Abstract, Ecological Society of America Annual Meeting, Spokane, WA, July, 1999.

KING, ANTHONY W.*, R. J. NORBY, C. A. GUNDERSON, and S. D. WULLSCHLEGER. Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA. Modeling canopy nitrogen distribution and photosynthetic response to elevated CO₂.

The Hirose-Werger model of canopy nitrogen distribution assumes an exponential decline of leaf nitrogen content with depth. Differential allocation of nitrogen to the top of the canopy where more light is available should lead to greater canopy photosynthesis than a uniform distribution, especially in a dense canopy. The distribution of leaf nitrogen observed in sweetgum before exposure to elevated CO₂ in the Oak Ridge FACE Experiment was consistent with the Hirose-Werger model, albeit suboptimal. We parameterized a model of canopy photosynthesis with these observations. In the model, leaf nitrogen influences both photosynthetic capacity and maintenance respiration. Simulations with doubled CO₂ predict a 35% increase in daily canopy photosynthesis. The increase is reduced by an increase in leaf area index (LAI), but reallocation of nitrogen to the top of the canopy can compensate. A shift to uniform distribution of leaf nitrogen still allows for a 20% increase in canopy photosynthesis. The model results generate hypotheses that will be tested with results from the Oak Ridge FACE Experiment. If there are costs in allocating nitrogen to the top of the canopy, elevated CO₂ might allow "relaxation" toward a more uniform distribution. Without costs, losses of canopy photosynthesis to increased LAI might be compensated for by reallocation of nitrogen. First year experimental results support model assumptions, but longer term results are required for conclusive tests of the hypotheses.