As the foundation of food webs, plant functional trait variability is consequential for associated biotic communities and ecosystem processes. Plant adaptation to the local abiotic and biotic environment can generate intraspecific trait variability (ITV) across populations, and these traits can in turn structure associated biotic communities. However, interannual variation in abiotic factors can not only mediate the strength of selection from year to year, but also the direction of selection maintaining ITV. We established a series of common gardens of Artemisia californica containing 20-45 populations sourced along a 5° latitudinal cline of northward-increasing aridity. The first set of gardens contains 20 populations located in a southern and northern garden planted in 2011, whereas the second set contains 30-45 populations located in a southern, central, and northern garden planted in 2017. We tested for local adaptation over multiple years with respect to 3 fitness correlates, reproductive output, survival, and plant biomass. Moreover, we sought to identify the physiological basis of local adaptation by measuring a suite of plant water-relations traits, as well as putative herbivore defense traits and associated arthropod and phyllosphere communities.
Results/Conclusions
Overall (across 2011-2018) we found strong evidence of local adaptation, with northern populations demonstrating greater survival and reproduction than southern populations in a northern garden, and vice versa in the southern common garden. Nevertheless, these patterns reversed for the southern common garden in an extremely wet year (2019), where high mortality (76%), likely from a fungal outbreak, resulted in higher survival for northern populations. With respect to the mechanisms underlying local adaptation, we find genetically-based clines in plant traits consistent with adaptation to the aridity gradient. Furthermore, we also document genetically-based clines in herbivore defense traits and, accordingly, population variation and clinal patterns in the plant-associated biotic community composition. In summary, local adaptation to an aridity gradient drives genetically-based clinal variation in species traits and associated communities, but the strength and direction of selection can nevertheless vary among years due to extreme climatic events.