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

PnET (net photosynthesis / evapotranspiration) is a nested series of
models of carbon (C), water and nitrogen dynamics in forest
ecosystems. The models can be used to predict transient responses in
net primary production (NPP), carbon and water balances, net N
mineralization and nitrification and N leaching losses, resulting from
changes in climate, N deposition, troposheric ozone and land use as
well as variation in species composition. The models use generalized
representations of physiological and ecosystem processes and were
designed to be run with relatively few input parameters that require
no calibration. At the core of each version is a multi-layered canopy
model of phenology and photosynthesis (PnET-Day) that was designed to
simulate daily canopy carbon exchange.
The PnET-II model is a canopy- to stand-level model of monthly
carbon and water balances for temperate and boreal forest ecosystems.
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.