CITY TREES

IRVING C. ROOT, CHARLES C. ROBINSON.

We rate a tree in a forest as potential lumber according to the texture of its wood and clean bole, its rapidity of growth, market value, and availability. City trees have price tags on them, of course (perhaps as much as $20 per inch of trunk diameter), but we appraise them largely on intangible values of shade and beauty. We judge the city tree by the shape of its canopy, its habit of growing tall and slender or small and spreading, its spring bloom or fall color, the shape and size of its leaves, and its evergreen or deciduous nature. Important, too, is whether it has any tendency to break in storms, whether it is a clean tree or a dirty one, its susceptibility to insect pests and disease, and its ability to adapt itself to the artificial conditions of cities.

No single item distinguishes a city more than its green areas, and probably our first reaction to a community is to its abundance or lack of trees. The shade they give from the sun makes the summer heat more tolerable, and filters for grateful eyes the dazzling reflection from masonry and concrete. A city of monumental buildings, like Washington, particularly needs trees to interrupt the reflected light. The landscape architect uses trees to soften hard building lines and accentuate vertical or horizontal details.

The home owner plants trees to give scale and proportion to desired features and to delight his family and neighbors with spring bloom and fragrance, green coolness in summer, color in autumn, and interesting branch-and-twig patterns in winter.

Trees form vistas, frame views, and define park areas. They can screen out undesirable sights, and separate active from passive recreation. They border our city lakes and streams and cast their reflections in our pools. Groups of trees are a back drop, a cyclorama.

For all their beauty, city trees are no weaklings. It is sometimes surprising how they can survive the artificial and adverse conditions under which they grow. Smoke and gases, physical injuries, the disrupted water table, hard-packed soil, lack of humus renewal and mulch, inadequate root space, reflected heat from pavements and buildings, and glacial blasts of air through the wind tunnel formed by street and buildings, all make their lot hard.

Because trees manufacture their food by the action of sunlight on elements in the leaf, any substantial accumulation of soot or residual oil from the air will screen out sunlight and retard this process of photosynthesis, resulting in a weakening of the tree from starvation. Trees such as ailanthus, horsechestnut, hackberry, American ash, ginkgo, poplars, sycamores, willows, lindens, and elms are all tolerant of soot and smoke. Others, like the sugar maple, sourgum, sweetgum, and honeylocust, can grow well only if the air is unpolluted.

No one seems prepared to say why one tree and not another can stand smoke and soot. It may be related to the effect of smoke and soot-impregnated soil on the mycorrhiza and their relation to root growth and feeding. Why some trees are more affected than others might be a fertile field for research. Perhaps investigation would show us how to grow sugar maples in smoky, soot-laden air where now they cannot survive. Perhaps some simple treatment of the soil or tree may some day make this possible.

Another factor that seems to affect the ability of a tree to withstand smoke and gases is the nature of its leaf surface. A rough, heavy, or sticky leaf will accumulate more soot and residual oil than a smooth or waxy one, and the latter is more easily cleaned off by rainfall and wind.

City trees are subject to physical injuries from many sources. Seldom is a sewer, drainage, or utility line put in on a tree-lined street but that some damage is done to the roots. Because the health of a tree is in direct proportion to the extent and effectiveness of its feeder roots, great care should be exercised that a minimum of damage be done to the tree roots during construction. This damage might not kill the tree, but it might so debilitate it that it would become easy prey to insects and disease.

The power and telephone companies used to expect their line-clearing crews to hack off the tops and sides of trees for line clearance. The branches exposed to the sun by the sudden removal of the protecting canopy were vulnerable to sunscald, which cracked the tender bark and permitted the inroads of disease and insect pests. Fortunately such butchering is on the wane, and few cities permit it today. More and more underground conduits are used; they eliminate unsightly poles and wires and do away with the necessity for any type of drastic tree pruning. Many trees are killed annually by illuminating gas from underground gas lines. One should be suspicious of chlorosis or yellowing of leaves and of any other signs of the unexplained declining health of a tree when it is located near an underground gas line.

Additional hazards of city trees are the bumps and splintering from vehicular accidents, the thoughtlessness of the boy who breaks off branches and gouges with his new knife or ax, and girdling by squirrels in a small park.

