Using branch and basal trunk sap flow measurements to estimate whole-plant water capacitance: a caution |
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Authors: | Stephen S O Burgess Todd E Dawson |
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Institution: | (1) School of Plant Biology, University of Western Australia, 35 Stirling Highway, Crawley, WA, 6009, Australia;(2) Cooperative Research Centre for Plant-Based Management of Dryland Salinity, 35 Stirling Highway, Crawley, WA, 6009, Australia;(3) Department of Integrative Biology, University of California, Berkeley, CA 94720, USA |
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Abstract: | Thermometric sap flow sensors are widely used to measure water flow in roots, stems and branches of plants. Comparison of
the timing of flow in branches and stems has been used to estimate water capacitance of large trees. We review studies of
sap flow in branches and present our own data to show that there is wide variation in the patterns and timing of sap flow
of branches in different parts of the crown, owing to the course of daily solar illuminance. In contiguous forest, east-facing
and upper branches are illuminated earlier than west-facing and lower branches and most capacitance studies do not include
adequate information about branch sampling regimes relative to the overall pattern of crown illuminance, raising questions
about the accuracy of capacitance estimates. Measuring only upper branches and normalising these results to represent the
entire crown is dangerous because flows at the stem base likely peak in response to maximum crown illuminance (and transpiration)
and this will differ compared to the timing of peak flows in upper branches. We suggest that the magnitude of flow lags between
branches and stems needs further study, with careful attention to branch position and method application before a robust understanding
of capacitance, particularly in woody tissues of large trees, can be formed. We did not detect flow lags in the world’s tallest
and largest tree species Sequoia sempervirens and Sequoiadendron giganteum, despite measurement along large pathlengths (∼57 and 85 m), which raises questions as to why large flow lags are often recorded
for much smaller species. One conspicuous possibility is the different methods used among studies. Constant-heating methods
such as the thermal dissipation probe (and also heat balance methods) include heat capacitance behaviour due to warming of
wood tissues, which delays the response of the sensors to changing sap flow conditions. We argue that methods with intrinsic
heat-capacitance present dangers when trying to measure water-capacitance in trees. In this respect heat pulse methods hold
an advantage. |
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Keywords: | Branch sap flow Capacitance Stem water storage Flow lags Water transport Heat pulse Thermal dissipation probe Heat balance gauge Heat storage Cohesion-tension theory |
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