Water Movement in Trees
Kim D. Coder
Professor Silvics/Ecology
Warnell School of Forest Resources
The University of Georgia
April 1999
A tree allocates life-energy to survive and thrive in an environment
which never has optimal resources. What essential resources are available
are usually present in too low, too high, or unavailable concentrations.
Trees continue to react to environmental changes with internal adjustments
selected for efficient use of tree food while minimizing energy loss to
the environment.
The more limiting essential resources are, or the larger energy costs
are for a tree for any circumstance, the greater the stress. Trees only
have a limited set of responses to any stressful situation as recorded
in their genetic material. Trees can only react to stress in genetically
pre-set ways. The eventual result of site limitations and stress will be
death, but effective management, damage control, and minimizing stress
can provide for long tree life.
Essential Water
Of all the resource components of stress impacting tree survival and growth,
water stress is the most prevalent. Water is the single most important
substance for tree life, comprising 80% of tree substance. All the life
processes of a tree take place in water-food making, food transport, food
storage, food use, and defense. Water is a reagent in chemical reactions,
a chemical bath for other reactions, a transporter, a hydraulic pressure
liquid, a coating, buffer, and binder. Water is a universal liquid workbench,
chemical scaffold, and biological facilitator.
Water is essential for tree life. As such, it is aggressively gathered,
carefully guarded, and allowed to slowly escape in exchange for work energy.
The largest single use of water in a tree is for transport of essential
materials from roots to leaves. This transport is called the "transpiration
stream" and occurs in columns of dead xylem cells within the last few annual
rings of the tree. Living cells surrounding this xylem lift system assist
with monitoring the water stream. Clearly, water is critical to this basic
process.
Pulling Bonds
Because of water's chemical properties, and their modification when materials
are dissolved or suspended, water sticks together and adheres to various
surfaces. Water has an affinity for sticking closely to other water molecules.
This property is why drops of water placed on a wax (hydrophobic) surface
bead-up rather than flattening out and covering the surface. In this case,
water would rather stick to other water molecules than to the wax surface.
Using your finger, you can "pull" water droplets over the waxy surface
and consolidate them into larger drops.
Trees utilize water's special chemical features in many ways, most noticeably
in transporting materials into the root and then on to the leaf. Water
is pulled in long chains into the root, up through narrow xylem columns
or channels, and to the leaf surface where it evaporates into the perpetually
dry air. Water evaporates as bonds between molecules are broken by energy
concentrated at the liquid water surface.
Sticky Water
As one water molecule is exposed at the wet surface, it is still bound
to surrounding water molecules. Because of the temperature (sensible heat
or energy) and humidity in the atmosphere, surface water molecules are
pulled away from the liquid surface. This pull breaks water connections
to other molecules, and at the same time pulls these once-connected neighboring
molecules onto the surface. These water molecules, in turn, evaporate into
the air generating an evaporative "pull" at the water surface and down
through the water column to the roots and into the soil.
One way to consider water in the tree is as a tightly connected stream
moving from the soil pores and surfaces, into the root, up the stem, out
to the leaf surface and into the air. The water is a continuous line all
held together by water's affinity for sticking to other water molecules.
This "stickiness" allows water to be pulled to the top of the tallest of
trees against gravity, conduit resistance, and complex pathways.
The faster the evaporation from leaf surfaces, the more energy is exerted
to pull the water to the leaf. Too much exertion, and the continuous line
of water filled with billions of molecular bonds, is pulled apart. Water
column breakage can be catastrophic for the tree because once broken, transport
stops. Too much resistance in the soil or too rapid (high energy) evaporation
at the leaf, can quickly snap ascending water columns.
Sensing Stress
As water moves from the soil through the roots and into the leaves, it
carries with it essential elements, nutrients, and chemical messages. As
water and elements move from root to shoot, growth regulators are added
by the roots and by neighboring cells along the water columns. Through
this chemical communication link, the shoots of the tree can react to the
status of the roots. The shoots can then produce their own growth regulator
and ship it along living cells to the farthest root tip. The shoots of
a tree continually update growth processes in response to root functions,
and the tree roots continually modify life processes in response to shoot
functions.
In addition to growth regulation signals providing environmental supply
and demand information in the tree, leaves have an additional sensor. Leaves
are the center of the evaporative load on water columns throughout the
tree. Leaves can close or open leaf valves (stomates) for taking in carbon-dioxide
gas required in photosynthesis to make food. When the stomates are open,
carbon-dioxide can move into the leaf, but water rapidly evaporates and
escapes the leaf. For average conditions in a yard tree, 5-10 water molecules
evaporate from the leaf for every 1 carbon-dioxide captured. As water availability
declines, leaves sense and respond by closing down stomates and photosynthetic
processes.
Getting Physical
Water loss in trees is primarily a physical process. There are few points
of biological control that override the physical process of water movement.
The soil, soil/root interactions, vascular system, and leaf all provide
resistance to water movement. Increased resistance to water movement makes
water less available at the leaf. Water movement resistance is based upon
the surfaces and structure which water must move through, not biological
life functions. The engine that powers water movement in trees is the dryness
of the air and the rate of evaporation through the stomates. Anything that
effects atmospheric demand for water, and stomate loss rates and control,
would affect water movement in the tree.
