Climate change effects on vegetation distribution and carbon budget in theUnited States

The Kyoto protocol has focused the attention of the public and policymarker s on the earth's carbon (C) budget. Previous estimates of the impacts of ve getation change have been limited to equilibrium "snapshots" that could not capture nonlinear or threshold effects along the trajectory of change. New models have been designed to complement equilibrium models and simulate ve getation succession through time while estimating variability in the C budg et and responses to episodic events such as drought and fire. In addition, a plethora of future climate scenarios has been used to produce a bewilderi ng variety of simulated ecological responses. Our objectives were to use an equilibrium model (Mapped Atmosphere-Plant-Soil system, or MAPSS) and a dy namic model (MCl) to (a) simulate changes in potential equilibrium-vegetati on distribution under historical conditions and across a wide gradient of f uture temperature changes to look for consistencies and trends among the ma ny future scenarios, (b) simulate time-dependent changes in vegetation dist ribution and its associated C pools to illustrate the possible trajectories of vegetation change near the high and low ends of the temperature gradien t, and (c) analyze the extent of the US area supporting a negative C balanc e. Both models agree that a moderate increase in temperature produces an in crease in vegetation density and carbon sequestration across most of the US with small changes in vegetation types. Large increases in temperature cau se losses of C with large shifts in vegetation types. In the western states , particularly southern California, precipitation and thus vegetation densi ty increase and forests expand under all but the hottest scenarios. In the eastern US, particularly the Southeast, forests expand under the more moder ate scenarios but decline under more severe climate scenarios, with catastr ophic fires potentially causing rapid vegetation conversions from forest to savanna. Both models show that there is a potential for either positive or negative feedbacks to the atmosphere depending on the level of warming in the climate change scenarios.