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Harvard Forest REU Symposium Abstract 2010

  • Title: Azimuthal Variations in Sap Flux Density Suggest a Trade-off Between Axial Efficiency and Tangential Connectivity in Tree Vascular Systems
  • Author: Lisa N Chen (Harvard College (Harvard University))
  • Abstract:

    Different trees pull water at different rates as a function of xylem conduit size and associated vascular axial resistance. It is not known, however, whether axial resistance is correlated with hydraulic variation around the azimuth. In tree species with lower axial to tangential resistance, it is easier for water to move vertically instead of horizontally: this behavior predicts that greater variation in sap flow should be found in tree species with more hydraulically efficient vascular systems. Azimuthal variation of sap flux density measured using Granier’s Thermal Dissipation Probe (TDP) system can serve as a proxy for how water is mixing at the trunk of a tree. Four or more probe pairs of Granier’s TDP system were inserted into red oaks (Quercus rubra), white pines (Pinus strobus), and red maples (Acer rubrum). Measurements were taken from May through July 2010 in Petersham, MA. Mean, variance, and coefficient of variation (CV) were computed for and compared between the three species. Results suggest the CVs of oaks are highly significantly different (p < 0.01) from that of pines or maples. The variances of all species were also found to be significantly different (p < 0.05) from each other. This is consistent with current knowledge about the vasculature of trees, and suggests that the highly efficient axial transport system supported by oaks comes at the price of reduced tangential communication. Such studies provide greater nuances in our understanding of different vascular evolutionary strategies of trees, as trade-offs are made between a tree’s ability to pull large amounts of water for greater photosynthetic output and its ability to provide a more homogenous hydraulic environment that has enough flexibility to deal with the dynamics of cavitation.

  • Research Category: Physiological Ecology, Population Dynamics, and Species Interactions