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.

Date: 2015-01-02; view: 1573