|North American Flora|
Forest is the native vegetation of almost half of mainland Canada and the United States. Before European settlement, forest lands dominated the eastern, and much of the northern, part of the continent.
Grasses covered a large part of the continental interior. Desert vegetation is native in the South-west, tundra in the far north. Overmuch of the continent, however, human activity has virtually eliminated native vegetation.
Coniferous Forests of the West
Along the Pacific coast from Alaska to Northern California, evergreen coniferous forest grows luxuriantly, watered by moisture-laden winds blowing from the ocean. This lowland forest includes some of the largest and longest-lived trees in the world. North of California, characteristic trees include Sitka spruce, western hemlock, and western red cedar.
Douglas fir, one of the major timber species in North America, is also common. The northwest coastal coniferous forest is sometimes called temperate rain forest because, in its lushness, it resembles the tropical rain forests. Many of the trees of the coastal forest have been cut for timber.
In California, the dominant coastal conifer species is the coast redwood. The tallest tree in the world, coast redwood reaches 330 feet (100meters) and can live two thousand years.
Coniferous forest also grows along the Cascade Mountains and the Sierra Nevada. Trees of the Cascades include mountain hemlock and subal pine fir at high elevations and western hemlock, western red cedar, and firs somewhat lower.
Sierra Nevada forests include pines, mountain hemlock, and red fir at high elevations; and red and white firs, pines, and Douglas fir somewhat lower. Ponderosa pine is dominant at low mountain elevations in both of these Pacific ranges.
The giant sequoia, long thought to be the largest living organismon earth, grows in scattered groves in the Sierra Nevada. (The largest organism actually may be a very old tree root-rot fungus that covers 1,500 acres in Washington State.)
Although shorter than the coast redwood, the giant sequoia is larger in trunk diameter and bulk. It can reach 260 feet (80 meters) tall and 30 feet (10 meters) in circumference.
Coniferous forest also dominates the Rocky Mountains and some mountainous areas of Mexico. In the Rockies, Englemann spruce and subalpine fir grow at high elevations, and Douglas fir, lodgepole pine, and white fir somewhat lower. Ponderosa pine grows throughout the Rockies at low elevations and is a dominant tree in western North America.
Boreal Coniferous Forest
|Boreal Coniferous Forest|
Taiga is the most extensive coniferous forest in North America, covering nearly 30 percent of the land area north of Mexico. It grows across Alaska and Canada and southward into the northern Great Lakes states and New England. White spruce and balsam fir dominate much of the Canadian taiga.
Eastern Deciduous Forest
A forest of mainly broad-leaved, deciduous trees is the native vegetation of much of eastern North America. Narrow fingers of this forest, growing along rivers, penetrate westward into the interior grasslands. Early settlers from Europe cut most of the eastern forest, but second-growth forest now covers considerable areas.
The plants are closely related to plant species of the temperate deciduous forests in Europe and Asia. In contrast, the plants of other biomes in North America are generally not closely related to the plants that occur in the same biomes elsewhere in the world, although they look similar.
In the eastern deciduous forest, maple and oak are widespread—maples especially in the north, oaks in the south. There are major subdivisions within the forest.
These include oak and hickory forests in Illinois, Missouri, Arkansas, eastern Texas, and also in the east—Pennsylvania, Virginia, and West Virginia—where oak and chestnut forest formerly predominated; beech and maple forest in Michigan, Indiana, and Ohio; and maple and basswood forest in Wisconsin and Minnesota.
The forest in parts of Michigan, Wisconsin, Minnesota, and New England contains not only deciduous trees but also evergreen conifers, including pines and hemlock. Vast native pine stands in the Great Lakes states have been cut for lumber.
Plant diseases have changed the composition of the eastern forest. American chestnut was once an important tree but has now nearly disappeared as a result of an introduced fungal disease. Dutch elm disease is similarly devastating American elms.
In many parts of the Southeast, people have replaced the native vegetation with fast-growing species of pines for timber production. In Mexico, tropical rain forests are prominent on the west coast, in the south and east, and in Yucatán. On the south coasts of Mexico and Florida, swamps of mangrove trees are common.
