Physics Photo of the Week

September 25, 2009

Tall trees and water
The sequoyah trees in Sequoyah National Park, California are some of the tallest living things in the world.  Many of these trees are over 200 feet tall.  Examining this photo carefully a person can be seen walking near the base.

All life on Earth needs water, and these trees need lots of it.  It is estimated that over 8000 gallons of water flow through one of these large trees in a day to be transpired (evaporated) out of the leaves in the crowns.  Physicists and botanists have long wondered what mechanism delivers water from the roots and soil to the tops of the trees.  There is no active pump at the base of the trees, nor in the tissues of the trees.  Capillary action alone cannot lift a water column to such great heights.  When any tree dies, the porous material still remains, but the flow of water stops and the dead tree dies out - even if the cause of death of the tree is due to flooding!

Hydrostatically, if the top of the trees were vacuum, the water height could not reach above about 32 feet above the water table in the ground.  A pump at the top of a water column could only lift water to the height given by the barometric pressure. 

The effects that bring water to the tops of tall trees are completely passive - i. e. there no cellular pumps in the tree tissues.  Water is needed in the leaves for photosynthesis.  Carbon dioxide is also needed and obtained from the atmosphere.  Carbon dioxide enters leaves in the pores - stomates - on the undersides of leaves.  When a stomate is opened to admit carbon dioxide, about 6 times as many water molecules escape due to evaporation - or transpiration.  Water molecules stick to each other due to the polar nature of the molecule.  The evaporation of water molecules actually pulls the water through the narrow xylem tubes in the structure of the tree.  Water also adheres to the cell walls of the xylem tubes through a similar bonding with polar molecules that bind water molecules to each other.  This evaporative effect is the main energy source to bring the water to the great heights in tree tops.  In fact, there is a tensile stress in the narrow water columns - often refered to as "negative pressure".  If the evaporation through the leaves is too great due to excessive sunlight or drought, the stomate close, evaporation is reduced, and the tension in the water column can continue to deliver the water, but at a slower rate.

The cells containing the xylem tubes are dead cells.  How is it that when a tree dies, it dries out completely - even when the tree is standing in a flooded area such as a beaver pond?  Presumably, the stomates on the leaves fail to close when the leaf dies, thus allowing the water in the tree to dry out faster than the water can be replenished.  When deciduous trees shed their leaves, the vascular system is sealed off at the leaf stem before the leaf is released, thus preventing water loss in the winter.

I wish to thank Dr. Amy Boyd, a botany professor at WWC for helping me to understand water transport in trees and for suggesting the following references:

Coder, K. D. "Water Movement in Trees",
University of Georgia School of Forest Resources Extension, Publication FOR 99-007 (1999).

Koch,
George W., Stephen C. Sillett, Gregory M. Jennings, and Stephen D. Davis, "The limits to tree height", Nature, 428, pp 851-853 (2004).



Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to dcollins@warren-wilson.edu. 

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