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Rap~d and simultaneous changes in temperature, precipitation and the atmospheric concentration of CO, are predicted to occur over the next century. Simple, well-validated models of ecosystem function are required to predict the effects of these changes. This paper describes an improved version of a forest carbon and water balance model (PnET-11) and the application of the model to predict stand-and regional-level effects of changes in temperature, precipitation and atmospheric CO2 conceniraiion. PnET-ii is d s u ~~p i e , y e ~~e l d i i ~e d , l~lu~lii~iy ii~~ie-btep ~nociel of water and carbon "vlances (gross and net) driven by nitrogen availability as expressed through foliar N concentration. Improvements from the orig~nal model include a complete carbon balance and improvements in the prediction of canopy phenology, as well as in the computation of canopy structure and photosynthesis. The model was parameterized and run for 4 forestkite con~binations and validated against available data for water yield, gross and net carbon exchange and biomass production. The validation exercise suggests that the determination of actual water availability to stands and the occurrence or non-occurrence of soilbased water stress are critical to accurate modeling of forest net primary production (NPP) and net ecosystem production (NEP). The model was then run for the entire NewEngland/New York (USA) region using a 1 km resolution geographic information system. Predicted long-term NEP ranged from -85 to +275 g C m-2 yr" for the 4 forest/site combinations, and from -150 to 350 g C m-' yr-' for the region, with a regional average of 76 g C m-2 yr-l A con~bination of increased temperature (+6OC), decreased precipitation (-15%) and increased water use efficiency (2x, due to doubling of CO,) resulted generally in increases in NPP and decreases in water yield over the region.
Aber et al. (Sun,) studied this question.