Climate Change Effects on NPP and Water Yield

PnET-II is a simple, generalized, monthly time-step model of water and carbon balances (gross and net) driven by nitrogen availability as expressed through foliar N concentration. Improvements from the original model include a complete carbon balance and improvements in prediction of canopy phenology, as well as in the computation of canopy structure and photosynthesis. The model was parameterized and run for 4 forest/site combinations 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 soil-based 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 New England/New York (UAS) 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^2 /yr for the region, with a regional average of 76 g C /m^2/yr.

A combination of increased temperature (+6 deg C), decreased precipitation (-15%) and increased water use efficiency (2X, due to doubling of CO2) resulted generally in increases in NPP and decreases in water yield over the region.