Effects of CO2 and light on tree phytochemistry and insect performance

Direct and interactive effects of CO2 and light on tree phytochemistry and insect fitness parameters were examined through experimental manipulations of plant growth conditions and performance of insect bioassays. Three speci es of deciduous trees (quaking aspen, Populus tremuloides: paper birch, Bet ula papyrifera; sugar maple, Acer saccharum) were grown under ambient (387 +/- 8 mu L/L) and elevated (696 +/- 2 mu L/L) levels of atmospheric CO2, wi th low and high light availability (375 and 855 mu mol x m(-2) x s(-1) at s olar noon). Effects on the population and individual performance of a gener alist phytophagous insect, the white-marked tussock moth (Orgyia leucostigm a) were evaluated. Caterpillars were reared on experimental trees for the d uration of the larval stage; and complementary short-term (fourth instar) F eeding trials were conducted with insects fed detached leaves. Phytochemical analyses demonstrated strong effects of both CO2 and light on all foliar nutritional variables (water. starch and nitrogen). For all spe cies. enriched CO2 decreased water content and increased starch content, es pecially under high light conditions. High CO2 availability reduced levels of foliar nitrogen. but effects were species specific and most pronounced f or high light aspen and birch. Analyses of secondary plant compounds reveal ed that levels of phenolic glycosides (salicortin and tremulacin) in aspen and condensed tannins in birch and maple were positively influenced by leve ls of both CO2 and light. In contrast, levels of condensed tannins in aspen were primarily affected by light, whereas levels of ellagitannins and gall otannins in maple responded to light and CO2, respectively. The lone-term bioassays showed strong treatment effects on survival, develo pment time, and pupal mass. In general. CO2 effects were pronounced in high light and decreased along the gradient aspen > birch > maple. For larvae r eared on high light aspen, enriched CO2 resulted in 62% fewer survivors. wi th increased development time, and reduced pupal mass. For maple-fed insect s, elevated CO2 levels had negative effects on survival and pupal mass in l ow light. For birch, the only negative CO2 effects were observed in high li ght, where female larvae showed prolonged development. Fourth instar feedin g trials demonstrated that low food conversion efficiency reduced insect pe rformance. Elevated levels of CO2 significantly reduced total consumption, especially by insects on high light aspen and loa: light maple. This research demonstrates that effects of CO2 on phytochemistry and insect performance can be strongly light-dependent, and that plant responses to t hese two environmental variables differ among species. Overall, increased C O2 availability appeared to increase the defensive capacity of early-succes sional species primarily under high light conditions, and of late successio nal species under low light conditions. Due to the interactive effects of t ree species, light, CO2, and herbivory, community composition of forests ma y change in the future.