Plant adaptation to a resource cline has consequences for plant-herbivore interactions

Background/Question/Methods   Geographic variation in ecologically important plant traits can influence the strength of plant-herbivore interactions across a species range. Determining differences in such interactions along environmental gradients can help predict community-level consequences of plant responses to environmental change. We examined geographic variation in the response to predicted climate change of Artemisia californica, a keystone species in California's coastal sage scrub habitat. Cuttings of A. californica were collected from five populations across 700 km of this species range, characterized by a four-fold precipitation gradient. Cuttings were planted in a common garden experiment where we manipulated precipitation and assessed variation in tradeoffs between plant growth and defense among these populations, and the implications of this variation for herbivore communities. We quantified plant resistance to rabbits, a ubiquitous herbivore throughout California's natural habitats, and to a specialist aphid species found throughout A. californica's range.

Results/Conclusions   We found clinal variation in ecologically important traits of A. californica, including growth rate and resistance to herbivores. Growth rate was fastest for southernmost populations and decreased northward in accordance with increasing precipitation of field site from which the plants were collected. Southernmost genotypes were twice the volume of northernmost genotypes after one year's growth. A. californica source populations differed in the proportion of plants damaged by rabbits with northernmost plants experiencing three times more herbivory than southernmost plants. As plant volume increased the extent of rabbit damage decreased, indicating that some correlate of plant size influences the feeding choices of rabbits. While population level differences associated with adaptation to clinal variation did lead to differences in resistance to rabbit herbivory, we did not observe a tradeoff in resource allocation to growth and defense as the slowest growing plants were also the most susceptible to rabbit herbivory. Per capita aphid population growth rate differed based on plant source population, but these differences did not follow a cline. Taken together, these data provide evidence of strong local adaption to climate across this species' range, as well as extended consequences of this adaptation for A. californica's associates.