Evidence of Climate-associated Local Adaptation and Phenotypic Plasticity Across Populations of a Widely-distributed California Oak, Quercus lobata

Brandon W.S. MacDonald, University of California Los Angeles, Department of Ecology and Evolutionary Biology

Luke Browne, University of California Los Angeles, Department of Ecology and Evolutionary Biology (currently Yale University)

Jessica W. Wright, USDA Forest Service, Pacific Southwest Research Station

Victoria L. Sork, University of California Los Angeles, Department of Ecology and Evolutionary Biology

The ability to tolerate temperature or fluctuations in temperature between seasons is key to the survival of a plant and to the sustainability of populations. The environment will select for individuals who have the highest fitness in their local environment resulting in locally adapted populations. Environmental heterogeneity may also favor individuals who can tolerate a range of conditions. Temporal climatic heterogeneity within a location can select for the evolution of phenotypic plasticity, defined as the ability of a genotype to produce multiple phenotypes in different environments that may optimize plant fitness in those conditions. We tested the hypotheses of local adaptation and phenotypic plasticity by examining leaf traits from 5,488 four-year old Quercus lobata trees, a wide-spread California oak, planted into two common gardens maintained by the USDA Forest Service. These gardens were established with acorns harvested from over 600 trees across the species’ range in 2012. We measured several leaf traits known to be associated with plant response to climate. We found that these leaf traits showed genetic differentiation among maternal families and location of origin in a pattern that correlates with climatic variation, which is evidence of local adaptation.  For two traits, leaf lobedness and trichome density, we also found significantly higher levels of plasticity in progeny from climates with a greater difference between winter and summer temperatures. Furthermore, maternal families with higher plasticity in trichome density tended to display lower levels of growth in the common gardens, suggesting a potential trade-off between plasticity and growth. These studies provide evidence of climate-associated selection on leaf traits and support for the hypothesis that seasonal climatic heterogeneity of a site has favored the evolution of phenotypic plasticity in some leaf phenotypes.