© 1996 Heron Publishing—Victoria, Canada
Interactive effects of atmospheric carbon dioxide and leaf nitrogen concentration on canopy light use efficiency: a modeling
analysis
B. E. Medlyn
School of Biological Science, University of New South Wales, Sydney, NSW 2052, Australia / Received March 2, 1995
Summary
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.
Keywords:
canopy model, elevated CO2, photosynthetic downregulation.
