Home Is Where Your Trees Are

Tree Structure. Trunk. Part 2

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. 1.) 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.

structure in cross-section
Cambium layer. Phloem. Cork cambium. Outer bark.

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. 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.