Note: This website is no longer being updated and is being maintained for archive purposes by the Department of Botany, National Museum of Natural History, Smithsonian Institution. Please see About the Project for further details.

Link to North America map of regional study sites
North America map

Link to Middle America map
Middle America map

Link to South America
South America map

Link to Centres of Plant Diversity home page

Link to Smithsonian Institution

Link to Department of Botany, National Museum of Natural History
Botany

Link to North America Regional Overview
North America Regional Centre of Endemism

North America

Centres of Plant Diversity and Endemism

Sites have been selected for Data Sheet treatment in the U.S.A. and adjacent parts of Mexico over those in Canada and Greenland because of their relatively higher levels of plant diversity and endemism. Some of the important centres of plant diversity and endemism, although lying predominantly in the U.S.A., extend into Canada. This explains in part the relatively larger number of endemics in southern as compared to northern Canada.

REGIONS
Canada
Pacific Northwest
Intermountain region
Appalachian mountain region
Interior Low Plateaux province
South-eastern Coastal Plain
Desert Areas
References

Canada

The Wisconsin glaciation destroyed much of the Canadian flora, resulting, overall, in a relatively young flora. There are, however, a number of local centres of plant diversity and endemism, as listed below. Most of these centres probably represent refugia. Some are also active centres of evolution which need protection. Centres of evolution are important to the study of evolutionary biology and biogeography and are gene reservoirs for plant and animal breeding. They are defined as geographical areas in which biological populations are actually or potentially undergoing evolutionary change, especially where this change is of greater than average rate or extent (Argus and McNeill 1974). They also can contain high endemism or morphologically and physiologically distinct populations in a restricted area. Areas in British Columbia and Ontario merit protection due to their high plant diversity and greatest number of both rare and endemic taxa.

Centres of Plant Diversity and Endemism

Centres of plant diversity and endemism in Canada include:

NA1. Ellesmere Island

NA2. The Arctic Islands (Brassard 1971)

NA3. Baffin Island (Brassard 1971)

NA4. Central Yukon Plateau (Cody 1971)

NA5. Mackenzie Mountains (Cody 1971)

NA6. Lake Athabasca, Saskatchewan, sand-dune region (Argus and McNeill 1974)

NA7. Western Newfoundland (Fernald 1926; Argus and McNeill 1974)

NA8. Torngat Mountains (Abbe 1936)

NA9. The Queen Charlotte Islands (Calder and Taylor 1968)

NA10. Rocky Mountains (Packer 1971)

NA11. Gulf of St Lawrence (Morisset 1971; Argus and McNeill 1974)

Return to Top

Pacific Northwest

The Pacific Northwest of North America should be considered a single floristic unit, due largely to the uniform moist climate (Brooks 1987). It encompasses Oregon, Washington and northern California in the U.S.A. and southern British Columbia in Canada, and corresponds roughly to the Vancouverian province of Takhtajan (1986). Whittaker (1960, 1961), Franklin and Dyrness (1988) and Kruckeberg (1969, 1984a-c, 1992) have described the geology, floristics and ecology of the region. Detling (1968) covers the historical background of the flora. The Pacific Northwest is rich in centres of plant endemism, with some of these centres also extending into Idaho and California (Siddall, Chambers and Wagner 1979). It is also known for its areas of serpentine-associated endemism. These may be found at: Bralorne and the upper Tulameen River in British Columbia; Double Eagle Lakes and the Sumas, Twin Sisters and Wenatchee Mountains in Washington; and Fields Creek, Baldy Mountain and the Klamath/Siskiyou Mountains in Oregon. In total, perhaps 500 to 600 species are restricted or nearly restricted to the Pacific Northwest (Thorne 1993).

Centres of Plant Diversity and Endemism

U.S.A.

NA12. Owyhee region, Oregon to Idaho

Leslie Gulch (in Oregon) is a drainage of about 90 km² at the centre of the Owyhee region with a high number of unusual, rare and endemic plants. It is an area from which normal vegetation is excluded. This entire region has a complex stratigraphy of volcanic rocks and sediments, with ashtuff as the typical substrate. It is thought likely that many endemics that occur here are pioneers adapted to recently exposed habitats and might be competitively excluded from more normal substrates.

