| Plant defense against herbivores includes both resistance and tolerance. Resistance limits damage, while tolerance counteracts the negative effects of damage. These defenses confer fitness benefits, but theory predicts costs of defense expressed as tradeoffs between defense and other fitness components such as growth and reproduction. Theory also predicts tradeoffs between resistance and tolerance. These tradeoffs and cost-benefit dynamics help maintain genetic variation for defense. Additionally, genetic variation for, and costs of, resistance and tolerance are influenced by environmental conditions. I examined resistance and tolerance in twelve genotypes of aspen (Populus tremuloides ) growing under two soil nutrient conditions. This common garden experiment revealed genetic variation for resistance and tolerance under both low- and high-nutrient conditions, but no resistance-tolerance tradeoff. I found costs of resistance and tolerance, particularly under high-nutrient conditions. This is the first documentation of costs of tolerance in a woody species. Further, I assessed the relationship between tolerance and key plant traits. Under low-nutrient conditions, I found a positive correlation between tolerance and stem mass before damage. Under high-nutrient conditions, tolerance correlated positively with the production of stem mass in response to damage.; Additionally, I explored the induction of resistance factors (condensed tannins and phenolic glycosides) in aspen. Induced resistance may be a cost saving strategy. I found a tradeoff between inducible and constitutive levels of phenolic glycosides that may have developed due to costs of resistance. In indeterminately-growing saplings defoliated in early summer, I observed simultaneous chemical induction and compensatory growth later in the season. The herbivore-resistant compensatory growth that I found likely limits additional damage in trees that have already experienced herbivory and provides an example of the complementary roles of resistance and tolerance in aspen. |
