Although genetic variability and resource availability both influence plant chemical composition, little is known about how these factors interact to modulate costs of resistance, expressed as negative correlations between growth and defense. We evaluated genotype x environment effects on foliar chemistry and growth of quaking aspen (Populus tremuloides) by growing multiple aspen genotypes under variable conditions of light and soil nutrient availability in a common garden. Foliage was analyzed for levels of nitrogen, phenolic glycosides and condensed tannins. Bioassays of leaf quality were conducted with fourth-stadium gypsy moth (Lymantria dispar) larvae. Results revealed strong effects of plant genotype, light availability and nutrient availability; the importance of each factor depended upon compound type. For example, tannin concentrations differed little among genotypes and across nutrient regimes under low light conditions, but markedly so under high light conditions. Phenolic glycoside concentrations, in contrast, were largely determined by genotype. Variation in phenolic glycoside concentrations among genotypes was the most important factor affecting gypsy moth performance. Gypsy moth biomass and development time were negatively and positively correlated, respectively, with phenolic glycoside levels. Allocation to phenolic glycosides appeared to be costly in terms of growth, but only under resource-limiting conditions. Context-dependent trade-offs help to explain why costs of allocation to resistance are often difficult to demonstrate.

This research tested the long-term effects of defoliation on aspen chemistry and growth in relation to genotype and nutrient availability. We grew saplings of four aspen genotypes in a common garden under two conditions of nutrient availability, and subsequently subjected them to two levels of artificial defoliation. Artificial defoliation suppressed plant growth, and saplings of the four genotypes did not show evidence of genetic variation in tolerance to defoliation. Phenolic glycoside concentrations did not respond to defoliation, but were influenced by genotype and nutrient availability. Condensed tannins responded to defoliation and varied among genotypes. Although defoliation affected condensed tannins, plant quality was not altered in a manner important for gypsy moth performance. Regression analyses suggested that phenolic glycoside concentrations accounted for most of the variation in insect performance. The lack of a strong response important for herbivores was surprising given the severity of the defoliation treatment (nearly 100% of leaf area was removed). In this study, plant genotype was of primary importance, nutrient availability was of secondary importance and long-term induced responses were unimportant as determinants of insect performance.

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