NA13. Wenatchee Mountains, Washington

The Wenatchee Mountains form a series of spurs along the eastern side of the central Washington Cascades. Portions of these mountains provided refugia along the southern margins of the Pleistocene continental ice sheet. Many mountain tops and some valleys not only escaped alpine glaciation, but were presumably sufficiently isolated for speciation to occur. Additionally, these mountains have the most extensive ultramafic (primarily serpentine and peridotite) rock outcrops in Washington (Sheehan and Schuller 1981). In combination, these factors resulted in the highest concentration of rare and endemic plant taxa in the state.

NA14. Olympic Mountains, Washington

The Olympic Mountains in north-west Washington are isolated from other mountains of comparable size. Although most of this range was glaciated during the Pleistocene, the few peaks and ridges that escaped glaciation provided refugia for some constituents of older floras. These former refugia are responsible, at least partly, for relatively high levels of endemism (Sheehan and Schuller 1981). About 20 species are restricted to the Olympic Peninsula (Jones 1936).

Canada

NA15. Southern British Columbia, primarily the Queen Charlotte Islands

The vegetation largely belongs to the Cordilleran Forest province of the U.S.A., which reaches a short distance into Canada. Of the 47 taxa endemic to British Columbia, only 14 are restricted to the province itself. These occur primarily on the Queen Charlotte Islands (and also on north-western Vancouver Island) of southern British Columbia, which provided refugia during the Pleistocene and thus possess a relictual flora (Argus and Pryer 1990).

California

For additional information on the flora of California and the California Floristic Province, see Abrams (1925, 1926), Campbell and Wiggins (1947), Howell (1957), Stebbins and Major (1965), Ornduff (1974), Raven (1977), Raven and Axelrod (1978), Barbour (1988), Barbour and Christensen (1993), Hickman (1993), Thorne (1993) and Yatskievych and Spellenberg (1993).

Centres of Plant Diversity and Endemism

U.S.A., Mexico

NA16. California Floristic Province

U.S.A.

NA16a. Big Bear Valley and Baldwin Lake area (San Bernardino National Forest)

This area contains relicts of a mesic coniferous and deciduous forest which was widespread throughout the Northern Hemisphere in the middle of the Tertiary period (Axelrod 1958).

Mexico

NA16b. Guadalupe Island

 Located 265 km off the mainland of Baja California, Guadalupe Island is known for its high degree of endemism. The flora consists of 164 native species, of which 31 are endemic, including two endemic genera (Baeriopsis and the extinct Hesperelaea) (Raven and Axelrod 1978).

U.S.A.

NA16c. Klamath-Siskiyou region

NA16d. Santa Lucia range and Monterey Peninsula

This area contains high endemism.

U.S.A., Canada

NA16e. Serpentine flora (western)

U.S.A.

NA16f. Kern Plateau and southern Sierra Nevada

U.S.A., Mexico

NA16g. Vernal Pools

U.S.A.

NA17. Inyo region of California and Nevada

This is considered both the floristically richest region in transmontane California and the most important centre of endemism east of the mountains in California. Of the 454 species occurring only within this region in California, at least 199 are not found elsewhere outside of the state. Also, there are at least 43 endemic species within the Inyo region and over 25 other endemic species that range a short distance into Nevada (Raven 1977).

NA18. White Mountains

The White Mountains of California and Nevada are of exceptional botanical interest due to the presence of the oldest known living plant (Pinus longaeva). The vegetation is distinctly Great Basin in character consisting of desert scrub, pinyon woodland, subalpine forest and alpine zones. Within these zones nearly 1100 vascular plant taxa are known to occur (Morefield 1988). The desert ranges such as those in the southern White Mountains of eastern California have edaphic endemics confined almost entirely to limestone soils derived from dolomite (Lloyd and Mitchell 1973).

NA19. Death Valley

The extremely dry Death Valley, with temperatures frequently approaching 52°C, is part of the Mojave Desert. The Mojave covers the southern tip of Nevada and the eastern edge of California. Its unique appearance comes from the characteristic regular spacing of the creosote bush. Death Valley itself contains a rich flora, while endemism on limestone in the deep canyons flanking the valley is another important feature. In addition, many genera occur in California only within this region.

