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

Increasing concentrations of atmospheric CO2 will interact with other environmental factors to influence the physiology and ecology of trees . This research evaluated how plant phytochemical responses to enriche d atmospheric CO2 are affected by the availability of soil nitrate (NO 3-) and how these chemical changes, in turn, alter the performance of a tree-feeding folivore. Seedlings of three deciduous tree species-qua king aspen (Populus tremuloides), red oak (Quercus rubra), and sugar m aple (Acer saccharum)-were grown in ambient (355 mu L/L) or elevated ( 650 mu L/L) CO2 in combination with low (1.25 mmol/L) or high (7.5 mmo l/L) soil NO3- availability. After 60 d, foliage was analysed for chan ges in nutrients and allelochemicals likely to be influenced by the av ailability of CO2 and NO3-. Penultimate gypsy moth larvae (Lymantria d ispar) were reared on foliage (aspen and maple) to determine how perfo rmance would be affected by host chemical changes. Using the framework of carbon-nutrient balance (CNB) theory, we tested three hypotheses r egarding the impact of CO2 and NO3- availability on plant chemistry an d insect performance: (1) nitrogen-based compounds will decrease, and carbon-based compounds will increase in response to elevated CO2 and/o r low NO3-; (2) aspen will exhibit the greatest change in C:N ratios, and maple the least; and (3) phytochemical changes will influence gyps y moth performance, with larvae fed aspen being affected more than tho se fed maple. Concentrations of nitrogen and soluble protein decreased , whereas concentrations of starch, condensed tannins, and ellagitanni ns increased, in response to elevated CO2 and/or low NO3-. Responses o f simple carbohydrates and phenolic glycosides were variable, however, suggesting that foliar accumulations of ''dynamic metabolites'' do no t follow the predictions of CNB theory as well as do those of stable e nd products. With respect to Hypothesis 2, we found that absolute (net ) changes in foliar C:N ratios were greatest for aspen and least for o ak, whereas relative (proportional) changes were greatest for maple an d least for aspen. Thus, Hypothesis 2 was only partially supported by the data. Considering Hypothesis 3, we found that elevated CO2 treatme nts had little effect on gypsy moth development time, growth rate, or larval mass. Larvae reared on aspen foliage grown under elevated CO2 e xhibited increased consumption but decreased conversion efficiencies. Gypsy moth responses to NO3- were strongly host specific: the highest consumption and food digestibility occurred in larvae on high-NO3- asp en, whereas the fastest growth rates occurred in larvae on high-NO3- m aple. In short, our results again only partially supported the predict ed pattern, They indicate, however, that the magnitude of insect respo nse 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 observ ed relatively few true interactions (i.e,, nonadditive) between carbon and nitrogen availability vis a vis foliar chemistry and insect perfo rmance. Tree species, however, frequently interacted with CO2 and/or N O3- availability to affect both sets of parameters. These results sugg est that the effects of elevated atmospheric CO2 on terrestrial plant communities will not be homogeneous, but will depend on species compos ition and soil nutrient availability.