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Harvard Forest Symposium Abstract 2005

  • Title: Diurnal stem diameter variations: a new tool for detecting sap flow
  • Primary Author: Sanna Sevanto (Harvard University)
  • Additional Authors: Noel Michele Holbrook (Harvard Forest); J. William Munger (Harvard University); Steven Wofsy (Harvard University)
  • Abstract:

    Transpiration is an important factor in the energy fluxes of forest ecosystems and trees are the main transporters of water from the soil to the atmosphere in forest-covered areas. However measuring transpiration directly is not easy since water molecules tend to stick on the surfaces of the measurement chambers or pipelines.


    Tree stem diameter varies diurnally because of transpiration induced tension in the sap. The diameter is largest just before sunrise in the morning and smallest in early afternoon when transpiration rates are highest. Xylem diameter variations are exceedingly closely linked with transpiration (Perämäki et al. 2001). Even the small variations in transpiration rates (and thus water tension) resulting from e.g. changes in cloud cover are observable in xylem diameter (figure 1). The daily amplitude of the variation is determined by the difference in transpiration rate and soil water uptake. The higher the transpiration rate and drier the soil, the higher the water tension and larger the amplitude.


    In this project we have been measuring diurnal diameter variations at Harvard Forest on maple and birch trees. The plan for summer 2005 is to introduce these measurements on more species including hemlock and to compare diameter variations with sap flow and water vapor flux measurements and to determine parameters that would scale diameter variations to tree-level transpiration rates for different species.


    We will also make manipulation experiments both in field and laboratory conditions to find out the real potential of diameter variation measurements in detecting xylem-phloem interactions and sugar transport related issues. Sevanto et al. 2002 and 2003 suggested that by observing the dynamics between diameter variations measured below and on bark, information on these topics could be gained.


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