1. Climate is a major evolutionary force in driving adaptive differentiation and plasticity in plant function. Xylem anatomy and hydraulic architecture are critical to water use, growth, and responses of trees to drought and thus important in delimiting their ecological niches. How wood properties have been shaped through evolution by their climatic origins and the importance of plasticity for species persistence remain open questions critical to understanding plant responses to changing climate.

2. We measured 11 wood anatomical traits for 18 Quercus (oak) species in arboreta that span contrasting climates (California-US, southwestern France, and central England). We investigated coordinate evolution of xylem anatomical properties with climatic niche and intraspecific variation in relation to growth environment.

3. Species originating from climates with drier summers had traits associated with higher resistance to drought– higher density of vasicentric tracheids (VT), lower vessel hydraulic diameter (Dmh), and lower hydraulic conductivity. Species that evolved in climates with drier, hotter summers or colder winters had higher numbers of VT than those from mesic climates, supporting hypotheses that VT are critical for water-transport during drought. We found limited intraspecific variation in xylem traits associated with growth environment– only four traits (pit fraction, VT, vessel density, and Dmh)– differed among gardens.

4. Xylem traits showed high lability across the phylogeny, consistent with evidence for parallel sympatric adaptive radiation and global diversification of the oaks. Our results provide evidence for the physiological mechanisms that underlie adaptation to changing environments and responses to climate change.