The Terrestrial Ecosystem Model (TEM version 4) is a process-based ecosystem model (Raich et al., 1989; McGuire et al. 1992, 1993, 1996a, 1996b; Melillo et al., 1993, 1995 ) that describes carbon and nitrogen dynamics of plant and soils for non-wetland ecosystems of the globe. The TEM uses spatially referenced information on climate, elevation, soils, vegetation and water availability as well as soil- and vegetation-specific parameters to make monthly estimates of important carbon and nitrogen fluxes and pool sizes. Hydrological inputs for TEM are determined by a water balance model (Vorosmarty et al. 1989) that use the same climatic data and soil-specific parameters as used in TEM. The TEM operates on a monthly time step and at a 0.5 degrees latitude/longitude spatial resolution.
In TEM, annual primary production (NPP) is the difference between carbon captured from the atmosphere as gross primary production (GPP) and carbon respired to the atmosphere by the vegetation. Gross primary production is calculated as a function of light availability, air temperature, atmospheric CO2 concentration, moisture availability and nitrogen supply. The nitrogen uptake in the model is controlled by the stoichiometric C:N ratio of biomass production. The carbon-nitrogen status of the vegetation cause the model to allocate more effort towards either carbon or nitrogen uptake. Plant respiration is a function of vegetation carbon(i.e. biomass) and air temperature. In TEM, decomposition is a function of the one soil organic carbon compartment, temperature and soil moisture. The carbon and nitrogen pool sizes of vegetation and soil are affected by dynamic carbon and nitrogen fluxes (NPP, litterfall C, decomposition, litterfall N, net N mineralization, N uptake, etc.). Elevated CO2 may have either a direct or indirect effect on GPP. A direct consequence of elevated atmospheric CO2 is to increase GPP via a Michaelis -Menton (hyperbolic) relationship. Elevated CO2 may indirectly affect GPP by altering the carbon-nitrogen status of the vegetation to increase effort towards nitrogen uptake.
For simulating mature ecosystems at "equilibrium" as required by the VEMAP activity (VEMAP Members, 1995), TEM assumes equilibrium conditions are reached when: 1) annual fluxes of NPP, litterfall carbon, and decomposition are balanced; 2) the annual fluxes of net nitrogen mineralization, litterfall nitrogen, and nitrogen uptake by vegetation are balanced; and 3) nitrogen inputs are equal to nitrogen losses from the ecosystem.
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