Ecotypic differences in drought-coping ability in an endemic California oak

Abstract

The frequency and severity of drought in the Western United States have significantly increased. California endemic blue oaks (Quercus douglasii) are predicted to be negatively impacted by extreme drought and are already experiencing dieback in the driest areas of their distribution. To explore whether there is drought-adaptive variation among blue oak populations, we conducted a greenhouse common garden drought experiment with seedlings from five sites along a range-wide precipitation gradient. We investigated seedling performance under experimental drought (fluorescence/maximum fluorescence [Fv/Fm], stomatal conductance, and leaf desiccation). We measured physiological and morphological traits, including average leaf area, specific leaf area (SLA), leaf margin morphology, C:N, and carbon isotope discrimination (δ¹³C, a proxy for water use efficiency). We used generalized linear mixed models to understand the relationship between seedling performance and traits, and the mean annual precipitation (MAP) of the seedling source site. We found significant relationships between source site MAP and Fv/Fm, percent of green leaves, and plant stomatal conductance, with plants from drier source sites performing better under the experimental drought. Seedlings from drier sites also had a lower C:N ratio, consistent with adaptation to more arid environments. Our study points to population-level variation in seedling drought adaptation. Climate-forward conservation strategies that conserve or leverage drought-adaptive genetic resources from the species' threatened dry range edge could support climate change resilience in a future drier environment.