The central plains of North America, a wide swath from the Texas coast north to Saskatchewan, Canada, were once a vast grassland, the prairie. The climate there is too dry to support trees, except along rivers.
From west to east, there is a transition from the more desert like short-grass prairie (the Great Plains), through the mixed-grass prairie, to the moister, richer, tall-grass prairie.
Grassland soil is the most fertile in North America. Instead of wild prairie grasses, this land now supports agriculture and the domesticated grasses corn and wheat.
The tall-grass prairie, which had the best soil in all the grasslands, has been almost entirely converted to growing corn. Much of the grassland that escaped the plow is now grazed by cattle, which has disturbed the land and aided the spread of invasive, nonnative plants.
Other outlying grasslands occur in western North America. Between the eastern deciduous forest and the prairie is savanna, a grassland with scattered deciduous trees, mainly oaks. Savanna also occurs over much of eastern Mexico and southern Florida.
Scrub and Desert
In the semiarid and arid West, the natural vegetation is grass and shrubs. Over a large part of California, this takes the form of a fire-adapted scrub community called chaparral or Mediterranean scrub, in which evergreen, often spiny shrubs form dense thickets.
The climate, with rainy, mild winters and hot, dry summers, is like that around the Mediterranean Sea,where a similar kind of vegetation, called maquis, has evolved. However, chaparral and maquis vegetation are not closely related genetically. Humans have greatly altered the chaparral through overgrazing of livestock and other disturbances.
The North American deserts, which are located between the Rocky Mountains and the Sierra Nevada, cover less than 5 percent of the continent. Shrubs are the predominant vegetation, although there are many species of annuals. Desert plants, commonly cacti and other succulents, are sparsely distributed.
In Southern California, Arizona, New Mexico, West Texas, and northwestern Mexico, there are three distinct deserts. The Sonoran Desert stretches from Southern California to western Arizona and south into Mexico.
A characteristic plant of the Sonoran is the giant saguaro cactus. To the east of the Sonoran, in West Texas and New Mexico, is the Chihuahuan Desert, where a common plant is the agave, or century plant.
North of the Sonoran Desert, in southeastern California, southern Nevada, and northwestern Arizona, is the Mojave Desert, where the Joshua tree, a tree-like lily, is a well-known plant. It can reach 50 feet (15 meters) in height.
Creosote bush is common in all three deserts. To the north, the Mojave Desert grades into the Great Basin Desert, which is a cold desert, large and bleak. The dominant plant of the Great Basin Desert is big sagebrush. Plant diversity there is lower than in the hot American deserts.
Deserts dominated by grasses rather than shrubs once occurred at high elevations near the Sonoran and Chihuahuan Deserts. Much of this area has been overtaken by desert scrub, including creosote bush, mesquite, and tarbush. Cattle grazing may have been a factor in this change. Desert is very fragile; even one pass with a heavy vehicle causes lasting damage.
Tundra vegetation grows to the northern limits of plant growth, above the Arctic Circle, in Canada. The flora consists of only about six hundred plant species. In contrast, tropical regions that are smaller in area support tens of thousands of plant species.
Arctic tundra is dominated by grasses, sedges, mosses, and lichens. Some shrubby plants also grow there. Most tundra plants are perennials. During the short Arctic growing season, many of these plants produce brightly colored flowers.
Like desert, tundra is exceptionally fragile, and it takes many years for disturbed tundra to recover. Tundra also occurs southward, on mountain tops, from southern Alaska into the Rocky Mountains, the Cascades, and the Sierra Nevada. This alpine tundra grows at elevations too high for mountain coniferous forest.
Along the coasts of the Atlantic Ocean and the Gulf of Mexico, the soil is saturated with water and is very salty. Tides regularly inundate low-lying vegetation.
The plants in these salt marsh areas consist mainly of grasses and rushes. Marshes are a vital breeding ground and nursery for fish and shellfish. They play an important role in absorbing and purifying water from the land. Coastal marshes are being lost to development at a rapid rate.