Tracheids and vessels contribute to xylem hydraulic efficiency and safety of five California Quercus species (Fagaceae) along an elevation gradient

Climate change studies predict that mediterranean-type climate regions around the world will experience more intense and protracted drought events in the coming years, which could have significant effects on the already seasonally drought-stressed plant communities in these regions. Many studies have quantified recent drought-associated plant mortality and dieback, including in plant communities of California. Although the effect of water stress on plant structure and hydraulic function has been studied, the different roles of xylem tracheary element types (i.e., vessels and tracheids) within some woody angiosperms and the contribution of these cells to plant hydraulic function have not been a central focus in the context of water stress. Particularly of interest are the water-stress responses of California native oak (Quercus) species, which occupy a wide variety of plant communities throughout California, ranging from low water and high temperature foothill woodland communities to high water and low temperature mixed conifer forests. These oak species are also of interest because of their xylem hydraulic transport network that includes both vessels and tracheids. The relative contribution to plant hydraulic function and water stress response of these different types of conduits within an organism have never been separated. It has been hypothesized that these xylem structural features, particularly the presence of tracheids in a species with vessels, may contribute to xylem hydraulic function under conditions of low water availability. I examined the vulnerability to embolism of five different oak species of the southern Sierra Nevada mountains of California. I also considered the effect of elevation on xylem conduit structure and hydraulic function in these species.