SHADE TREES FOR THE ROCKIES

LAKE S. GILL.

Getting trees to grow along with the settlements and cities of the southern Rocky Mountain region has been all the more impressive because trees are naturally absent from most of the area.

The region Arizona, Colorado, New Mexico, and Utah is largely a high plateau 4,000 to 8,000 feet in elevation and broken by mountain ranges that often exceed 10,000 feet. In southern Arizona and southwestern New Mexico, roughly the area drained by the Gila River and its tributaries, the plateau is 1,000 to 4,000 feet in altitude. The plateau is largely treeless, although a few species occur along stream banks, and usually the foothills support an open woodland forest of low pinyons and junipers. Coniferous forests, broken occasionally by stands of quaking aspen, cover the sides of the mountains.

Normal precipitation is less than 16 inches a year, only about half of which falls during the growing season. In the Gila Basin the average annual rainfall is less than 12 inches, although there is proportionally more rain in the winter than in other parts of the region below 8,000 feet elevation. Precipitation is 20 to 30 inches in the mountains. Cold winters and hot summers are the rule except in the Gila drainage, where winter temperatures are usually mild. Wide changes in daily temperatures occur throughout the region, especially during the winter months. Late frosts are the rule. Searing winds are common. Most of the soil is alkaline; much of it is low in nitrogen and poor in physical characteristics. Often an impervious layer of hardpan lies close to the surface.

Under such conditions, the early settlers deserve great credit for introducing new trees. Today Salt Lake City, Denver, and Phoenix are outstanding examples of large cities that have been beautified by shade trees despite natural odds against them.

At first the plains poplar and the common hackberry, both native, were commonly planted. More recently, the plains poplar has lost favor because of its space requirements, its expansive and high water-consuming root system, and its untidy habit of shedding "cotton." The common hackberry is still widely used in difficult locations.

The black locust and boxelder were among the first introductions to survive the vicissitudes of climate and soil with minimum care. Later the Siberian elm joined them. These three cannot be surpassed in their ability to produce quick shade, stand abuse, and endure unusually unfavorable climatic and soil conditions.

Today the list of shade trees that can be grown successfully in the southern Rocky Mountains is indeed impressive. Most of them require supplemental irrigation. Sometimes the soil in which they are planted must be carefully selected or specially prepared, either with a view toward reducing alkalinity or of penetrating underlying hardpan to permit better drainage and root growth. Under the most severe conditions only the hardier kinds will live, but even in the northern part in protected locations a number of the more tender species that cannot be recommended for the whole region can be grown.

Following are brief descriptions and supplemental notes of the more common trees that have been successfully planted in the region. Most of the broadleaved evergreens suitable only for the extremely mild winters of the Gila River Basin have been omitted.

THE DECIDUOUS trees are listed first. The tree-of-Heaven ailanthus, often called simply ailanthus, is an aggressive tree that can thrive where many other kinds would perish. It grows rapidly with some care, but it is relatively short-lived and provides little shade from its thin, loose crown. It is recommended as a street tree where low moisture, poor soil, and excessive heat preclude the use of more attractive species. It is almost immune to smoke and soot injury but is easily broken by snow and high wind. Only seed-bearing trees should be planted, as the pollen-bearing form, the male, has an offensive odor when it blooms. It may be planted throughout the region except in the high mountains or parts of the plateau where winter temperatures are low.

Green ash, although smaller than white ash, is the preferred street tree. It has darker foliage, is more resistant to drought, and seems to have fewer enemies. It is suitable for the plateau but it is not recommended for the mountains.

Velvet ash, especially the smooth or Arizona form, is well adapted to the Gila drainage area. It will not stand protracted cold, but it resists drought and can grow in strongly alkaline soil.

White ash has a rather oval crown and light-green and moderately dense foliage. It will stand abuse but is sensitive to drought. The oystershell scale often attacks it. It is not recommended for the mountains or the Gila Basin.

