LONG-TERM RESPONSE OF NUTRIENT-LIMITED FORESTS TO CO2 ENRICHMENT - EQUILIBRIUM BEHAVIOR OF PLANT-SOIL MODELS

Established process-based models of forest biomass production in relat ion to atmospheric CO2 concentration (McMurtrie 1991) and soil carbon/ nutrient dynamics (Parton et al. 1987) are integrated to derive the '' Generic Decomposition and Yield'' model (G'DAY). The model is used to describe how photosynthesis and nutritional factors interact to determ ine the productivity of forests growing under nitrogen-limited conditi ons. A simulated instantaneous doubling of atmospheric CO2 concentrati on leads to a growth response that is initially large (27% above produ ctivity at current CO2) but declines to <10% elevation within 5 yr. Th e decline occurs because increases in photosynthetic carbon gain at el evated CO2 are not matched by increases in nutrient supply. Lower foli ar N concentrations at elevated CO2 have two countervailing effects on forest production: decreased rates of N cycling between vegetation an d soils (with negative consequences for productivity), and reduced rat es of N loss through gaseous emission, fire, and leaching. Theoretical analysis reveals that there is an enduring response to CO2 enrichment , but that the magnitude of the long-term equilibrium response is extr emely sensitive to the assumed rate of gaseous emission resulting from mineralization of nitrogen. Theory developed to analyze G'DAY is appl icable to other published production-decomposition models describing t he partitioning of soil carbon among compartments with widely differin g decay-time constants.