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Harvard Forest Symposium Abstract 2006

  • Title: Estimating sap flow using diurnal stem diameter variations
  • Primary Author: Sanna Sevanto (Harvard University)
  • Additional Authors: Michael Daley (Boston University); Noel Michele Holbrook (Harvard Forest); Justin Pettijohn (Boston University); Nathan Phillips (Boston University)
  • Abstract:

    According to the cohesion-tension theory, the ascent of sap in trees is driven by transpiration-induced pull on a continuous water column extending from the roots to the leaves. This pull creates a tension on the water column, which is held continuous by the cohesive forces between water molecules.


    Tree stem diameter varies diurnally as a result of the varying water tension in side the stem. Maximum diameter is found in the morning just before sunrise and the minimum in the afternoon when transpiration rates are highest. The magnitude of the variation is a good indicator tree water use and can be linked to tree-level transpiration rates as well as soil water availability. Water tension and sap flow rates are connected to diameter variations via physical properties of the wood material. Thus, measuring any two gives information on the interaction of the structure and function of the water conducting tissue.


    In this study we measured diurnal diameter variations (Fig. 1) and sap flow, with heat dissipation method, on three broad-leaved species (red oak, red maple and black birch). Our aim was to find relationships between sap flow rates and diameter variations for different species in the same stand as well as determine effects of different structural properties on the tension-flow rate relation. Our results show that in well-watered conditions the ratio of diameter variation and sap flow rate (per sapwood area) is a constant (not a function of sap flow rate) that varies between species in accord with the theory (Fig. 2). From that constant we can estimate the elasticity and conductivity of the xylem tissue. They link the wood structure to the water tension and enable us to use the easy-to-maintain diameter variation measurement alone for estimating sap flow.


  • Research Category: Forest-Atmosphere Exchange
    Physiological Ecology, Population Dynamics, and Species Interactions