An Ecosystem Model of Carbon, Nitrogen and Water Balances in Forests

Central to the model are relationships between foliar nitrogen and maximum leaf photosynthesis, and between actual photosynthesis and stomatal conductance. Together, these provide a link between the carbon and water balance portions of the model in that transpiration is determined as a function of canopy photosynthesis. The model has been applied and validated regionally in the northeastern US and elsewhere using input parameters that reflect typical deciduous and coniferous forests.

PnET-II outputs include annual net primary productivity (NPP), wood production and runoff under current and climate change conditions (+6 degrees C, -15% precipitation and 2X CO2). Climate change predictions are presented as ratios of future to current values. The PnET-CN model adds live biomass, litter and soil organic matter compartments to the structure of PnET-II and simulates temporal changes in both plant and soil C and N pools.

Nitrogen is included in all compartments and fluxes, as well as N mineralization and nitrification, plant N uptake and leaching losses to produce complete cycles for water, carbon and nitrogen. Canopy and soil processes interact through C:N ratios in foliage and litter. Foliar N concentrations vary with the availability of C and N in plants.

Tissue C:N ratios are reflected in litter, and high C/N ratios decrease net N mineralization. PnET-CN can be applied transiently where site history information is available. PnET-CN outputs presented here are regional predictions of equilibrium N cycling.