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 at mospheric CO2 concentration are likely to be constrained by nutrient l imitations. However, the interactive effects of nitrogen nutrition and CO2 concentration on growth are difficult to define because both fact ors affect several aspects of growth, including photosynthesis, respir ation, and leaf area. By expressing growth as a product of light inter cepted and light use efficiency (epsilon), it is possible to decouple the effects of nutrient availability and CO2 concentration on photosyn thetic rates from their effects on other aspects of plant growth. I us ed measured responses of leaf photosynthesis to leaf nitrogen (N) cont ent and CO2 concentration to parameterize a model of canopy radiation absorption and photosynthesis, and then used the model to estimate the response of E to elevated CO2 concentration for Pinus radiata D. Don, Nothofagus fusca (Hook. f.) Orst. and Eucalyptus grandis W. Hill ex M aiden. Down-regulation of photosynthesis at elevated CO2 was represent ed as a reduction in either leaf N content or leaf Rubisco activity. T he response of epsilon to elevated CO2, which differed among the three species, was analyzed in terms of the underlying relationships betwee n 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 availa bility and CO2 on growth are thus likely to be the result of either di fferences in down-regulation of photosynthesis at low and high N avail ability or interactive effects of CO2 and N availability on other aspe cts of plant growth.