South of the thirty-sixth parallel, the Southern Maritime Pine Belt extends along the Atlantic coast with a breadth varying from 100 to 200 miles, crosses over the Florida peninsula, and stretches along the Gulf coast as far as the flood plains of the Mississippi ; it reappears in Louisiana over a small area, but gradually merges into the deciduous Mississippi Forest. The characteristic tree of this region is the long-leaved pine (Pinus palustris), and the southern cypress is also extensively found. The first of these trees, although coniferous, requires a somewhat warmer climate than the white pine, and is usually found on drier soils ; the cypress, on the contrary, generally grows in swampy localities and near the coast.
The Deciduous Forest of the Mississippi basin and the Atlantic coast extends inland from the Atlantic coast between the thirty sixth and fortieth parallels, and northwards from the Gulf of Mexico ; it covers all the country, not already described, east of the lower Mississippi, and of a line drawn from its confluence with the Ohio to the southern extremity of Lake Michigan, together with the trans-Mississippi states of Arkansas, Missouri, and Louisiana, and parts of Texas and Indian Territory. The characteristic trees of this region, with its high summer temperatures, are the broad-leaved summer-green trees, such as chestnuts, oaks, hickories, walnuts, and tulip trees.
On the Pacific coast the Northern Forest extends south as far as the fifty-eighth parallel. Here begins the Pacific Coast Forest which runs southwards in a narrow strip to the fiftieth parallel, where it extends inland and embraces the region of heavy rainfall around Puget Sound. South of it, the Coast Forest covers the well-watered mountain slopes, as far inland as the crests of the Cas cades and Sierra Nevada, and as far south as the thirty-fifth parallel, beyond which it runs along the high ridges of the southern Coast Range to the boundary of the United States. This forest is com posed chiefly of coniferous trees ; in the north there are the Alaska cedar, the tide-land spruce, and the hemlock ; south of latitude 54° the red fir appears and is the most characteristic tree of the coastal region as far as latitude 43° ; in the interior the yellow pine is the principal tree, but the red fir, the hemlock, and the red cedar are still important. In California the chief trees are the redwood and the red fir along the coast ranges, the sequoia, the sugar pine, the red fir, and the yellow pine along the Sierra Nevada at an elevation of 4,000 to 8,000 feet, and oaks in the valleys between.
The Interior Forest includes all the wooded areas between the extreme east of the Coast Forest and the eastern crests of the Rocky Mountains, and between the Northern Forest and Mexico. This
forest is thin and poor, when compared with that along the coast, and is found on high mountain slopes which catch the rain, and along the river courses. On the east of the Sierras and Cascades, and on the west of the Rocky Mountains, especially in the south, there is to be found mountain mahogany, yellow pine, spruce, and white pine.
The unforested areas of North America fall into three main groups—the tundras, the grasslands, and the deserts and semi deserts. The tundras lie to the north of the limits of tree growth, and in Canada are generally known as the Barren Grounds. The extremely short vegetative season, after the snow has melted and the ground has thawed, prevents the development of higher forms of plant life, and mosses and lichens are the dominant species.
Under favourable conditions there are also many shallow rooted plants and berry bearing bushes, all forming a continuous covering, but elsewhere large tracts are bare.
The grasslands cover a much more extensive area, occupying the whole country between the Atlantic and the Pacific Forests. Two distinct types must, however, be recognised. A large region, which lies east of the line separating the Prairie Plains and the Great Plateaus of North America, and west of the Atlantic forests as already described, as well as the northern part of the unforested area belonging to the Canadian section of the Great Plateaus, consist of mixed woodland and grassland, the latter predominating. This is the debatable area between woodland and grassland. The rainfall is sufficient for the former, and it is believed, indeed, that the whole prairie region was at one time forested, but that, the trees being once destroyed, grass obtained the mastery owing to the fine adjustment of climatic and edaphic conditions. Whatever be the true solution of this question, and it has been the subject of much controversy, the region under con sideration now forms the greatest natural meadow in the world, the grasses growing in close formation. To the west of it, the country is an original steppe, except along the valleys of the rivers, where trees are sometimes found. From east to west this steppe becomes more xerophilous in character ; in the east it approaches meadow, and in the west desert, the controlling factor being the increasing scarcity of moisture.
Large areas in Oregon and Idaho, Nevada, Utah, Arizona, and New Mexico are unforested, and over the greater part of this region sage bush, which covers hundreds of thousands of square miles, is the prevailing vegetation. It is here that there is a nearer approach to desert conditions than in any other part of the United States. Further south these same conditions extend over Lower California and a considerable part of the Mexican plateau.