The need for plants to acquire CO2 for photosynthesis results in the passive loss of large amounts of water vapor from their leaves, which must be continuously replaced by the xylem, a transport tissue consisting of thousands of dead, hollow conduits running from the roots to the leaves. Hydraulic architecture determines the capacity and efficiency with which the xylem is able to supply water to the leaves, and thus limits the rate at which plants can gain carbon and grow. In many woody plants, the xylem conduits serve dual functions of water transport and mechanical support, leading to a partial sacrifice of hydraulic efficiency for mechanical stability. The goals of this research are to classify and understand the diversity of hydraulic architecture across major plant groups and growth forms by evaluating the extent of trade-offs of hydraulic against mechanical functions of xylem. This will be accomplished by determining how several key vascular network traits such as hydraulic conductivity, conduit number and area, and water transport velocity change from near the base of the plant to its uppermost branches. These results will be plotted on graphs to generate hydraulic landscapes. The positions of different plant types on these landscapes will indicate the extent to which the mechanical support function of xylem constrains its hydraulic function. Palms and vines are expected to be more hydraulically efficient than trees because, unlike trees, their xylem only transports water and does not provide structural support. Within temperate trees, a negative relationship between hydraulic efficiency and the proportion of wood devoted to xylem conduits relative to other cell types is expected. Finally, restrictions on xylem structure imposed by freezing should make temperate trees less hydraulically efficient than tropical trees. This research will enhance understanding of intrinsic constraints on the growth and productivity of forests and other natural and managed ecosystems. Broader impacts of this work include training and mentoring opportunities for a postdoctoral scientist, undergraduate and graduate students, was well as for Latin American students who will participate in the work to be carried out in Panama at the Smithsonian Tropical Research Institute.