Water movement and evaporation is a function of temperature and energy
in the environment. The evaporative pull from the leaf surfaces move water
from around soil particles and into the root. Water is not moved by "pumping,"
"suction," or "capillary action." Water in trees move by sticking together
and being dragged to the leaf surface where evaporation from the stomate
(transpiration) generates a "pulling" force on the water columns. Water
also evaporates from all tree surfaces - buds, bark, lenticels, fruit,
etc.-but the leaves have the only major tree-controlled system for modifying
water loss.
Increasing Tension
As water is pulled up to the tree tops and the resistance to soil-water
movement increases (uptake slows), a tension or negative pressure develops
in the water columns. Water continues to move in the tree from the relatively
low tension, easily moved soil water, to the high tension, rapidly evaporating
leaf water. As leaf loss continues to exceed root uptake, more tension
develops in the water columns. The greater the water tension in the leaf,
the less efficient and damaged the photosynthetic support system becomes.
As evaporative forces become too great and water tensions too large, leaves
will close-down their stomates to prevent damage and conserve water.
The tension in the water columns, even after leaves have closed stomates
and are no longer actively evaporating water, still provides energy to
pull water into the root and up the stem. When water tensions are reduced
enough by roots catching-up to leaf water loss, the stomates may reopen.
Water columns can be simply compared to extended rubber bands that are
pulled on as leaf evaporation exceeds root uptake of water. The potential
energy from the extended rubber band can pull together items, just like
a water column under tension can continue to pull in more water after the
stomates are closed.
Taking A Break
The consequences of water movement in trees produce two interesting results:
siestas and night refilling. During bright, sunny, hot days when the sun
is high enough from the horizon to cause the stomates to open, transpiration
increases until it out-runs the root's ability to keep-up. As water column
tension increases, a point is reached by mid-day when the tree closes many
stomates on many leaves for several hours. The water column tension continues
to pull in water from the soil and as tension values decline, stomates
begin to reopen. Trees take siestas in the middle of the day to minimize
water loss and improve resource efficiency.
As the sun nears the horizon and night approaches, stomates are closed
in trees. Water tensions still remain high from the day. Water column tensions
continue to pull-in water from the soil over night. Just before dawn, the
tree is as rehydrated (water filled) as it will be that day-without rain
or irrigation. Trees refill at night.
Stomates
Trees act as conduits through which soil water passes into the atmosphere.
Instead of water evaporating at the soil surface using sunlight energy,
the tree provides an elevated surface for water evaporation and energy
impact. At the junction between tree and atmosphere is the ideal place
to position a biological control valve-the stomate. Across the entire water
column system, the leaf stomate is the only place actively controlled by
the tree to manage water movement.
Stomates are hydraulic valves, usually active only on leaf undersides.
By definition, a stomate is the hole in a leaf epidermis initiated by pressure
difference between two surrounding guard cells. Generically, stomates are
the valve system components taken all together. Some stomates are protected
with clumps of trichomes (tree hairs), some are surrounded with layers
or deposits of wax, and some are imbedded deep into the leaf away from
the surface. Stomates are positioned and designed to take-in carbon-dioxide
for food production and minimize water loss at the same time.
Dry Air
When surrounding cells (guard cells) are pumped-up with water, an open
gap appears between. The gap produced allows access to the internal portions
of the leaf not protected by a cuticle or dead cells. Carbon-dioxide can
dissolve into the water-saturated walls and be used in food making. Water
from the saturated walls evaporates quickly and escapes from the stomate.
The physiological health of the guard cells including supplies of sugars,
starch, potassium, and water all influence the opening of the stomate.
As water contents decline in a leaf, stomates can not be opened.
The evaporative force to move water through a tree is generated by the
dryness of the air. The ability of the air to evaporate water depends upon
the water content gradient between the air and leaf surface. At 98% relative
humidity (moist!) in the air at 70°F, the air is still 100 times dryer
than the inside of the leaf. Trees are always losing water. Only in fog,
which is 100% relative humidity, would water not evaporate from the leaf.
In addition, temperature provides energy for evaporation. For every 18°F
increase in temperature, almost twice the amount of water evaporates from
the tree.
Speed
Water movement through a tree is controlled by the tug-of-war between the
water availability and movement in the soil versus the water loss from
the leaves. The normal seasonal rate of water movement to the top of some
trees can be rapid. For example, water movement in feet per hour in ring
porous trees are: red oak = 92, ash = 85, hickory = 62, elm = 20;--in diffuse
porous trees: black walnut = 13, willow = 10, yellow poplar = 9, maple
= 8, magnolia = 7, beech = 4;--and, in conifers: pine = 6, hemlock = 3.
Conclusions
Water movement and control in trees can be summarized as a physical process
of evaporation-controlled by temperature and humidity-being utilized to
move essential materials from root to shoot. This process is partially
biologically controlled by opening and closing leaf valves called stomates.
Water is the most critical of the site resources trees must gather and
control. Stomates help conserve water while allowing for food production.
Stomates help convert atmospheric evaporative pull in a supply highway
of the tree.