Plasticity drives geographic variation and trait coordination in blue oak drought physiology

Our ability to predict drought stress across the landscape remains limited. This uncertainty stems in part from an incomplete understanding of within-species variation in hydraulic physiology, particularly coordinated variation across multiple traits. This variation reflects genetic differentiation among populations (ecotypic variation) and phenotypic plasticity. We examined among-population differentiation in morphological and hydraulic traits in California blue oak (Quercus douglasii) using a 30 year-old common garden. We compared trait differentiation and trait-trait coordination in the garden to wild phenotypes from the original source populations. We found remarkably limited among-population differentiation in all traits in the common garden but considerable site-to-site variation in the field that could rarely be explained with site climate variables. Trait-trait relationships were also stronger in the field than in the garden, particularly links between leaf morphology, leaf hydraulic efficiency and stem hydraulic efficiency. Only four trait-trait relationship were present in both the wild and garden, but 12 of 45 relationships showed significant wild phenotypic correlations, with strong coordination among leaf and stem hydraulic efficiency apparently mediated by leaf size. Ultimately, we found limited evidence for ecotypic variation but considerable geographic in phenotypic integration in the wild, suggesting considerable acclimation potential in the face of climate change.