Return to Top

Intermountain Region

This area, as described in Cronquist et al. (1972), covers the dryland region between the Sierra Nevada on the west and the Rocky Mountains on the east, and between the moister country of the Pacific Northwest on the north and warmer drylands to the south. It includes all of Utah, most of Nevada (see Tidestrom 1923) and portions of Idaho, Oregon, Arizona and California. It includes the hydrographic Great Basin and a considerable portion of the geologic Colorado Plateau. The region coincides to a large extent with the Great Basin Floristic province, as defined by Gleason and Cronquist (1964). Although the Intermountain region is mountainous, its mountains are discontinuous and surrounded by desert.

Although the region occurs in the arid to semi-arid part of the U.S.A., the flora shows great diversity, because the many mountain ranges contain habitats humid enough to support woodland and forest vegetation. Thus, the flora ranges from alpine to xeric to subtropic. The region as a whole has at least 215 endemic species and 58 endemic varieties and subspecies (Cronquist et al. 1972). For more detailed information on the Intermountain region and its endemic taxa see also Tidestrom (1923), Graham (1937), Kearney and Peebles (1960), Franklin and Dyrness (1988), Welsh, Atwood and Reveal (1975), Welsh (1979), Mozingo and Williams (1980), Steele et al. (1981), Washington Natural Heritage Program (1990), Skinner and Pavlik (1994), Rutman (1990), Barbour and Christensen (1993), Thorne (1993) and Yatskievych and Spellenberg (1993).

The area encompassed in the Intermountain Floristic region includes all or parts of four major physiographic provinces or subprovinces: Colorado Plateau, Central Rocky Mountains, Great Basin (forms nearly three-quarters of the total Intermountain region) and the Snake River Plains. Each of the following divisions has at least one section containing a number of endemic taxa: Great Basin division (the Calcareous Mountains section is the richest area in this division for plant endemism); Wasatch Mountains division (only one section, by the same name, has some endemic taxa); the Uinta Mountains division (comprising only the Uinta Mountain section); and the Colorado Plateau division (Canyon Lands and Utah Plateaux sections).

Centres of Plant Diversity and Endemism

U.S.A.

NA20. Colorado Plateau division

A large system of plateaux and intervening valleys encompassing an area of around 1,341,620 km2. This system contains some of the world's most scenic river canyons. The areas of high endemism in this division include the following two sections. These three areas, in Utah alone, have 120 endemics (Shultz 1993).

NA20a. Canyon Lands section

This section lies almost entirely in south-eastern Utah, covering an area of 61,515 km². It is characterized by a broad desert plain broken by deep canyons. The Colorado River cuts diagonally across the section in a deep, rugged canyon. It is the richest section in the Intermountain region for endemic taxa (approximately 90 species).

NA20b. Utah Plateaux section

This area consists of nine fault-blocks or individual plateaux, lined up in three series separated by flat-bottomed valleys, occupying 31,080 km². It is the second richest section in the Intermountain region for endemic taxa (approximately 40 species).

NA21. Central Rocky Mountains

The branch of the Laramide uplift that trends southward from Idaho into Utah and Colorado and comprises the central Rocky Mountain province. Considerable work has been done recently at the University of Wyoming to survey the flora of Wyoming and the adjacent Middle Rocky Mountain region. The following ecological regions have been found to have high levels of endemism: Absaroka/Owl Creek (9); Bighorn/Pryor Mountains (6); Colorado Front Range/Laramie Range/Medecine Bow Mountains (4); Uinta Mountains foothills (9); Wyoming Basin (17).

NA21a. Uinta Mountains

This range, located mainly in north-eastern Utah along the Wyoming boundary, is the largest east-west trending mountain range in the Western Hemisphere (Fenneman 1938). The Uinta Mountains section covers an area of approximately 12,175 km2. Many peaks are over 3660 m in elevation and are composed mainly of quartzites. The upper portions of the range were extensively glaciated. The portion of this basin in Utah has 25 endemics.

