Physics Photo of the Week
Tall trees and water
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
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
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
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).
Photo of the
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