Herbivory serves as a powerful stressor shaping plant evolution, and, as a result, plants have evolved a variety of defense strategies to either reduce the amount of herbivore damage received (i.e. resistance) or mitigate the fitness costs of herbivore damage (i.e. tolerance). Moreover, local adaptation to the abiotic environment can mediate plant responses to herbivory. To this end, environmental gradients can serve as powerful tools to understand how plant adaptation along abiotic clines shapes plant defense strategies. We established a common garden of Artemisia californica sourced from 20 populations along a 5° latitudinal gradient, where precipitation increases northward. The resource availability hypothesis (RAH) predicts that resources determine plant growth rate and defense, resulting in a growth-resistance trade-off. Moreover, an increasing push to fold tolerance into the RAH has led to predictions of a tolerance-resistance trade-off. So, we predicted A. californica plants sourced from arid environments to exhibit slower growth, greater resistance, and less tolerance compared to northern populations that have evolved to historically more mesic environments. To test this, we artificially damaged half of the plants within each population and surveyed each plant for vertebrate herbivore damage to quantify genetic variation in tolerance and resistance to herbivory.
Results/Conclusions
Within a southern common garden, we found strong latitudinal variation in aboveground biomass with southern plants larger than northern plants on average, consistent with patterns of local adaptation. While many studies have documented positive relationships between growth rate and resource availability, both across and within species, A. californica plants from mesic environments did not have higher growth rates. This is likely due to local adaptation leading to a trait-environment mismatch for northern plants in a southern common garden. For resistance, populations sourced from northern environments lost a greater proportion of their biomass relative to southern populations, thus exhibiting lower resistance. However, absolute biomass removal correlated with plant size, with larger plants receiving greater damage, which suggests that plant quantity and not quality might be driving herbivore foraging patterns. In contrast to resistance, we found no genetically-based cline in tolerance to herbivory. However, a drought-induced mortality event reduced the experimental design to eight populations, thus limiting the power for this test. Because tolerance translates herbivore damage into the currency of plant fitness, assessing tolerance and resistance in tandem is necessary to understand how plant defense strategies mediate the impacts of herbivores and how herbivores influence patterns of local adaptation.