NA21b. Wasatch Mountains section

For nearly 320 km, the Wasatch Mountain range, with peaks up to 3660 m in elevation, trends north-south in Utah and south-eastern Idaho. Various rivers and creeks have dissected the steep western escarpment of these mountains into a number of U-shaped canyons. Also several areas along the range were glaciated. Fifteen taxa are known only from this section.

Return to Top

Appalachian Mountain Region

The Appalachian mountain region, composed primarily of crystalline rocks, is considered one of the most important areas of plant endemism in the eastern U.S.A. It is part of the Piedmont Upland geological province, a belt of metamorphic rock extending from Trenton, New Jersey to Alabama (Brooks 1987). This, in turn, forms part of a very long belt of ultramafics extending through New York state, the Gaspé Peninsula, Newfoundland and the northern tip of Quebec in the Ungava.

The Southern Appalachians are unique in that they represent the oldest land mass in eastern North America thought to have been relatively unaffected by marine waters or continental glaciation. They are also thought to have served as "...a source of inoculum for the development of the floras of the south-eastern coastal plains and the revegetation of the glaciated terrain to the north" (Sharp 1970). Holt (1970) covers the distributional history of the flora. Wood (1970) states that "of the approximately 100 genera endemic to eastern North America, 52 are represented in the southern Appalachian region...". Harper (1947) listed 188 species of vascular plants found in the southern Appalachians. About half of them are definitely narrow Appalachian endemics, while some of the others range westward to the sandstone of the Cumberland Plateau in Kentucky and Tennessee. Many of the taxa on this list are endemic to shale barrens or to limestone or dolomitic valleys and cliffs.

Centres of Plant Diversity and Endemism

U.S.A.

NA22. Shale barrens

The term was first used by Steele (1911) to designate a peculiar type of plant habitat found in the mid-Appalachians and extending northward along the Virginia-West Virginia border, across Maryland and into central Pennsylvania. These barrens developed on areas with outcrops of certain types of hard shaly rock. The barrens are particularly well-developed from central Pennsylvania to south-western Virginia and adjacent West Virginia, where they occur at elevations of 305-610 m (Keener 1970). Their characteristics include a generally southern exposure, usually a steep slope on a low hill, a stream undercutting the base and sparse vegetation growing on a mantle of thin weather-resistant rock.

A scrubby growth of pines, oaks and other woody plants, with herbaceous species scattered beneath, typically occupies these barrens (Henry 1954). The peculiar flora includes a number of endemic and near-endemic taxa (Keener 1970).

See Platt (1951) for a complete list of shale barren plants, including 97 angiosperm taxa. See also Henry (1954) for another treatment of floristic aspects of the endemic flora. Localities of the Virginia-West Virginia barrens, primarily along a narrow strip of shale outcropping along both sides of the states' border (where a considerable number of endemics is found), are covered by Core (1940); for locations of the Pennsylvania barrens, see Henry (1954).

NA23. Great Smoky Mountains

Several peaks with elevations of over 1830 m occur in the Blue Ridge and Smoky Mountains of North Carolina and easternmost Tennessee. Many of the numerous southern Appalachian endemics and near-endemics have patchy distributions and may occur at fairly high altitudes (Reed 1986). This area is now well protected as the Great Smoky Mountains National Park, established in 1934 and covering 2092 km².

NA24. Piedmont rock outcrops in Georgia, South Carolina and Alabama

Outcrops of ancient granitic rock in the Piedmont, known as "flat-rocks" or "cedar rocks," are "without parallel in eastern North America" and support an unusual flora with one-third (17) of the outcrops' 44 most characteristic taxa being endemic, an unusually high percentage for such a small area (Braun 1955). The escarpment region, owing to its location at the edge of the Piedmont, has served as a pathway for migrations during the numerous periods of climatic change that have occurred in the Tertiary and Pleistocene. Its gorges and their surrounding areas now have a significant number of southern Appalachian endemics.

U.S.A., Canada

NA25. Serpentine flora (eastern)
see Data Sheet on North American serpentine flora (NA16e and NA25).

