Hydraulic function and conduit structure in the xylem of five oak species

Many plant lineages, including oaks (Quercus spp.), have both vessels and tracheids as hydraulically conductive cells within their xylem. The structure of these co-occurring conduit types and their contribution to plant hydraulic function have been relatively little studied. We hypothesized that vasicentric tracheids contribute to hydraulic function under conditions of low water availability. We predicted that within a species, oaks growing at drier and warmer low elevation sites would have more tracheids and be more embolism resistant compared to those growing at moister and colder higher elevation sites. We also predicted that across species, lower elevation oaks would have increased tracheid abundance within their xylem. Five oak species differed in many xylem traits, including vessel diameter and length, tracheid size and abundance, embolism resistance, and hydraulic conductivity. Tracheids were most abundant in the xylem of the highest elevation species at sites that receive winter snow and freezing temperatures. Vessels were relatively vulnerable to embolism as confirmed with multiple methods, including centrifuge vulnerability curves, micro-CT scans of native stem samples, and single vessel air injection. Theoretical conductivity calculations indicated that tracheids account for 5.7-15.5% of conductivity in hydrated stems, with tracheids likely increasing in importance as large diameter vulnerable vessels embolize. The occurrence of both vessels and vasicentric tracheids in the xylem of oaks may enable them to function within highly seasonal climates. Tracheids, though often overlooked, may be particularly important in maintaining conductivity throughout much of the year when water potentials decline from seasonal maximums and following freeze-thaw events.