Climate Change Effects on NPP and Water Yield

These datasets were formed from the ratios of two runs of PnET , a forest
ecosystem model (see Parent DIF). It represents a grid of pixels in New
England and eastern New York at spatial resolution of 60 arc seconds
(about 2 kilometers). One model run used ambient levels of CO2, precipitation
and temperature, and the other run used modified levels. Rapid and simultaneous
changes in temperature, precipitation and the atmospheric concentration of
CO2 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 model is an improved version of a forest carbon and water balance model
(PnEt-II) and the application of the model predicts stand-level and regional-
level effects of changes in temperature, precipitation, and atmospheric CO2
concentration. 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.