Boxelder develops into a tree with a ragged crown of fairly light-green foliage. Although the boxelder is extremely drought-resistant and able to withstand abuse, it is recommended only for places where more desirable trees will not grow. It is the breeding place of the boxelder bug, which in some years becomes objectionable in the houses nearby. Aphids commonly attack boxelder, and in the cities the honeydew that drops from them is objectionable on pavements. It will grow anywhere in the region. The Arizona form is best adapted to the Gila River drainage.

Northern catalpa, a relatively small tree, possesses a globe-shaped crown of large, heart-shaped leaves. It stands drought fairly well but not cold. Its flowers are large and attractive, but some persons object to it as a street tree because of the litter of the fallen flowers in early summer and the seed pods in autumn. It may be planted throughout the plateau, although it is not recommended for the extreme northern parts or for elevations above 5,000 feet.

American elm has been planted extensively on the plateau for many years. It is not very tolerant of alkali and should have plenty of irrigation to thrive. It is heavily attacked by the European elm scale. It is subject also to two serious epidemic diseases, the Dutch elm disease, which recently was found in Denver, and phloem necrosis, a killing virus disease that has not yet been found in the region. American elm should be used sparingly until controls are available for the diseases.

Siberian elm, which was introduced as Chinese elm and is still often called that, can persist despite drought, poor soil, and abuse, and at the same time provide shade and greenery. It is naturally scrubby in habit, but it can be trained while young into a fairly attractive street tree. It has been over-planted in some cities. Its root system is prone to invade tile sewers and to heave pavements and curbs. It produces seed profusely and the seedlings may become extremely noxious weeds in nearby gardens. It is not subject to the epidemic diseases described for American elm but is highly susceptible to the cotton or Ozonium root rot. Often the European elm scale attacks it.

The common hackberry is usually a small tree with a spreading, flat crown of dense, light-green foliage. It is excellent for use under adverse growing conditions in hot, dry climates. The foliage does not tend to turn yellow in alkaline soils. It is recommended for all parts of the region except in the mountains.

Thornless honeylocust forms a thin, irregular crown that provides light shade. It is one of the last trees to leaf out in the spring and one of the first to lose its foliage in the fall a distinct shortcoming in a region of long, hot summers. But it is drought-resistant, hardy, and capable of withstanding abuse once it is established. It is not recommended for the mountains.

Linden is not extensively planted here, although it is recommended as a good street tree in some of the larger cities for places where it will receive fertilization and irrigation. It is subject to sunscald following transplanting. American linden and the littleleaf linden are the two preferred species for the high plateau. Local authorities should be consulted regarding suitability of linden in parts of northwestern Colorado.

Black locust is a tree that will stand the rigors of drought, poor soil, and abuse. It produces showy, white, fragrant flowers in the spring, but some persons object to its ragged appearance, early leaf fall, and the litter of its flowers and seed pods. In some localities it is severely attacked by the locust borer. The thornless variety is preferable, but various pink-flowering forms are gaining in popularity.

Norway maple is a handsome tree with a dense, globelike crown of dark-green foliage. It is favored as a street tree in the larger cities in places where irrigation and fertilization are possible and space is ample for root development. It is less subject to snow damage than silver maple and is moderately resistant to smoke injury. The Schwedler variety has bright red leaves in the spring, which later turn deep green. This variety is less likely to suffer from yellow foliage in alkaline soil than other maples. Norway maple is subject to sunscorch or leafscald, especially when the root system has not sufficient room to develop properly. It is best suited to the high plateau and is not recommended for the Gila River drainage. It is best to consult local authorities regarding the use of Norway maple and its varieties, as the species has proved to be tender in some localities.

Red mulberry is a somewhat larger tree than Russian mulberry but is less resistant to drought and cold.

Russian mulberry develops a crown of dense, dark-green foliage. It is fairly resistant to drought but is subject to winter injury. The fruits attract birds but also make a litter on walks and streets. The male, or staminate, trees do not produce fruit. This tree should not be planted in the mountains or in the colder parts of the high plateau.

Oak may be grown successfully in a few restricted areas in the region, notably in the foothills of northern Colorado. The limiting factor is soil alkalinity, which, besides giving the foliage a yellow cast, inhibits normal development for a naturally slow-growing tree.

Bur oak appears to be best adapted to the plateau as a whole, but even with the best of care and conditions it grows slowly.