Home Is Where Your Trees Are

Tree Structure. Trunk. Part 1

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.