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Centres of Plant Diversity: the Americas

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Centres of Plant Diversity: North America Regional Overview


Total Land Area:
.21,500,000 km2.
Population (1994): 287,713,408a.
Maximum altitude: 6194 m (summit of Mount McKinley, Alaska).
Natural Vegetation: .Major types include: coniferous forest, in Alaska, Canada, around the Great Lakes and on the upper slopes of the higher eastern and western mountains, but mainly on Pacific Coast Ranges, Cascades, Sierra Nevada, and Rocky Mountains south to Mexico; tundra, across north of continent and on high mountains; temperate deciduous forest, in east from central Minnesota to Quebec, south to Texas; grassland (prairie), in mid-continent; desert, mainly in south-west; subtropical vegetation, including mangroves, in Florida.
Number of vascular plants: c. 20,000 species.
Number of endemic species: c. 4198 (est. 20%).
Number of genera: 2350.
Number of endemic genera: 900 (or nearly endemic).
Number of vascular plant families: 210.
Number of endemic families: 2.
Important plant families: Gramineae, Leguminosae, Pinaceae, Rosaceae, Ericaceae, Cucurbitaceae.
Source: a Famighetti (1995); flora statistics based on Thorne (1993); N. Moran 1995, pers. comm.
Useful plants
Centres for Plant Diversity and Endemism


The area covered in this region includes the U.S.A. (excluding Hawaii), Canada and Greenland.


The U.S.A. (including Hawaii; see Volume 2 of Centres of Plant Diversity for Hawaiian Islands, CPD Site PO6), is one of the largest countries in the world. As a whole, it extends over more than 50 degrees of latitude, from north of the Arctic Circle in northern Alaska to southernmost Hawaii, and over 120 degrees of longitude, from the east coast of Maine to the westernmost part of the Aleutian Islands. The geographic centre of the 48 contiguous states is at about 37°N, 96°W (in Kansas). To the north, these 48 states are bordered by Canada. The east coast extends from Maine to Florida along the Atlantic Ocean, then west along the north coast of the Gulf of Mexico to the Mexican border at the Rio Grande. The west coast extends from Washington through California along the Pacific Ocean to the Mexican border. Continental U.S.A. consists of a vast central plain, with high mountains in the west and hills and low mountains in the east. The highest point is Mount McKinley in Alaska, with an elevation of 6194 m; in the conterminous states it is Mount Whitney in California, with an altitude of 4418 m. The lowest point is in Death Valley, California at 86 m below sea-level.

Alaska, the largest state, is separated from the rest of the U.S.A. by Canada, which borders it to the south and east; it lies about 800 km north-west of the state of Washington. The Arctic Ocean lies to the north, the Chukchi and Bering Seas to the west, and the Pacific Ocean to the south. Alaska is mountainous and dominated by the Brooks Range to the north and the Alaska and Coastal ranges to the south.


Canada comprises the northern half of the continent (except Alaska) and the arctic islands. It is a confederation of 10 provinces and 2 territories. Covering a land area of 9,976,186 km², Canada is the largest country in the Western Hemisphere. It is bounded on the north by the Arctic Ocean, on the south by the U.S.A., on the east by the Atlantic Ocean, and on the west by Alaska and the Pacific Ocean. The highest point is Mount Logan, at an elevation of 6050 m, in south-western Yukon Territory. The Hudson Bay, an inland sea about 1370 km long by 965 km wide, is a major feature of eastern Canada. Continental Canada consists of interior and arctic plains and lowlands with the immense Canadian Shield to the east and the Cordilleran mountain system to the west. Cold temperate boreal climates dominate over much of Canada, with polar and subpolar climates to the north and cool temperate ones in the south and west. An oceanic climate prevails off the west coast from the Queen Charlotte Islands to Vancouver Island (Brouillet and Whetstone 1993).


Greenland is the world's largest island, with an area of 2,175,600 km². Most of it lies within the Arctic Circle. It is located east of Canada, separated by Baffin Bay and the Davis Strait. The coast is intersected by deep fjords and is much influenced by sea-ice. The country is largely mountainous (highest elevation 3700 m), but there are some areas of gentle relief. Although the climate is primarily Arctic, it is sub-Arctic in some sheltered valleys in the far south.


The U.S.A. (excluding Hawaii) has a population of 259,542,408 (mid-1994 estimate). Since 1990, the population has increased by about 2.8 million people per year. The population density is 26 people per km². Regarding distribution, 79.5% of the population live in metropolitan areas, more than half of these live in just one of 41 metropolitan areas with populations of at least 1 million. Slightly over half of the population (56.4%) lives in the South and West. Between 1990 and 1993, the population in the West grew by 6.2% and in the South by 4.7%. The five most populous states are California (31.2 million), New York (18.2 million), Texas (18 million), Florida (13.7 million) and Pennsylvania (12 million). The five states with the lowest populations are Delaware (700,000), North Dakota (635,000), Alaska (599,000), Vermont (576,000) and Wyoming (470,000) (Famighetti 1995).

Canada has a population of about 28,114,000 (mid-1994 estimate). The population density is 2 people per km². About 77% of the population is urban. The most populous provinces are Ontario (10 million) and Quebec (nearly 7 million).

Greenland, long a possession of Denmark, became independent in 1979. At that time Greenlandic place names came into official use. The correct official name for Greenland is Kalaallit Nunaat. The island has a population of some 57,000 people (mid-1994 estimate), with a natural rate of increase of 1.3% per annum (IUCN 1992).


