Water Movement in Trees
Kim D. Koder, Proffessor Silvics/Ecology, Warnell
School of Forest Resourses, The University of Georgia
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 tree 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.
The Bugwood Network
College of Agricultural and Environmental Sciences/Warnell School of Forest
Resourses, University of Georgia Tifton GA, USA |
