Interactive effects of atmospheric carbon dioxide and leaf nitrogen concentration on canopy light use efficiency: A modeling analysis

Potential increases in plant productivity in response to increasing atmospheric CO₂ concentration are likely to be constrained by nutrient limitations. However, the interactive effects of nitrogen nutrition and CO₂ concentration on growth are difficult to define because both factors affect several aspects of growth, including photosynthesis, respiration, and leaf area. By expressing growth as a product of light intercepted and light use efficiency (ε), it is possible to decouple the effects of nutrient availability and CO₂ concentration on photosynthetic rates from their effects on other aspects of plant growth. I used measured responses of leaf photosynthesis to leaf nitrogen (N) content and CO₂ concentration to parameterize a model of canopy radiation absorption and photosynthesis, and then used the model to estimate the response of ε to elevated CO₂ concentration for Pinus radiata D. Don, Nothofagus fusca (Hook. f.) Ørst. and Eucalyptus grandis W. Hill ex Maiden. Down-regulation of photosynthesis at elevated CO₂ was represented as a reduction in either leaf N content or leaf Rubisco activity. The response of ε to elevated CO₂, which differed among the three species, was analyzed in terms of the underlying relationships between leaf photosynthesis and leaf N content. The response was independent of leaf N content when photosynthesis was down-regulated to the same extent at low and high leaf N content. Interactive effects of N availability and CO₂ on growth are thus likely to be the result of either differences in down-regulation of photosynthesis at low and high N availability or interactive effects of CO₂ and N availability on other aspects of plant growth.