Vegetation and animal life

The vegetation of the United States is marked by great diversity.

At the time European settlement began, about one-half of the United States was covered by forests. Today, because of extensive human modification, about 30 percent of the country’s land area is forested Grasslands and other natural vegetative cover decreased as the continent was settled.

Northern Alaska, located in 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 needle-leaf trees, notably spruces and firs, dominate a vast but slow-growing evergreen forest, known as the taiga. South of the taiga the growing season is still longer and more tree species can survive. Here is found a mixed forest containing both needle-leaf and broadleaf trees, and including pines, maples, elms, birches, and oaks, as well as hickory, beech, and sycamore.

Still farther south, in western Oregon and Washington, a true rain forest appears as the dry periods shrink to less than a few weeks in midsummer. This luxuriant forest consists primarily of a great variety of needle-leaf trees: Douglas firs, true firs, hemlocks, cedars, spruces, and pines, each occupying its own preferred elevation zone here, and together constituting the second richest forest resource for the nation. The coastal forests of Alaska have fewer species than the rich rain forest to the south but a faster growth than the taiga to the north.

The Gulf of Mexico coast is even warmer and favors 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.

Death Valley, which lies below sea level, is but one of the many nearly barren lowlands.

ANIMALS

Animals depend, either directly or indirectly, on plants for their survival. Each major vegetation region thus has its own characteristic kinds of animal life. In the Arctic areas and regions of mountain tundra are found burrowing marmots, ground squirrels, cold-water fish such as grayling and trout, and an occasional bear. Alaskan coastal waters are the habitat of a number of large mammals, including walrus and fur seals. Caribou and elk spend summers on the tundra but migrate into the conifer forest for winter. Many birds migrate even farther, going from the polar regions to the Tropics each winter. The mountainous western states, especially Alaska, are the last refuges in the United States of most big-game animals. Here may be found elk, pronghorn, moose, deer, bighorn sheep, mountain goats, timber wolves, and, in a few remote areas, brown bears. The Kodiak bear, the largest carnivore in North America, is found in Alaska.

The hardwood forests of the eastern United States contain moose, black bears, deer, foxes, raccoons, skunks, squirrels, and a diversity of small birds. Along the Gulf of Mexico coast live larger and more colorful birds such as pelicans, flamingos, and green kingfishers, as well as alligators and warmwater fish such as catfish. Several varieties of venomous snakes are also found here.

Bison (buffalo) are popularly associated with the grasslands, although in fact they once ranged over most of eastern North America before being nearly exterminated by hunting; they now exist only in captivity or in protected areas. Gophers, rabbits, prairie dogs, ferrets, ants, and other burrowing creatures are best suited to the grasslands, which were once swept by fires.

The deserts have few plants, fewer small animals, and almost no large animals. Kangaroo rats, lizards, and wide-ranging birds are typical animals in this harsh region.

The animals of Hawaii include many endemic species (those not found elsewhere), but many of these have been driven to near extinction by human alteration of the habitat and by competition from introduced animals. Hawaii’s only indigenous mammal is the bat.

Дата добавления: 2015-11-14