Page 9 - CBT 2018
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Tension-cohesion theory and drought
Tension-cohesion theory explains how trees normally transport water from
the ground to the canopy. Surface tension is generated at the evaporating leaf surface and transmitted across a continuous water column within the tree through the cohesion of water molecules to one other and their adhesion to walls of the water-conducting xylem cells.
The failure of this hydraulic system
is believed to explain why trees die during a drought. As soil dries and vapor pressure decreases, the tension exerted in the tree’s water column overcomes the tensile strength of water (about 10 MPa), introducing bubbles. This cavitation process, if not repaired, can cause bubbles to accumulate
and eventually cut off water supply
to the canopy, killing the tree. Thus, predicting large-scale forest mortality events requires an understanding of the underlying physiological controls of this hydraulic process, and that knowledge can be used to stage interventions and protect vulnerable ecosystems. [2–4]
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