North American flora

North America can be broadly divided into two major botanical regions: the Atlantic North American region, with its northern coniferous and temperate deciduous forests, and the drier Pacific North American region, with its northern evergreen oak forests, temperate redwood forests, and southern chapparal and semi-desert vegetation. Smaller botanical regions within the continent include the subtropical southern tip of Florida, the tundra vegetation of the Arctic and sub-Arctic region in the extreme north, and the desert regions of the southwest.

Atlantic North American region This region has two main components: the Canadian forest area and the deciduous forests of the USA. The former, which is the North American portion of the northern coniferous forest belt, contains, unlike its European counterpart, a wide range of species belonging to many genera (Abies, Chamaecyparis, Juniperus, Larix, Picea, Pinus, Thuja, and Tsuga). Only one species, Picea glauca, is found throughout the whole region. The deciduous forests of eastern North America also contain a huge and complex variety of plant species.

Important plants that originated in the Atlantic North American region include the sugar maples (such as Acer saccharum), the hickories and pecans (Carya species), and medicinal plants such as witch hazel Hamamelis virginiana. Well-known garden plants from this region include Michaelmas daisies (Aster species), black walnut Juglans nigra, magnolia Magnolia grandiflora, sumac Rhus typhina, and the locust tree Robinia pseudoacacia.

The North American prairies also fall within this region. Within them are a number of vegetation zones, such as tall-grass prairie, mixed prairie, and short-grass prairie, which succeed one another from east to west as the climate becomes drier. The main grasses are species of Andropogon, such as A. scoparius and A. gerardii, although Stipa comata, Buchloe dactyloides, and Bouteloua gracilis are important.

Pacific North American region This region has about 300 unique native genera, mostly concentrated in California. Species from this region that have been introduced as garden plants elsewhere include the Californian poppy Eschscholzia californica, Arbutus menziesii, the tree lupin Lupinus arboreus, and the Oregon grape Mahonia aquifolium. California's winter-rain belt is distinctive for the richness of its species, with many oak and cypress species. In the north, evergreen oak forests dominate, while in the south there is the chaparral, a shrubby community dominated by Arbutus, Arctostaphylos, and Ceanothus. The chaparral is the American equivalent of the European maquis, but unlike the maquis, it is a natural climatically controlled vegetation.

North of San Francisco are the warm temperate redwood forests of Sequoia sempervirens. This species grows to a height of over 100 m/330 ft, higher even than the Sequoiadendron of the Sierra Nevada. East of the Sierra Nevada, at altitudes of 1,300 m/4,265 ft, are the sagebrush semi-deserts of Artemisia tridentata on the heavier soils with Pinyon (Pinus species and Juniperus) on stony ground. True coniferous forests of Pinus flexilis and P. aristata begin at 2,000 m/6,550 ft and the treeline (above which trees cannot grow) is reached above 3,000 m/9,850 ft, with Picea englemannii and Abies lasiocarpa.

Desert The driest region is the Sonoran Desert of northern Mexico, where the most characteristic vegetation is cactus forest, dominated by candelabra cacti such as the saguaro Carnegiea gigantea, the largest of all cacti. They can store up to 3,000 l/660 imp gal of water and live for over a year without water absorption. Wide, flat areas carry the drought-resistant creosote bush Larrea divaricata that spreads its roots over a wide area just below the ground, and puts out its leaves only when rain falls. In the mountains, the cactus desert gives way to a shrub desert with leaf-succulents such as Agave and Yucca, and this in turn becomes evergreen oak forest. Above these, there is a belt of Pinus ponderosa.