Adaptive and plastic variation of wood anatomical properties among oak species

Clarissa G. Fontes, Ecology, Evolution and Behavior, University of Minnesota, Saint Paul, MN, Béatrice Chassé, Arboretum des Pouyouleix, Saint-Jory-de-Chalais, France, David A. Coomes, Department of Plant Sciences, University of Cambridge, Cambridge, United Kingdom, Anna L. Jacobsen and R. Brandon Pratt, Department of Biology, California State University, Bakersfield, Bakersfield, CA, Jeannine Cavender-Bares, Ecology, Evolution, and Behavior, University of Minnesota, Saint Paul, MN

Background/Question/Methods
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 whether plasticity of these traits—expressed in contrasting local environments—dominates the influence of their origins remain open questions critical to understanding plant responses to changing climate. We measured 11 wood anatomical traits for 18 Quercus (oak) species sampled in arboreta that span contrasting climates in California-US, southwestern France, and central England. Using a phylogenetic comparative approach, we investigated the importance of species climatic niche and growth environment on intra- and interspecific variation of xylem anatomical properties.
Results/Conclusions
Species originating from climates with drier summers had traits associated with higher resistance to drought, including higher number of vasicentric tracheids (VT), lower vessel hydraulic diameter (Dmh), and lower hydraulic conductivity. We found little evidence for intraspecific variation in xylem traits, and only four traits—pit fraction, VT, vessel density, and Dmh—were significantly different among the three locations. Interestingly, pit traits were the only ones in which growth environments explained more trait variability than climate of origin, highlighting the key role these traits play in helping plants acclimate to different environmental conditions. Species that evolved in drier and hotter climates had higher numbers of VT than species that originated in mesic climates, supporting previous hypotheses that these understudied tracheids are critical for water-transport during drought in the oaks. Finally, the interaction between climate of origin and environment explained most of the variation in these xylem traits, indicating that the degree of plasticity in response to the growth environment depends on the climate of origin and may itself be adaptive.