UCLA

Contemporary climate change is expected to outpace the migration of long-lived trees, underscoring the importance of understanding how populations respond to changing environments through standing variation of traits. Intraspecific trait variation is shaped by both local adaptation and phenotypic plasticity, but an adaptive trait must be genetically based, have a clear function, and confer a fitness advantage. To determine how leaf traits and drought response strategies contribute to adaptedness in valley oak (Quercus lobata Née), a California endemic tree spanning a wide climate gradient, we combined common garden and greenhouse experiments. First, using eleven-year-old trees in two common gardens, we tested whether leaf morphological traits are genetically differentiated, associated with maternal climate, related to leaf physiology, and under contemporary selection. We found that leaf morphological traits were genetically differentiated, and that plasticity shaped both leaf morphology and physiology. Leaf thickness emerged as the most prominent trait measured in shaping adaptedness, being genetically differentiated, climate-associated, consistently related to physiology, and under selection favoring thinner leaves in both gardens. Second, to test whether trees from drier regions have evolved trait values related to greater drought tolerance, we compared leaf morphology, foliar nutrients, diurnal gas exchange, and leaf water potential between trees from the wetter northern and drier southern regions of the species’ range. We found that the southern trees produced smaller, thicker, denser leaves and had lower photosynthetic rates and stomatal conductance in both gardens. However, water use efficiency, foliar nutrient content, and leaf water potential did not differ between maternal regions. Lastly, to determine whether stomatal behavior under drought is genetically diverged, we conducted a greenhouse drought experiment on seedlings from both regions across a controlled drought cycle. We found that stomatal closure was the primary drought response in both regions, but that southern seedlings closed their stomata more than northern seedlings, while photosynthetic biochemistry did not differ between regions. Collectively, this work 1) identifies leaf thickness as a key trait shaping adaptedness in valley oak, 2) demonstrates that local adaptation has shaped leaf structure, photosynthetic capacity and diurnal patterns of stomatal conductance, consistent with greater drought tolerance in drier regions, and 3) provides evidence that the degree of stomatal closure in response to drought may be genetically differentiated, showing that selection will act on physiology that can facilitate greater drought tolerance. Overall, this work underscores the importance of distinguishing genetically based and plastic trait variation when considering climate-informed seed sourcing and management of long-lived trees.