Xylem anatomy traits of Quercus species

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.