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 relation 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 determine the productivity of forests growing under nitrogen-limited conditions. A simulated instantaneous doubling of atmospheric CO2 concentration leads to a growth response that is initially large (27% above productivity at current CO2) but declines to <10% elevation within 5 yr. The decline occurs because increases in photosynthetic carbon gain at elevated CO2 are not matched by increases in nutrient supply. Lower foliar N concentrations at elevated CO2 have two countervailing effects on forest production: decreased rates of N cycling between vegetation and soils (with negative consequences for productivity), and reduced rates 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 extremely sensitive to the assumed rate of gaseous emission resulting from mineralization of nitrogen. Theory developed to analyze G'DAY is applicable to other published production-decomposition models describing the partitioning of soil carbon among compartments with widely differing decay-time constants.