegetation types and have similar estimates o f area for forests (42 to 46% of the conterminous United States), gras slands (17 to 27%), savannas (15 to 25%), and shrublands (14 to 18%). The biogeochemistry models estimate similar continental-scale net prim ary production (NPP; 3125 to 3772 x 10(12) gC yr(-1)) and total carbon storage (108 to 118 x 10(15) gC) for contemporary conditions. Among t he scenarios of doubled CO2 and associated equilibrium climates produc ed by the three general circulation models (Oregon State University (O SU), Geophysical Fluid Dynamics Laboratory (GFDL), and United Kingdom Meteorological Office (UKMO)), all three biogeography models show both gains and losses of total forest area depending on the scenario (betw een 38 and 53% of conterminous United States area). The only consisten t gains in forest area with all three models (BIOME2, DOLY, and MAPSS) were under the GFDL scenario due to large increases in precipitation. MAPSS lost forest area under UKMO, DOLY under OSU, and BIOME2 under b oth UKMO and OSU, The variability in forest area estimates occurs beca use the hydrologic cycles of the biogeography models have different se nsitivities to increases in temperature and CO2. However, in general, the biogeography models produced broadly similar results when incorpor ating both climate change and elevated CO2 concentrations. For these s cenarios, the NPP estimated by the biogeochemistry models increases be tween 2% (BIOME-BGC with UKMO climate) and 35% (TEM with UKMO climate) . Changes in total carbon storage range from losses of 33% (BIOME-BGC with UKMO climate) to gains of 16% (TEM with OSU climate). The CENTURY responses of NPP and carbon storage are positive and intermediate to the responses of BIOME-BGC and TEM. The variability in carbon cycle re sponses occurs because the hydrologic and nitrogen cycles of the bioge ochemistry models have different sensitivities to increases in tempera ture and CO2. When the biogeochemistry models are run with the vegetat ion distributions of the biogeography models, NPP ranges from no respo nse (BIOME-BGCwith all three biogeography model vegetations for UKMO c limate) to increases of 40% (TEM with MAPSS vegetation for OSU climate ). The total carbon storage response ranges from a decrease of 39% (BI OME-BGC with MAPSS vegetation for UKMO climate) to an increase of 32% (TEM with MAPSS vegetation for OSU and GFDL climates). The UKMO respon ses of BIOME-BGC with MAPSS vegetation are primarily caused by decreas es in forested area and temperature-induced water stress, The OSU and GFDL responses of TEM with MAPSS vegetations are primarily caused by f orest expansion and temperature-enhanced nitrogen cycling.