Spatial and temporal variation in selection and consequences for plant intraspecific trait variation and associated biotic communities

Jordan R. Croy and Jessica D. Pratt, Ecology and Evolutionary Biology, University of California, Irvine, Irvine, CA, Kailen A. Mooney, Ecology and Evolutionary Biology, University of California, Ivine, Irvine, CA

Background/Question/Methods
Plant intraspecific variation is a form of hidden biodiversity that can structure associated biotic communities and influence ecosystem processes, and thus identifying the processes that generate and maintain this diversity is a central goal of evolutionary ecology. One important source of intraspecific variation is plant local adaptation along environmental gradients and uncovering the selecting agents can provide a framework for predicting future evolutionary change. Also, interannual variation in environmental conditions can not only alter the strength of selection from year to year, but also the direction of selection. Our goal was to test for plant local adaptation to an environmental gradient and link this evolutionary process to the ecological interactions between plants and their associated biotic communities. We established two series of common gardens of Artemisia californica containing 20 (est. 2011) and 45 (est. 2017) populations sourced from a 5° latitudinal cline of northward-increasing aridity. The first series included a garden located at the northern and southern end of the species’ distribution, whereas the second series included gardens located at northern, southern, and central locations within the species’ distribution. We then quantified plant performance, measured a suite of plant traits related to plant water relations and herbivore defense, and collected and characterized plant-associated arthropod and phyllosphere communities.
Results/Conclusions
Overall (across 2011-2018), we found strong evidence of local adaptation. Comparing non-local (transfer distance > 750 km) to local populations (transfer distance = 0 km) across all gardens, we found that flower production, survival, and above-ground biomass decreased 68%, 67.5%, and 33%, respectively. Interestingly, an extremely wet year (2019) within one of our southern common gardens led to substantial mortality (76%) and reversed the direction of selection, such that northern populations performed better than southern populations. Although, we do not know the exact cause of mortality, we suspect that soil properties in combination with an abnormally wet year led to a fungal outbreak. With respect to the mechanisms underlying local adaptation, we found 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 community composition. In summary, we provide evidence that plant local adaptation to an aridity gradient drives genetically-based clinal variation in traits and associated communities, but the strength and direction of selection can nevertheless vary among years due to extreme climatic events.