Myths and Discovery in Sap Ascent for Tall Trees

Our first lesson in practical physics as preschoolers may have been that water flows downhill. So how does water reach the top of tall trees? The more physics we think we know, the more improbable sap ascent becomes. For trees up to about 10 meters in height, some combination of wicking, osmotic pressure from dissolved salts and sugars, and suction from evaporation would seem to meet the need. But what about coastal redwoods and other giants that are ten times that tall? Water reaches the top of tall trees by flowing downhill, at least in thermodynamic terms.  Come discover the myths, physics, and beauty of sap ascent.

Kevin SmithKevin T. Smith is a plant physiologist and project leader with the USDA Forest Service in Durham, New Hampshire. Smith’s research centers on the effects of environmental disturbance on tree biology, especially tree growth and the response of trees to injury and infection. Part of this research involves understanding how trees survive, grow, and die in the stressful urban environment. Research tools include dendrochronology, forest pathology, and biological chemistry. Smith has received national awards for research excellence, is an Affiliate Professor of Plant Biology at the University of New Hampshire, and has authored more than 50 publications in scientific journals, book chapters, and magazine articles.

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