PnET is a simple, lumped-parameter, monthlytime-step model of carbon and water balances of forests built on two principal relationships: 1) maximum photosynthetic rate is a function of foliar nitrogen concentration, and 2) stomatal conductance is a function of realized photosynthetic rate. Monthyly leaf area display and carbon and water balances are predicted by combining these with standard equations describing light attenuation in canopies and photosynthetic response to diminishing radiation intensity, along with effects of soil water stress and vapor pressure deficit (VPD). PnET has been validated against field data from 10 well-studied temperate and boreal forest ecosystems, supporting our central hypothesis that aggregation of climatic data to the monthly scale and biological data such as foliar characteristics to the ecosystem level does not cause a significant loss of information relative to long-term, mean ecosystem responses. Sensitivity analyses reveal a diversity of responses among systems to identical alterations in climatic drivers. This suggests that great care should be used in developing generalizations as to how forests will respond to a changing climate. Also critical is the degree to which the temperature responses of photosynthesis and respiration might acclimate to changes in mean temperatures at decadal time scales. An extreme climate change simulation (+3° C maximum temperature, -25% precipitation with no change in minimum temperature or radiation, direct effects of increased atmospheric CO ignored) suggests that major increases in water stress, and reductions in biomass production (net carbon gain) and water yield would follow such a change.

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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.

Five methods (Thornthwaite, Hamon, Jensen‐Haise, Turc, and Penman) for estimating potential evaporation for a reference surface (PEr) were compared to four methods (Priestley‐Taylor, McNaughton‐Black, Penman‐Monteith, and Shuttleworth‐Wallace) for estimating surface‐dependent potential evaporation (PEs) using three cover types at each of seven locations from Fairbanks, Alaska, to San Juan, Puerto Rico. For annual PE the PEs methods generally agreed with the PEr methods, but for many locations, differences among methods were hundreds of millimeters per year. No methods were consistently low or high. Three of the PEs methods depend strongly on maximum leaf conductance, for which Körner [1994] provided satisfactory values by cover type. Potential interception ∥PEi∥ can only be estimated appropriately for all cover types by the Shuttleworth‐Wallace method. Use of 5‐day or monthly input data did not greatly degrade results, so use of monthly data to generate PE estimates appears warranted in global water balance models.