Plants can evolve resistance and/or tolerance strategies to minimize the negative fitness effects of herbivore damage. Mechanisms responsible for the observed diversity of plant defenses in nature are poorly understood. We examined how environmental heterogeneity and succession influences the expression of resistance and tolerance traits of an old-field perennial, Canada goldenrod (Solidago canadensis). We predicted that if environmental heterogeneity is an important mechanism influencing the selection for locally adapted defense traits, then defense expression should co-vary spatially with environmental conditions and change over the course of succession. We quantified the spatial structure of goldenrod resistance, tolerance, fitness traits, and herbivore damage within replicate early- and late-successional fields and assessed whether that structure was correlated with a edaphic conditions and the neighboring plant community. We then propagated replicates of the same genotypes in a common garden to experimentally measure resistance, tolerance, fitness, costs, and selection differences among genotypes. Lastly, we tested the prediction that resistance and tolerance are differentially favored by selection in early- and late-successional stages. For this experiment, we transplanted resistant, tolerant, and susceptible genotypes into replicate early- and late-successional fields and measured fitness, defense levels, costs, and selection coefficients.
Results/Conclusions
Field surveys revealed that defense traits were strongly spatially structured (i.e., autocorrelated), with late-successional fields being more strongly structured than early-successional fields. Aspects of the plant neighborhood were often correlated with defense levels, but the relationships varied with field successional stage. Damage was positively correlated with ground vegetative cover in early-successional fields, and tolerance traits were positively correlated with canopy cover and stem density in late-successional fields. In the common garden, where neighbor effects were absent, we found no cost to resistance but a significant cost to tolerance in genotypes from the early-successional fields. Results from the in situ experiment suggested that selection for resistance was stronger in early-successional fields while selection for tolerance was stronger in late-successional fields. Together these studies suggest that S. canadensis defense expression may be highly influenced by the context of their environment and may shift from a resistance to tolerance strategy as their environment changes during old-field succession. This is the first study to examine plant resistance and tolerance to herbivory in a spatial and temporal context and suggests that environmental heterogeneity may be an important mechanism promoting the diversity of plant defense expression in natural landscapes.
