UCLA

Plants have evolved to survive a wide range of environmental stressors. Current climate change models predict higher frequency and severity of California droughts. It is imperative to understand the impact that stress has on plant performance to predict changes in habitat distributions and abundance for all plants species. While most work examines broad scales across multiple genera, few studies analyze plant drought tolerance within a single genera or species. The highly diverse woody genus, Quercus (oaks), contains ~450 long-lived woody species that dominate the Northern Hemisphere, and now faces threats from climate change. My research explores gaps in plant-hydraulic theory by comparing key drought tolerance traits – minimum leaf surface conductance (gmin), stomatal conductance (gs) and leaf hydraulic conductance (Kleaf) – to climate variables, in closely related California Quercus species grown in a common garden. I show that gmin is driven by cuticle layer permeability and stomatal aperture. Furthermore, I found that native California oaks reduce their hydraulic capacity before stomata are fully closed to prevent water loss. I expanded on this work to explore the morphological and physiological leaf traits within a single species, Quercus lobata, grown in two common gardens. Valley oaks show local adaptation to their climate, with many of their physical traits being shaped by climate-driven selection. Morphological traits are more influenced by genetics, while ecophysiological traits are more adaptable to environmental changes. Some traits vary significantly among different oak families, and these differences are linked to the climate conditions of their maternal origins, indicating that the climate where the mother tree grew influences these traits. Lastly, I investigated biomass and nutrient allocation trade-offs in Q. lobata by analyzing above- and belowground biomass and carbon and nitrogen concentrations in juvenile trees. Valley oak families show plasticity in their root-to-shoot ratios and some nutrient traits when grown in common gardens, aligning with previous studies on resource allocation. However, genetic differentiation and climate selection were found in various biomass traits, but not in root-to-shoot ratios or nutrient traits. Overall, Valley oak uses a mixed strategy to adapt to climate conditions. Collectively, this research underscores the importance of studying plant drought tolerance, at both the genus and species levels, by seeking to clarify environmental drivers of oak trait responses.