A mower in the hands of a careless maintenance man can severely damage the bark and cambium layer at the base of trees; those cuts and bruises can become immediate focal points for infectious diseases like verticillium wilt and canker stain of our sycamores. Indeed, there are several instances on record where injuries by lawn mowers were responsible for the spread of canker stain and the subsequent loss of rows of fine old sycamores.

Another disadvantage under which city trees live is the lack of humus build-up provided by the decay of fallen leaves. Humus or duff, formed by decayed leaves, is nature's food for the tree and her protection for its feeding roots from the sun and drying wind. When we remove this humus or do not permit its manufacture, we are disturbing one of nature's processes for tree growth and vigor.

In large city parks like Fairmount Park in Philadelphia and Rock Creek Park in the District of Columbia, much of the total area is left naturalized and the fallen leaves are allowed to remain, decay, and form the humus that provides much of the tree's natural food and its mulch for moisture retention. Public opinion demands, however, that the small park square, the quadrangle, and parking space along the street be kept free of fallen leaves and other debris. As a practical matter, it would be almost impossible, even if desired, to allow a build-up of fallen and decaying leaves. Clogged gutters and drain pipes, fire hazard, dangerously slippery streets, to name only a few, make necessary their prompt removal along streets and in most parks.

In their natural habitat trees usually grow in soils and moisture conditions that are best suited to them. Thus (in New England, Middle Atlantic, and Midwestern States to which this discussion pertains) we find elms, pin oaks, and sweetgums in low-lying land along the streams and even in swamps. Tuliptrees seem to like the mountain valleys from which they spread to the low-lying ridges. Red and white oaks and sourgums, on the other hand, may be found on higher mountains where their roots have to go deep for water.

Too often in city planting, particularly along the streets, we put in elms, oaks, and honeylocusts, with but little regard to their preference of soil and moisture conditions. Trees that in their native environment search deeply for water are planted alongside those with shallow roots. It is a tribute to nature's adaptability that elms can thrive alongside the deep-rooted white or red oaks.

Were it not for this amazing adaptability, the selection of trees for urban use would be even more complex. It is true, though, that the nearer we can duplicate natural conditions of the soil and the water table, the better we can expect our tree to thrive.

The runoff of rainfall is high in cities almost 100 percent from paved areas. The ground has no chance to absorb and store up the moisture for future needs; most of the rainfall, rather, is immediately carried off into gutters and drains. Trees in sizable city parks seldom suffer from lack of moisture in periods of normal rainfall, but the street tree in a narrow parking never gets a fair share of water and cannot absorb the little it receives. An oak tree gives off some 120 tons of water in only one season through its leaves water that must be replenished from the soil and it seems almost miraculous that our street trees survive at all. An interesting observation is that in times of drought, street trees, which are conditioned to a constant substandard amount of moisture, fare better than those accustomed to adequate rainfall.

How can these adverse conditions be improved?

First, we must see that the tree we select for planting has a sufficiently large tree pit filled with good soil to accommodate potential roots for some years to come. The hole must have natural or artificial drainage to insure against wet feet and root suffocation. The variety selected should be environmentally suited to the designated site from the standpoint of exposure, elevation, and purity of air. It should be planted where physical hazards are few. If natural moisture is lacking, particularly during periods of drought, it must be watered. If it shows signs of starvation, it must be fed with inorganic fertilizer or organics like manure, tobacco stems, sludge, or tankage.

Use determines whether a tree is desirable or undesirable for city planting. A broad-spreading, low-hanging Chinese magnolia may be ideal as a specimen in a small city park or on home grounds but impossible as a street tree. A fastigiate English oak may be perfect for a narrow street but of limited use in the large park. Individual peculiarities may make certain trees undesirable for any urban use the female ginkgo, whose fruit has a bad odor, for example, or the silver maple, which breaks easily in wind and snow, or the boxelder, which has rapid but unsightly growth. Lombardy and Carolina poplars are out of favor because their roots fill sewer lines.

For street use, species or varieties should be avoided that are subject to disease and insect pests. Just as a contagious disease will tend to spread rapidly through a family whose members are in close contact with one another, so the Dutch elm disease, for instance, will spread rapidly through a concentrated group of city elms unless strong preventive measures are taken.

Dutch elm disease and phloem necrosis of elm and the canker stain of sycamores make unwise their widespread planting, particularly for cities.