Botanists speak of something called climax vegetation, which is defined as the assemblage that would grow and reproduce indefinitely at a place given a stable climate and average conditions of soil and drainage. For most of the inhabited portions of America today, that concept has little meaning. The "natural" vegetation, if it ever existed, has been so substantially removed, rearranged, and replaced that it seldom is found now. In the Southeast, for example, the original mixed broadleaf and needleleaf forests were cut and replaced by the economically more important needleleaf forests. The grasses of the plains and prairies are mostly European imports. Their native American predecessors are gone either because they offered an inferior browse for farm animals or because they could not withstand the onslaught of modern humanity and its imported weeds. Most of what climax vegetation remains is in the West and North.
There are several ways of creating vegetation regions. Perhaps the simplest is to divide the United States into three broad categories--forest, grasslands, and scrublands. Forests once covered most of the East, the central and northern Pacific Coast, the higher elevations of the West, and a broad band across the interior North. Forests of the Pacific coast, the interior West, the North, and a narrow belt in the Deep South were all needleleaf and composed of many different trees. Much of the Ohio and lower Mississippi River Valleys and the middle Great Lakes region was covered by a deciduous broadleaf forest.
Grasslands covered much of the interior lowlands, including nearly all of the Great Plains from Texas and New Mexico to the Canadian border. This is an area of generally subhumid climate where precipitation amounts are not adequate to support tree growth. An eastward extension of the grasslands, the Prairie Wedge, reached across Illinois to the western edge of Indiana, where precipitation is clearly adequate to support tree growth.
Scrublands usually develop under dry conditions. They are concentrated in the lowlands of the interior West. Actual vegetation varies from the cacti of the Southwest to the dense, brushy chaparral of southern California and the mesquite of Texas.
The tundra of the far North is the result of a climate that is too cold and too dry for the growth of vegetation other than grasses, lichens, and mosses. Tundra exists in small areas far southward into the United States, where climatic conditions at high elevations are inhospitable to tree growth. Northward, the altitudinal tree line is found at lower elevations until, eventually, the latitudinal tree line is reached.
Source: U.S. Department of State