A Tree Is A Pump, And A Fountain

That fifty-foot tree that stands in your front lawn—you think of it as a hard column of wood with rigid limbs branching into flexible boughs and finer twigs, from which the leaves stem out more or less toughly. It is, in short, a large and intricate complex of cellulose fibers.

Your picture is perfectly correct so far as the structural solids of your tree are concerned. But to comprehend the tree more fully, another picture is necessary. What you are looking at also is an invisible column of water. This column is moving constantly upward, dividing and subdividing as it rises into smaller streams and threads until, in the leaves, a continuous nimbus of moisture meets the atmosphere. The tree's whole structure is composed of roughly fifty per cent water, always in motion. During the growing season your tree is actually an unfailing fountain.

For the fountains that men contrive, an external force is needed to send the water up and throw it outward. This force can be gravity—the water being piped from a source higher than the jets—or it can be a pump which gives the water a pressure greater than gravity. Trees are so constructed that their pumping force is internal. They have no hearts, in the organic sense that animals have, to impel their circulations. What is called a tree's heart, the dense wood at its center, is inert. Like an animal's bones, its prime function is to support. Nevertheless, a tree has powerful inner pumping action: in fact, except for its inert heart and outer hide, the entire tree is a pump.

From its hairy feeder roots below, up through its trunk to topmost twigs and leaflets, its cells are so arranged that they imbibe moisture and, by osmosis (diffusion through membranes), elevate it from the ground to the crown, where air and light can act on it in the system's upper terminals. Evaporation from the leaf pores supplies an added pull to overcome gravity. The action is microscopic in its parts, but it proceeds so fast and constantly, with trillions of cells incessantly functioning as a bucket brigade, that the water pumped up by a good-sized tree may exceed 200 gallons a day.

The water is not put forth at the tree-fountain's top in fluid form or even, except momentarily on the leaves' surfaces, as a detectable vapor. Upon arrival in the leaves' external cells the droplets combine with carbon dioxide in the air to form carbohydrates—sugars and starches, which the leaves absorb as nourishment—and oxygen, much of which is released into the air. Oxygen is as vital to fauna as carbon dioxide is to flora. Thus, like all other plants, trees are potent aids to the health of the animal kingdom. They actually filter and enrich our very breath. This service, apart from the fiber and food and fuel that trees supply, may well have been suspected by primitive man and led him to such worshipful imaginings as Yggdrasil, the earth-sheltering, dewdropping Tree of All Existence. Modern man's understanding of plants as air conditioners is more practical. When he ventures away from earth in spaceships he plans to take along some tiny vegetable organisms called algae, to help purify his air supply and solve his food problem. These little "trees of existence" will ride and grow in tanks of water, every drop of which will have to be recaptured and recycled within the sealed vehicle.

Trees' tropism for water is one basic law of their lives (others are for air and light), since all their food must be in aqueous solution. Species vary widely in their need for moisture, from desert cactus to pondside willow. Some will go to extravagant lengths to slake their craving. The most impressive case of tree thirst I ever saw was a ninety-foot Carolina poplar, eight feet through the butt, whose owner sorrowfully called us in to take it down. This tree's enormous, brittle head towering over his house was a dire menace in every windstorm, but that was the least of the client's worries. Trouble was, he explained, that the giant had drunk dry not only his own well but also the wells of his neighbors.

The nearest neighbor's well was more than 200 feet from the condemned tree. Unbelieving, we investigated. Sure enough, the well was dust dry and the invading poplar roots that had sucked it so had formed a matted plug that choked the well-spring shut. When we cut the huge bole and counted its annual rings we found that this tree was only forty-seven years old instead of the century or more that it looked. We learned that it had been bought for twenty-five cents from an itinerant peddler of poplar "whips" and planted for future shade as a quick-growing yard tree. Through its lust for water and aggressiveness in finding it, the supposed blessing had become a curse on its vicinity.

A case where the merits were reversed was that of a patriarchal horsechestnut which shaded another client's south terrace. When he built a flagged patio there, he "potted" the tree with a low retaining wall a dozen feet out around the buttress roots. Within this wall he sprinkled topsoil, planted ivy, and diligently watered and fed his tree to keep it flourishing. All went well with the horse-chestnut, apparently, for several years. Then it began to die back throughout its whole crown. What had happened was not obvious, but our explorations exposed it.
Unable to find moisture beneath the heavy flagging, the tree's outer roots had atrophied while inner ones had multiplied and massed under the "pot' Here they became self-constricting, and entirely dependent on artificial drink and food, which were not enough. The solution: to drill holes and insert short lengths of pipe down through the flagstones, spaced widely around the "pot"; then, by frequent watering and feeding, to coax the horsechestnut's root system back outward to a normal pattern. (This system, with sieve caps over the pipe inserts and a cutting tool to clear the pipes when rootlets clog them, as they will, can be used to preserve feature trees rooted where a driveway must go.)