Return to Top

Interior Low Plateaux Province

The Interior Low Plateaux province includes southernmost Illinois and Indiana, south-easternmost Ohio, central and western Kentucky and central Tennessee and extends into northernmost Alabama. The general geological structure is similar to the Appalachian Plateau, except that altitudes are lower and relief is more subdued. The northernmost portion of the province underwent glaciation. There are two major elevations or domes, one centred in Kentucky and the other in Tennessee. Owing to the erosion of these domes, hills have been formed surrounding them. These are composed largely of sandstone and chert-rich layers of limestone. The centre of the Nashville dome is eroded into a basin. In a number of areas, limestone is exposed or, where moderately flat, is covered with only a very thin soil. In the Nashville area, such places are called barrens, while elsewhere in the province they are called glades. Rivers tend to be subterranean in limestone areas.

Centres of Plant Diversity and Endemism

U.S.A.

NA26. Cedar glades

One of the most unusual areas of rock-outcrop vegetation in the eastern U.S.A. is the cedar glades. Red cedar, Juniperus virginiana, is one of the most common woody species growing on the deeper soils in and around these openings. Neo- and palaeoendemics, as well as disjuncts and eastern species of western genera, are among the plant taxa characteristic of the glades. Although originally known from and predominantly found in central Tennessee, the glades have also been described from a number of localities in the eastern deciduous forest. These areas include eastern Tennessee, northern Alabama, north-western Georgia, the Ozark region of Missouri, and extreme south-western Virginia and Kentucky. As determined by Baskin and Baskin (1988) and others, the cedar glade flora includes some 30 endemic and near endemic taxa. Baskin and Baskin state that the unusually high number of endemics, combined with the presence of disjuncts, is indicative of the sites having served as centres of speciation for a long time.

Return to Top

South-eastern Coastal Plain

The south-eastern Coastal Plain, of low relief and dipping seaward, extends some 3540 km from Cape Cod south to Florida and west to Texas and for an additional 1609 km along the Gulf coast of Mexico (Shimer 1972). The underwater seaward extension forms the continental shelf. Overall, the plain is composed of poorly consolidated sediments of Cretaceous to Recent age. Elevations of hills and ridges reach only 60-90 m. The most recently formed features, which lie along the coast, include cliffs, sand-dunes, beaches, sandspits and sandbars. The area from Cape Cod to Long Island was glaciated. The vegetation can be classified as the southern mixed hardwood region of the deciduous forest. The flora includes several hundred endemic species, several endemic genera, and one endemic family, the monotypic Leitneriaceae (Thorne 1993). The states with important centres of plant diversity and endemism are North and South Carolina, Florida and Texas.

Florida contains a large number of endemic plant species due to three main factors: (1) most of Florida is a peninsula, relatively isolated from other land masses; (2) it projects southward from the continent, and there is no nearby land mass with climates similar to those of Florida's southern portion (except the Bahamas, which are relatively small and have very different soil from most of Florida); and (3) Florida has more sandy soils than any other state, as well as considerable areas of limestone in its central and southern portions (Harper 1914). Also, past geological factors have played a major role in determining the centres (Woodson 1947; Thorne 1949; James 1961). For example, the existence of an island refugium, which could have functioned as a centre of dispersal during the Tertiary, has been postulated. Howard (1954) indicated the areas within which these endemics are most concentrated. Muller et al. (1989) have provided current data on the centres and their endemic taxa. They list some 235 endemic and an additional 40 near-endemic vascular plant taxa.

Texas displays wide ranges in rainfall, soil type and elevation, resulting in great differences in habitat and vegetation from area to area. The extreme variation and isolation that occurs within Texas has produced both a large, very diverse flora and a high degree of endemism. Some 5480 known taxa of flowering plants and ferns occur here, including half of the grass species indigenous to the U.S.A. Furthermore, Texas is the only state in which are found, not only Rocky Mountain and eastern plants, especially oaks and pines, but also, in the state's southernmost section, many subtropical species.

Of the flowering plant and fern taxa, 464 are considered endemic to Texas, about 8.3% of the total (Correll and Johnston 1970). Important centres of plant endemism occur in five of the state's ten major vegetational areas: East Texas pinewoods; coastal prairie (Gulf prairies and marshes); Rio Grande plains (South Texas plains); Edwards Plateau; and Trans-Pecos region. The last two areas are especially important centres of plant diversity and endemism.

