COS 76-6 - Ecotypic differences in drought coping ability in an endemic California oak

Abstract: With anthropogenic change, the prevalence and intensity of drought in the Western US is increasing at an alarming rate. Blue oaks (Quercus douglasii) are endemic to California and occur across broad climatic gradients, have limited gene flow, fragmented habitat and long generation times. They are predicted to be negatively impacted by extreme drought and they are already experiencing die-back at the driest sites of their distribution, indicating an absolute dry climate threshold response. One way to build climate change resilience in oaks could be through assisted gene flow (AGF), the introduction of drought-tolerant genotypes in wetter environments projected to become drier in the future. We investigate underlying mechanisms of drought response in blue oak seedlings from six sites along a precipitation gradient in a greenhouse common garden drydown experiment. We expected that xeric ecotypes would show higher drought adaptive traits than the mesic ecotypes when exposed to drought. We collected acorns from six different sites across the species climatic distribution, from 25-100 cm average annual precipitation. . We grew the seedlings in a greenhouse in randomized block design for 6 months with ample watering, and then stopped watering. We measured Fv/Fm, stomatal conductance, leaf desiccation and survival as drought response variables throughout the dry down period, which lasted 12 weeks. We also measured specific leaf area, root:shoot ratio and water use efficiency. We used a GLMM model to understand the relationship between our drought response variables and average annual precipitation of source site. Plants from xeric sites showed lower drought stress as they maintained significantly higher values of Fv/Fm, slower leaf desiccation and higher survival. Understanding the unique population-level drought adaptations of blue oak and their susceptibility to drought could be key to protecting them from ‌climate change.