In the tree's ascending column of water are dissolved minerals from the soil. Chief of these are nitrogen, phosphorus, and potassium, which the tree must have, besides sugars and starches, in forms synthesized by leaf chemistry. At this peak point, in the leaves, the tree's water content becomes enriched sap. Now it must be redistributed downward to impart growth, energy and tensile strength to all parts of the tree. To see how this is done we must re-examine the tree's water column, and now we find that it is a two-way affair.

On the way up, the moisture takes an inner course through deep layers of xylem or sapwood cells, just outside the heartwood (which is old xylem cells grown inert). Surrounding this thick cylinder of sapwood is a thin outer one composed of tubular phloem cells through which the enriched sap is conducted earthward. Where the two parts of this pipe-within-a-pipe touch is called the cambium (exchange) layer. From it extend lateral fissures called medullary rays, through which both water and sap are transferred inward. This dual circulatory system (See Fig. i.) is present all the way from the slimmest leaf stem down through twig, branch, limb, and sturdy trunk into the tree's subterranean anatomy, the outbranching roots and rootlets.

Outside the cambium and phloem layers grow two layers of bark, the inner one corky and porous for air-breathing, the outer one also porous and fissured but hardened for protection. The bark layers are capable of expanding, sometimes by flaking off (as in sycamores and birches), to accommodate the tree's growth, which is in its girth as well as at its extremities. The tree swells by annual production of new xylem layers. These new cells are large in spring, becoming smaller toward autumn until growth pauses during winter dormancy. Each year's growth can be traced in the sapwood "rings" thus formed, marked off by the darker autumn cells. Darker also, as a rule, are the heartwood cells formed by aged sapwood. Fig. 1 shows in cross-section a tree's structure, which is continuous through all its members.

Cambium layer
Phloem
Cork cambium Outer bark

When growth stops for the winter, contrary to popular belief the tree's sap does not "go down into the roots/' It stays right where it is, stored in every part except the leaves of deciduous (shedding) trees, which wither and break off at the stem ends, where buds remain for new leaves next year. In fiercely cold winters, the moisture in a tree's extremities and outer tegument may freeze, with consequent damage to the containing cells. Roots suffer most in winters of deep frost under a scant snow blanket. Twigs and branches get hurt and frost cracks may open on limbs and trunks during cold snaps that follow "false spring" thaws. But as a rule trees have the hardihood to withstand the rigors of their accustomed climate: their sap stays fluid and viable, and is there to restart growth—in many species to put out blossoms—before the new leaves appear.

The greater importance and vulnerability of the downward flow of enriched sap, as contrasted to the upward water flow, are apparent. The phloem conduits are much thinner than the xylem conduits, and more exposed. Their burden is richer, containing all the tree's elaborated food, not just raw materials, as in the water column. External injury to the tree's cambium layer is thus much more serious than internal injury, to sapwood or heartwood. Trees even lightly "girdled"—cut or constricted all the way around— will die, not from the tops down, but from the bottoms up. Deprived of nourishment from above, the roots wither and cease sending up water to start the alimentary process.

Exceptional in this respect are palm trees, whose trunks can suffer circumference damage up to their breaking point without the trees' health diminishing. This is because the palm family's phloem conduits are arranged in scattered bundles throughout the stem instead of in a circle around it.

None of the moisture carried downward in the sapstream to the roots is returned into the soil. But in nature's economy, trees do reciprocate earth's gift of water by holding soil, and thus moisture, in place with their root meshes, and by lessening ground evaporation with their shade. This is why trees are planted around reservoirs, to check erosion and parching. Evergreens are most used for this purpose because they will grow fastest and densest with the least water requirement for themselves, and their roots run nearer the surface, where erosion begins.

Watershed plantings do not add to the water table through their upper parts except during fogs, when their contribution can be considerable. Gilbert White, England's first literary botanist, wrote as early as 1770 about the alembic action of trees in his own misty Hampshire. He noted that the best condensers are trees festooned with ivy, whose broad and evergreen leaves will drip puddles while the ground around stays powder-dry.

The next chapter, dealing with root systems, will make clear how to feed ailing trees, but your first concern should be with their water supply. Repeated droughts such as the East experienced in the 1950s can set trees back so severely that the effects persist for years. Even when a good growing year like i960 does come around, root systems may be so discouraged and stunted that your trees will respond slowly unless watered on a continuing basis. To safeguard species requiring ample moisture, like the maples and elms, a simple precaution is to set drainage tiles endwise into the ground, five or six around each sizeable tree, well out from the trunk. When the countryside starts to brown, fill these drinking tubes twice a week with the hose or watering can. Around younger trees, grade up a rim to retain the water as in a saucer when you sprinkle them. To do this for larger trees is laborious and unsightly, but a comparable effect can be obtained by putting shallow transverse dips across a tree-bearing slope when your grounds are graded. These will retard runoff water in times of plenty, and check erosion.

Too much water is as fatal to trees as too little. But if you have a chronic wet spot in your grounds, don't fill it, drain it. The effects on tree roots under it are suffocation and rot, which filling would only aggravate.

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