The United States is rich in the variety of its native forest trees, some of which, as the species of sequoia, are the most massive known. More than 1,000 species and varieties have been described, of which almost 200 are of economic value.
A coniferous forest of white and red pine, hemlock, spruce, and balsam fir extends interruptedly in a narrow strip near the Canadian border from Maine to Minnesota and southward along the Appalachian Mountains. There may be found smaller stands of tamarack, spruce, paper birch, willow, alder, and aspen or poplar. Southward, a transition zone of mixed conifers and deciduous trees gives way to a hardwood forest of broad-leaved trees. This forest, with varying mixtures of maple, oak, ash, locust, linden, walnut, hickory, sycamore, beech, once extended uninterruptedly from New England to Missouri and eastern Texas.
Pines, palmettos, and live oaks are replaced at the southern tip of Florida by the more tropical palms, figs, satinwood, and mangrove.
The alpine tundra, located in the conterminous United States only in the mountains above the limit of trees, consists principally of small plants that bloom brilliantly for a short season. Sagebrush is the most common plant of the arid basins and semideserts west of the Rocky Mountains, but juniper, nut pine, and mountain mahogany are often found on the slopes. The desert, extending from south-eastern California to Texas, is noted for the many species of cactus, some of which grow to the height of trees, and for the Joshua tree and other yuccas, creosote bush, and acacias.