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Abstract Two decades of widespread drought-induced forest mortality events on every forested continent have raised the specter of future unpredictable, rapid ecosystem changes in 21^stcentury forests. Yet our ability to predict drought stress, much less drought-induced mortality across the landscape remains limited. This uncertainty stems at least in part from an incomplete understanding of within-species variation in hydraulic physiology, which reflects the interaction of genetic differentiation among populations (ecotypic variation) and phenotypic plasticity in response to growth environment. We examined among-population genetic differentiation in a number of morphological and hydraulic traits in California blue oak (Quercus douglasii) using a 30 year old common garden. We then compared this genetic trait differentiation and trait-trait integration to wild phenotypes in the field from the original source populations. We found remarkably limited among-population genetic differentiation in all traits in the common garden, but considerable site-to-site variation in the field. However, it was difficult to explain trait variation in the field using site climate variables, suggesting that gridded climate data does not capture the drivers of plasticity in drought physiology in this species. Trait-trait relationships were also considerably stronger in the field than in the garden, particularly links between leaf morphology, leaf hydraulic efficiency and stem hydraulic efficiency. Indeed, while twelve of 45 potential trait-trait relationships showed significant wild phenotypic correlations, only four relationships showed both genetic and phenotypic correlations, and five relationships showed significantly different genetic and phenotypic correlations. Collectively, our results demonstrate limited ecotypic variation in drought-related physiology but considerable geographic variation in physiology and phenotypic integration in the wild, both driven largely by plasticity. 

Quantitative knowledge of xylem physical tolerance limits to dehydration is essential to understanding plant drought tolerance but is lacking in many long-vessel angiosperms. We examine the hypothesis that a fundamental association between sustained xylem water transport and downstream tissue function should select for xylem that avoids embolism in long-vessel trees by quantifying xylem capacity to withstand air entry of western North American oaks ( Quercus spp.). Optical visualization showed that 50% of embolism occurs at water potentials below −2.7 MPa in all 19 species, and −6.6 MPa in the most resistant species. By mapping the evolution of xylem vulnerability to embolism onto a fossil-dated phylogeny of the western North American oaks, we found large differences between clades (sections) while closely related species within each clade vary little in their capacity to withstand air entry. Phylogenetic conservatism in xylem physical tolerance, together with a significant correlation between species distributions along rainfall gradients and their dehydration tolerance, suggests that closely related species occupy similar climatic niches and that species' geographic ranges may have shifted along aridity gradients in accordance with their physical tolerance. Such trends, coupled with evolutionary associations between capacity to withstand xylem embolism and other hydraulic-related traits, yield wide margins of safety against embolism in oaks from diverse habitats. Evolved responses of the vascular system to aridity support the embolism avoidance hypothesis and reveal the importance of quantifying plant capacity to withstand xylem embolism for understanding function and biogeography of some of the Northern Hemisphere’s most ecologically and economically important plants. 

We quantified fire severity in the Tubbs Fire (Sonoma Co., CA, October 2017) across different vegetation types, and post-fire mortality and regeneration of tree species in permanent plots at the Pepperwood Preserve. The fire burned 14,895 ha, with > 25% in both medium and high severity. Chaparral and Pinus attenuata stands mostly burned at high severity, while other vegetation types experienced a fairly even distribution of fire severity. The fire killed 50% of saplings (dbh < 1 cm) and 27% of trees (dbh ≥ 1 cm), with higher mortality in high severity patches. Quercus agrifolia, Q. kelloggii, Arbutus menziesii and Umbellularia californica exhibited very high levels of topkill combined with basal resprouting. Pseudotsuga menziesii, which lacks resprouting ability, exhibited high mortality, especially in saplings at high severity. The results provide a baseline to examine potential vegetation change due to high-severity fire, especially in high-severity stands of P. menziesii. 

Summary Vulnerability to embolism varies between con‐generic species distributed along aridity gradients, yet little is known about intraspecific variation and its drivers. Even less is known about intraspecific variation in tissues other than stems, despite results suggesting that roots, stems and leaves can differ in vulnerability. We hypothesized that intraspecific variation in vulnerability in leaves and stems is adaptive and driven by aridity.We quantified leaf and stem vulnerability ofQuercus douglasiiusing the optical technique. To assess contributions of genetic variation and phenotypic plasticity to within‐species variation, we quantified the vulnerability of individuals growing in a common garden, but originating from populations along an aridity gradient, as well as individuals from the same wild populations.Intraspecific variation in water potential at which 50% of total embolism in a tissue is observed (P₅₀) was explained mostly by differences between individuals (>66% of total variance) and tissues (16%). There was little between‐population variation in leaf/stem P₅₀in the garden, which was not related to site of origin aridity. Unexpectedly, we observed a positive relationship between wild individual stem P₅₀and aridity.Although there is no local adaptation and only minor phenotypic plasticity in leaf/stem vulnerability inQ. douglasii, high levels of potentially heritable variation within populations or strong environmental selection could contribute to adaptive responses under future climate change.