Centres of Plant Diversity and Endemism

U.S.A.

NA27. Mesic savannas of North and South Carolina

In these savannas, graminoid diversity and herb diversity are generally very high. Common graminoids are Sporobolus teretifolius, Muhlenbergia expansa, Ctenium aromaticum, Andropogon spp. and Rhynchospora plumosa. Other indicative species are Lycopodium carolinianum, Lachnocaulon anceps and Xyris smalliana. However, the most distinctive taxa occurring here are insectivorous plants, including Drosera spp. (sundews), Pinguicula spp. (butterworts), Sarracenia spp. (pitcher plants) and the endemic Venus flytrap, Dionaea muscipula. The Venus flytrap is found only in savannas of the outer Coastal Plain of south-eastern North Carolina and at a few localities in South Carolina. Serious threats include human alteration and destruction of habitat, fire suppression and collecting and trade in insectivorous plants (Christensen 1988).

NA28. Apalachicola River drainage of north-western Florida (panhandle) and adjacent Georgia

The east side of the Apalachicola River is one of the classic areas of both endemics and rare plants, such as Torreya taxifolia, with its nearest relative in California, and the associated herb Croomia pauciflora, a member of a family (Croomiaceae) not elsewhere known outside of Asia. The flora contains many endemics and Tertiary relicts. The endemics occur primarily in the cool wet flatlands (savannas, seepage slopes and flatwoods).

NA29. Central highlands of Florida

NA30. Miami Ridge rocklands

Found in Dade and Monroe counties of Florida, the Miami Ridge rocklands contain a large number of endemic taxa, and support an endemic community. Dade County is considered to have a larger number of endemic plants than any other Florida county, with estimates ranging from 55-65 taxa (Muller et al. 1989).

NA31. Atlantic Coastal Plain

The area from south-eastern North Carolina south to north-eastern Florida between the coast and St John's River is an important centre of plant diversity. Many now feel that coastal North Carolina-Florida should be considered a separate region since numerous endemic plants occur in its habitats, including coastal hammocks, dunes, shell mounds, marshes and flatwoods. There are 73 species endemic to northern Florida.

NA32. Edwards Plateau

NA33. Trans-Pecos region, including Big Bend National Park

The Trans-Pecos region, an area of about 7,695,000 ha west of the Pecos River, consists of mountains and arid valleys. Some consider the extreme south-eastern portion to be part of the Edwards Plateau. The primary representation of the Chihuahuan Desert in the U.S.A. lies in this region (see NA35). About one out of every twelve species in the Texas flora occurs in the Trans-Pecos and nowhere else in Texas (Correll and Johnston 1970). Owing to the variety of available habitats, the flora is both rich and diverse; the primary vegetation types include creosote-tarbush desert shrub, grama grassland, yucca and juniper savannas, pinyon pine, oak forests and (more locally) ponderosa pine.

NA34. Gulf or coastal prairie

This is a nearly level plain of 3,847,500 ha lying along the coast of Texas. With an elevation of less than 45 m, it has numerous sluggish rivers, creeks, bayous and sloughs. It includes low flat woodlands, freshwater marshes, salt meadows and salt marshes. The natural vegetation consists of tall-grass prairie or post oak savanna. Cattle graze much of the marsh, with rangelands and farms in the uplands. Owing to human disturbance, much of the area has been invaded by certain trees, shrubs and cacti. This area contains many threatened plants.

Return to Top

Desert Areas

As stated in MacMahon (1979), low elevation "warm" deserts occupy the south-western portion of the U.S.A. and the northern quarter of Mexico: the Mojave and Chihuahuan deserts are warm temperate, while the Sonoran Desert is classified as subtropical. In contrast, the Great Basin Desert to the north, with 60% of its precipitation in the form of snow, is known as a cold desert. Except for a piece of the Great Basin Desert that overlaps onto the Columbia Plateau province and Baja California, considered a province in itself, all of the North American deserts are contained in the physiographic area referred to as the Basin and Range province.

