The topographical and biological variety of the United States has resulted in a wide range of habitats, accounting for the great number and diversity of native plants.
The eastern sector includes the extensive deciduous forest region from the Atlantic Coast westward to Minnesota, eastern Iowa, southern Missouri, and eastern Texas. From the northeast the deciduous forest stretches southward through the Appalachians but is replaced on the coastal plains by extensive evergreen pine forests. The southern tip of Florida is subtropical and has many Caribbean species. With decreasing precipitation as one moves westward, forest vegetation is replaced by grassland with a rich diversity of prairie flowers. Although sporadic remnants of tall-grass prairie occur as far east as Ohio, the vast continuous grasslands begin in Kansas and Oklahoma. As aridity increases to the west, tall—grass prairie tends to give way to mid—height grasses and, ultimately, to short—grass plains near the Rocky Mountains.
Near the Pacific Coast moist conditions prevail, with a drier, warmer climate to the south and wetter, cooler conditions to the north, creating a general division between plant species approximately at the California—Oregon border. In the mountains there are dense coniferous forests, most with few wildflowers, some surrounding meadows that bloom with a profusion of colorful species. The high Pacific mountains (the Sierra Nevada and the Cascade and Olympic Ranges),some with cold, windswept, tundra—like habitat on their ridges, receive most of the rainfall from the west. As a result, there are virtual rain forests on the slopes that face the prevailing winds, but a slight bit eastward, on the leeward side of the mountains, is a wide area of arid grasslands and deserts. To the north at lower elevations lies a cold desert with many gray shrubs, especially sagebrush. To the south the hot Mojave, Sonoran, and Chihuahuan Deserts have mostly Creosote Bush, an olive green shrub, and a completely different set of plants from those in the cold desert. Ponds, streams, and coastal salt marshes provide habitats for many kinds of marshland and aquatic plants. Species growing along the Pacific Coast and inland in low desert pools must be tolerant of salty conditions. Strongly selective habitats, such as sand along the coast or in the deserts, have their own peculiar sets of plants.
