EFFECTS OF CO2 AND NO3− AVAILABILITY ON DECIDUOUS TREES: PHYTOCHEMISTRY AND INSECT PERFORMANCE

Increasing concentrations of atmospheric CO2 will interact with other en- vironmental factors to influence the physiology and ecology of trees. This research evaluated how plant phytochemical responses to enriched atmospheric CO2 are affected by the avail- ability of soil nitrate (NO3-) and how these chemical changes, in turn, alter the performance of a tree-feeding folivore. Seedlings of three deciduous tree species-quaking aspen (Pop- ulus tremuloides), red oak (Quercus rubra), and sugar maple (Acer saccharum)-were grown in ambient (355 FL/L) or elevated (650 FL/L) CO2 in combination with low (1.25 mmol/L) or high (7.5 mmol/L) soil NO3- availability. After 60 d, foliage was analyzed for changes in nutrients and allelochemicals likely to be influenced by the availability of CO2 and NO3-. Penultimate gypsy moth larvae (Lymantria dispar) were reared on foliage (aspen and maple) to determine how performance would be affected by host chemical changes. Using the framework of carbon-nutrient balance (CNB) theory, we tested three hypotheses regarding the impact of CO2 and NO3- availability on plant chemistry and insect performance: (1) nitrogen-based compounds will decrease, and carbon-based compounds will increase in- response to elevated CO2 and/or low NO3-; (2) aspen will exhibit the greatest change in C:N ratios, and maple the least; and (3) phytochemical changes will influence gypsy moth perfor- mance, with larvae fed aspen being affected more than those fed maple. Concentrations of nitrogen and soluble protein decreased, whereas concentrations of starch, condensed tannins, and ellagitannins increased, in response to elevated CO2 and/or low NO3-. Responses of simple carbohydrates and phenolic glycosides were variable, however, suggesting that foliar accumulations of "dynamic metabolites" do not follow the predictions of CNB theory as well as do those of stable end products. With respect to Hypothesis 2, we found that absolute (net) changes in foliar C:N ratios were greatest for aspen and least for oak, whereas relative (proportional) changes were greatest for maple and least for aspen. Thus, Hypothesis 2 was only partially supported by the data. Considering Hypothesis 3, we found that elevated CO2 treatments had little effect on gypsy moth development time, growth rate, or larval mass. Larvae reared on aspen foliage grown under elevated CO2 exhibited increased consumption but decreased conversion efficiencies. Gypsy moth responses to NO3- were strongly host spe- cific: the highest consumption and food digestibility occurred in larvae on high-NO3- aspen, whereas the fastest growth rates occurred in larvae on high-NO3- maple. In short, our results again only partially supported the predicted pattern. They indicate, however, that the magnitude of insect response elicited by resource-mediated shifts in host chemistry will depend on how levels of compounds with specific importance to insect fitness (e.g., phenolic glycosides in aspen) are affected. Overall, we observed relatively few true interactions (i.e., nonadditive) between carbon and nitrogen availability vis a vis foliar chemistry and insect performance. Tree species, however, frequently interacted with CO2 and/or NO3- availability to affect both sets of parameters. These results suggest that the effects of elevated atmospheric CO2 on terrestrial plant communities will not be homogeneous, but will depend on species composition and soil nutrient availability.