Desert elevations range from -86 m in the Mojave to 1525 m in the southern Chihuahuan in Mexico; latitudes range from 22°N to 36.5°N. Consequently, these deserts collectively possess a wide range of vegetation types. In fact, most American deserts have a much greater diversity of species than do many temperate forests. All of these deserts possess endemic species. For general coverage of North American deserts, see MacMahon (1979) and Bender (1982).

Table 16 shows the approximate areas of North American deserts.

Centres of Plant Diversity and Endemism

U.S.A., Mexico

NA35. Chihuahuan Desert

The Chihuahuan Desert is of generally high elevations, with most sites between 1100 and 1500 m. It is the largest North American desert with a total land area of 450,000 km². Its lowest elevations, along the Rio Grande River, are near 400 m, while its highest elevations, in the south, reach up to 2000 m. Much of this desert is dominated by limestone or gypsum which often have endemic plants (MacMahon 1988). The mean annual temperature is 18.6°C, and precipitation ranges from 150 to 400 mm. It consists of three regions: the northernmost, Trans-Pecos region (see CPD site NA33), encompassing approximately 40% of the desert and including all of the sections in the U.S.A. and more than half of the desert areas of Chihuahua (Mexico); the Mapimian, or middle region, including Coahuila and parts of Chihuahua and Durango in Mexico; and the Saladan, or most southern region, including primarily the states of Zacatecas and San Luis Potosí in Mexico. The vegetation may be divided into eight primary subdivisions of desert scrub and woodlands: Chihuahuan Desert scrub, lechuguilla scrub, yucca woodland, Prosopis-Atriplex scrub, alkali scrub, gypsophilous scrub, cactus scrub and riparian woodland (Henrickson and Johnston 1986). Grasses are conspicuous in most areas of this desert, especially at higher elevations; common genera are Sporobolus, Muhlenbergia and Bouteloua. There are also many species of cacti; Opuntia phaeacantha and Echinocactus horizonthalonius cover large areas in dense stands.

U.S.A.

NA36. Great Basin Desert

This desert covers most of Nevada and Utah and south-western Colorado to northern New Mexico and Arizona. The part that extends north of 42°N latitude has been described as shrub steppe and is considered part of the Intermountain region. Its latitude and elevation (mostly above 1000 m) result in a temperate climate. Because of these factors, it is not a desert in the generally accepted sense, and should really be classified as a semi-desert with a shrub-steppe landscape. Artemisia spp. (sagebrush) are the most characteristic and widespread dominants constituting more than 70% of the cover (West 1988).

NA37. Mojave Desert

North of the Sonoran Desert is the Mojave, the smallest of the four deserts. It includes portions of southern California, southern Nevada, south-western Utah and north-western Arizona. The vegetation differs from that of the other deserts in that it is generally dominated by low-growing, usually widely spaced perennial shrubs, representing few species. Also, although cacti are present, they are generally of low stature. Five general vegetation types are found: (1) creosote bush in which the most common dominants are Larrea tridentata and Ambrosia dumosa, covering 70% of the desert; (2) shadscale (Atriplex confertifolia); (3) saltbush (Atriplex polycarpa); (4) blackbrush (Coleogyne ramosissima); and (5) Joshua tree (the usual dominant is Yucca brevifolia, with a total cover of 10.15%), associated with variable quantities of perennials at different elevations (Vasek and Barbour 1988). About 25% of this desert's plant species are endemic (nearly 80% of the approximately 250 annuals). Stebbins and Major (1965) listed 138 endemics.

U.S.A., Mexico

NA38. Sonoran Desert, including Baja California

The Sonoran Desert covers part of south-western Arizona and northern Mexico, as well as the Baja California peninsula. It is considered perhaps the richest of all North American deserts. Kearney et al. (1960) list endemic plant species found in the portion of the desert lying in Arizona and Bowers (1981) has an excellent annotated bibliography of local floras. See also Shreve and Wiggins (1964) for more information on the vegetation of the Sonoran Desert. (Part of this area is included in CPD Data Sheet MA11, Apachian/Madrean region of south-western North America.)

Return to Top

References for the North America regional overview

North | Middle | South

CPD Home  

Botany Home Page | Smithsonian Home Page