One of the commonest mountain trees is the aspen, popularly called the "quaking asp"; the characteristic shimmering action of its leaves in the slightest movement of air is caused by the fact that aspen leaves have flat stems. Male and female flowers are borne on separate trees, and since female trees are rare, aspens seldom grow from seeds. New trees usually sprout from underground roots of older trees, hence aspens generally grow in dense groves. The trees, however, require plenty of light, for they cannot live even in their own shade. In Utah, the value of aspens is largely scenic, their slender white trunks adding an attractive touch in any season, and in autumn their golden foliage makes a bright splash on the landscape at altitudes around 7,000 feet. The wood is too soft for lumber, but is extensively used for fuel and miscellaneous farm purposes. Aspens also shelter young pines and firs, and pave the way for more valuable forests.
Growing along streams, from low to medium elevations, are several species of cottonwood. Like the aspen, they are members of the poplar family. The green of these trees was a welcome sight to early explorers, for it frequently marked the end of a weary desert journey. Great numbers have been transplanted from their native areas to near-by settlements, and many small communities are visible for miles because of green cottonwoods lining the streets. The planting of these trees has often been regretted by residents, who are annoyed by the silkytufted seed carriers that get into their hair, on clothes, and in house screens. Attempts to uproot small cottonwoods from lawns usually result in the discovery of a network of roots extending from a tree across the fence. Probably the largest cottonwood tree in the State, more than eight feet in diameter, is at Moab.
Bordering the canyon streams are a number of species of shrubs which ordinarily do not reach the size of trees. Among them are red birch, mountain alder, hawthorn, and several varieties of willows. The wood of the willows, being soft and flexible, is used by Indians in basket-weaving. Hillsides are covered with dense thickets of maple, scrub oak, chokecherry, service berry, and mountain mahogany. Also called "buck brush," the latter, related to the true mahogany, is of value as a browsing brush for deer, and straighter specimens were used by pioneers for cabinet work. Its wood is one of the few kinds that will not float. Of the maples, the dwarf is the commonest Utah species. This shrubby tree is an outstanding feature of the mountain scenery at all times, but particularly in the autumn, when the first touch of frost turns its foliage into a blaze of brilliant color. The big-tooth maple, a hardwood variety, grows to larger size, and in some of the national forests is more than a foot in diameter and more than fifty feet high. The box elder, for which one of the northern counties was named, is another variety of maple common along the water courses. The sweet sap of this tree was sometimes used for syrup in pioneer days, but it has an undesirable attraction for insects, especially a black-and-red-winged plant bug with the objectionable habit of wintering in houses. Chokecherry often forms extensive growths on canyon slopes. It has showy white flowers that bloom in May and June. The fruit is edible and quantities of it are gathered for jellies and wines. Service (or "sarvis") berries, which are also edible, are sometimes dried by the Indians and mixed with meat to make pemmican.