PARENT SESSION
Oral Session #76: Plant Ecology: Nutrient Uptake, Movement, Use. Presiding: J. Schade.
Friday, August 10, 2001. 8:00 AM to 11:45 AM. Hall of Ideas G.
Coupling of canopy photosynthesis and nitrogen distribution in a pine forest predicted by a N turnover model.
Ellsworth, David1,2, Lai, Chun-Ta2, Kull, Olevi3, Katul, Gabriel2, 1 2 3
ABSTRACT- Carbon and nitrogen (N) cycles are tightly coupled in forest canopies because at the leaf-level there is a fundamental relationship between net CO2 assimilation capacity (A) and leaf N. The coupled N-irradiance distribution hypothesis of Hirose and Werger most widely employed to scale leaf-level A and N from the top of a forest canopy to all canopy positions presumes strictly parallel decreases in both N and maximum A with decreasing relative irradiance (Iz/Io) at progressively deeper canopy positions. While this approach has been used in many recent canopy photosynthesis models, it does not permit dynamic changes in leaf N with changing environmental conditions. We describe a mechanistic model of N distribution in forest canopies based on enzyme kinetics and turnover processes implemented within a coupled mass and energy exchange model. The model redistributes N according to the balance between supply of carbohydrates by photosynthesis and the demand for N by enzyme synthesis and turnover. We parameterized the model for a Pinus taeda forest (Duke Forest, NC) and tested the results against flux-tower data for this stand. Both the turnover model and the measurements showed a steeper decline in leaf N than was predicted from the coupled N-irradiance approach. While they differed in predicted N distributions in the canopy, the daily net CO2 flux (FCO2) from the N-irradiance approach was only 4% lower than daily FCO2 from the N turnover model, and both models reproduced the measured forest FCO2 well (r2 = 0.73 and 0.75 for the two models). Thus the previous, N-irradiance approach could be considered sufficient for simulating forest FCO2 though not under conditions where dynamic N adjustments are expected. Compared to alternate canopy N distributions with the same total canopy N, numerical simulations indicated that the turnover model reproduced the measured N distribution, maximized total daily canopy photosynthesis, and also maximized forest FCO2. The proposed canopy N turnover approach can be coupled with existing nutrient-uptake models, and may be a promising approach for predict the effects of rising atmospheric CO2 and nitrogen deposition on forest canopy C and N cycles.
KEY WORDS: canopy nitrogen, elevated CO2