Despite considerable research on plant defenses, we know very little about how temporal changes in the environment may influence resistance and tolerance levels or the costs and benefits of these defenses for long-lived plant species. We hypothesized that in successional habitats, predictable environmental changes should favor strong plasticity in defense expression, and that the costs, benefits, and levels of tolerance and resistance will change with environmental context. Using a widely distributed, old-field perennial, late goldenrod (Solidago altissima), we conducted a field experiment to test these predictions. We planted goldenrod genets exhibiting diverse levels of resistance and tolerance into replicate early- and late-successional fields (approx. 3 and 15 years in age, respectively) and experimentally measured resistance and tolerance levels and their associated costs and selection coefficients. We found a significant effect of successional stage but no effect of genotype or stage-genotype interaction on defense levels. Resistance levels were 41% greater and tolerance levels were 97% lower for genets planted in early-successional fields than those same genets planted in late-successional fields. There was a tradeoff between resistance and tolerance in early-successional fields, but the relationship between these two defenses was positive in late-successional fields. In early fields, we found no evidence of costs for increased resistance or tolerance levels but did find evidence of strong selection favoring increased resistance and decreased tolerance. In contrast, in late fields there was evidence of costsfor increased levels of resistance and tolerance, but no evidence of selection (positive or negative) for either defense. Our results suggest that the types of defenses that plants employ might change in qualitative ways during succession. Resistance may be more adaptive in early stages and despite a cost of tolerance in late stages, tolerance may be beneficial in mitigating the effects of both herbivory and environmental stresses (i.e., low light availability) that limit fitness in these fields. This study provides the first experimental evidence that succession can strongly influence defense expression and promote temporal variability in plant defenses. Fluctuating adaptive landscapes may have important implications for the evolution of plant traits and should be incorporated into future studies of plant defense.
Keywords: costs, plant defense, phenotypic plasticity, selection, Solidago altissima