In this study, we used the emergent model system Mimulus guttatus to explore the agents of selection that drive local adaptation across California’s coast-inland moisture gradient. We implemented a field reciprocal transplant experiment within agrofabric exclosures at sites in coastal and inland Sonoma County, California – minimizing the effects of aboveground stressors in an effort to elucidate their role in the evolution of local adaptation in this system. ASTER life-history modeling and generalized linear mixed modeling approaches were used to analyze survival and dry aboveground biomass as fitness proxies. Despite altering no edaphic conditions, we found that among coastal exclosures, inland replicates were significantly more fit than their control counterparts, essentially rescuing inland fitness outside of their native range. Exclosures provided no fitness advantage for either ecotype at the inland site, aside from a moderate but statistically significant increase in biomass among exclosed coastal replicates. While it is unlikely to know all the agents of selection limited by our exclosures, we found that the number of replicates that experienced herbivory were significantly lower across all exclosures at the coast site. An elemental analysis of Sodium (Na) using salt traps installed at each site also demonstrated a reduction in salt exposure within exclosures. It is likely that some combination of aboveground stressors, likely driven primarily by herbivory and/or salt stress, plays a continuing role in the evolution of coastal M. guttatus populations, thus providing a new understanding of how local adaptation is maintained in this model system.
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