At the time European settlement began, about 50 per cent of the United States was covered by forests; today, the figure is about 30 per cent. Similarly, grasslands and other natural vegetative cover decreased in extent as the continent was settled.
Northern Alaska, the northernmost part of the United States, is characterized by a windswept tundra, a region of lichens, mosses, hardy low shrubs, and flowering plants. Inland and to the south, the growing season lengthens and certain trees can survive. A few species of conifers, notably spruces and firs, dominate a vast evergreen forest, interspersed with lichen-covered rocky areas, grassy swamps, and aspen-choked fire scars. This forest, known as the taiga, stretches south-east from interior Alaska and reaches into northern New England and the Great Lakes region. South of the taiga the growing season is longer and more tree species can survive; the forest contains both conifer and deciduous trees, including pines, maples, elms, birches, oaks, hickory, beech, and sycamore. This type of mixed forest covered the region around the Great Lakes and most of the New England and Middle Atlantic states when European settlers arrived.
Still further south, the forest reaches its maximum diversity. The Great Smoky Mountains National Park in western North Carolina and eastern Tennessee contains more tree species than Europe. The Gulf of Mexico coast is even warmer than this mountainous area, but its plains and low hills do not support as complex a forest.
Moreover, the sandy soils and hot summers encourage fires, which suppress oaks and other hardwoods and favour the fast-growing pines that now represent the major forest resource of the nation. Other species found here include southern magnolia, pecan, red gum, and black gum (tupelo). A number of subtropical and tropical trees flourish in southern Florida. Along the coast of the Gulf of Mexico, salt marshes and groves of cypress and mangrove help to buttress the shore against the eroding forces of wind and water.
The diversity of the forest also decreases west of the Appalachian Mountains. First, the mountaintop spruces, firs, and mountain ashes disappear. Then rainfall decreases in quantity and reliability, and fires become more frequent. The lush hardwood forests of the Mississippi Valley slowly dwindle in size and complexity; oak-hickory forests give way to isolated stands of oak and then to tall grass prairies, which, before cultivation, occupied the present Corn Belt from Indiana to the eastern Great Plains. Further west, the climate becomes still drier, and the tall bluestem grasses yield to shorter grama and wheatgrass ranges. The grasses of the northern Great Plains grow only during the short summer and flower in late summer or early autumn. By contrast, the grasses of the southern Great Plains grow rapidly in spring, flower early, and then lie dormant during the hot, dry summers. Both kinds of grass become less productive as rainfall continues to diminish towards the west.
Shrubby sagebrush (in the north) and mesquite and juniper (in Texas) are typical invaders on poorer grasslands that have been overgrazed or protected from fires. A gradual transition to true desert vegetation is interrupted by the Rocky Mountains and other ranges, the elevation of which both increases rainfall and decreases temperature and evaporation. Trees become prominent on the lower and middle slopes. Hardy pines and junipers dominate at lower elevations, giving way to aspens, firs, and spruces at higher elevations.
Still higher, the spruces and firs become stunted and widely spaced. Above this zone is treeless tundra. Shrubby low-lying deserts alternate with forested (and occasionally tundra- or ice-capped) mountains across all of the Mountain states and into the Pacific states. This region is agriculturally productive only when massive investments are made in irrigation. Death Valley, which lies below sea level, is but one of the many nearly barren lowlands. Vegetation in these regions includes species such as sagebrush, juniper, piñon, rabbitbrush, mesquite, creosote bush, and yucca; the cactus “forests” that form a popular image of deserts are actually found on the slopes of mountain ranges in the Mojave Desert of southern Arizona and California. On the higher but still relatively dry Colorado Plateau are ponderosa and piñon pines.
The hot, dry summers and mild, moist winters of coastal southern California produce a distinctive shrub vegetation known as chaparral. Further north on the western slopes of the coastal hills and Sierra Nevada, where there is enough rain to permit rapid growth but a long enough dry season to discourage competition from numerous species, forests of giant sequoia and redwood grow. Still further north, in western Oregon and Washington, where the dry periods extend only to a few weeks in midsummer, a true rainforest appears, consisting primarily of a great variety of conifers. Douglas firs, true firs, hemlocks, cedars, spruces, and pines each occupy their own preferred elevation zone, and together constitute the second-richest forest resource for the nation. The coastal forests of Alaska have fewer species than the rich rainforest to the south but a faster growth than the taiga to the north.
The natural vegetation of Hawaii is conditioned by its isolation and by the interplay of its mountains and the moist trade winds. Forests dominated by guava trees on the windward (north-east) coasts of the islands graduate to a rich but swampy rainforest at moderate elevations, where the annual rainfall may exceed 10,000 mm (400 in). The high mountains support shrub forest, and patches of tundra are found on the summits of the highest peaks, Mauna Loa and Mauna Kea. The dry leeward (south-west) coasts are virtual deserts, with spiny koa and kiawe shrubs growing on the slightly wetter slopes.© "United States" © Emmanuel Buchot and Encarta
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