North America is the product of a very long geologic evolution which began early in the Archean (3750-3400 Ma). Plate-tectonic processes have largely dictated this evolution (Bally, Scotese and Ross 1989), including major plate movements during the Mesozoic and Tertiary. According to Bond, Nickerson and Kominz (1984), after the breakup of Pangea and Gondwana during the Middle Jurassic (175 Ma) the continents drifted away and were dispersed during the lower Palaeozoic (62.5-55.5 Ma). It is not known with certainty which continents originally bordered North America. However, the Palaeozoic folded belts of North America are continued in North Africa, Europe and Siberia. Much of North America and Greenland is underlain by Precambrian (3750-570 Ma) crust. The diversity and complexity of North American geology is illustrated in Thornbury (1965), Hunt (1967, 1974), Graf (1987), Bally and Palmer (1989) and Brouillet and Whetstone (1993).


The climate of North America is extremely diverse. Two geographical features have major effects on North American climates: the western Cordillera, trending north-south; and the Interior Plains to the east. The former constitutes a major obstacle to westerlies and trade winds, while the latter provides an interrupted path for the flow of arctic and tropical air masses. The major climatic regions of North America are: polar and subpolar; cold temperate boreal; cool temperate; warm temperate and subtropical; and tropical. See Brouillet and Whetstone (1993) for a detailed treatment.

Physiographic provinces

The division of North America into physiographic units has been detailed by various authors (Atwood 1940; Hunt 1967, 1974; Brouillet and Whetstone 1993). They include:

  • Atlantic Coastal Plain: a lowland area bordering the ocean into which it slopes and extending from Cape Cod south along the Atlantic Ocean and then west along the Gulf of Mexico into Yucatán. It is an area of generally low relief (altitudes less than 150 m), with some of the major hills and ridges rising only some 60 to 90 m. The surface of the Coastal Plain has been extensively reworked by coastal and fluvial processes over the last 2-3 million years. The Mississippi alluvial plain divides this province into the Atlantic and eastern Gulf coasts, the Mississippi flood plain, and the coastal band of Texas.
  • Appalachian Highlands: extending from the Maritime provinces of eastern Canada south-westward through the eastern U.S.A. It consists of an association of 3000 km long mountain ranges, plateaux and rolling uplands with altitudes ranging from 150 to 2300 m. The Highlands include several physiographic units: Maritime and New England, Adirondack Mountains, Piedmont Plateau, Blue Ridge, Valley and Ridge, and Appalachian Plateau.
  • Canadian Shield: geologically ancient area occupying much of Canada, extending from Baffin Island in the Arctic Ocean, south to northern Wisconsin and east to Labrador. Altitudes range up to 1500 m, but there is little local relief. To the north is found the Arctic Lowlands and Coastal Plain province.
  • Greenland Shield: an ice-cap (2500 km long, 1000 km wide and up to 3 km thick/avg. 300 m) covers 84% of the total land area, and an ice-free zone (generally very narrow, but sometimes broadening to 200-300 km) borders the coast (IUCN 1992).
  • St Lawrence Lowlands: between the Canadian Shield and the Appalachians. Characterized by flat limestone and sandstone outcrops. Elevations are sea level to 150 m.
  • Central Lowlands: gently rolling prairies forming a vast central plain extending from Ohio north-west to central Saskatchewan and south to the Gulf Coastal Plain. Elevations are 150 to 600 m.
  • The Great Plains Province: semi-arid western extension of the Central Lowlands, rising westward from 600 to 1500 m, and forming a broad belt extending from Rio Grande north almost to the Arctic Ocean, and from the Rocky Mountains in the west to the Central Lowlands in the east. Although appearing relatively flat, it slopes to the east.
  • Interior Highlands: between the southern end of the Central Lowlands and the Gulf Coastal Plain, south of glacial limits. It includes the Ozark Plateau, a rolling upland mostly above 300 m, and the Ouachita province, with altitudes of 150-800 m, and the Interior Low Plateau east of the Mississippi River, mostly below 300 m. It is an area composed mostly of sedimentary rock.
  • Rocky Mountains: Alaska, from the Brooks Range southward, to north-western Wyoming, where the mountain ranges are interrupted by a large plateau (not part of the province), and then continuing south into Mexico (the Sierra Madre ranges). The mountains of this province originated in the early Tertiary, primarily through anticlinal folding. The southern mountain ranges have altitudes of 1500-4265 m; the semi-arid Wyoming Basin has altitudes mostly of 1200-3000 m; the middle Rocky Mountains have altitudes mostly of 1525-2135 m and contain some semi-arid inter-montane basins (major ranges were glaciated, and snow and ice still cover most of the mountains above 2400 m); the Northern Rockies are highly irregular granitic mountains with altitudes mostly between 1220 and 2135 m.
  • Cordilleran or Intermontane Plateaux: a disjunctive province comprised of a series of plateaux extending from central Alaska southward into south-central Mexico. The Colorado Plateau section has the highest plateaux in the U.S.A., with its surface rising to 3355 m. It has numerous canyons and is semi-arid. The Basin and Range section, with its blocky mountains separated by desert basins, has altitudes from below sea-level to over 3660 m. However, relief between its mountains and basins is usually no more than 1525 m. The Columbia Plateau is primarily composed of lava flows. Altitudes are mostly below 1525 m. Although semi-arid, two major rivers, the Columbia and the Snake, traverse it.
  • The Pacific Border System: extends from western Alaska to Baja California, fronting the Pacific Coast. The ranges of the Cascade-Sierra Nevada province are north-south trending. The Cascades are a series of volcanoes, while the Sierra Nevada is composed of blocky granite. Altitudes reach 4270 m; western slopes are humid and eastern slopes semi-arid. The Pacific Border province consists of coastal ranges with altitudes mostly below 600 m. These ranges are separated from the Cascade-Sierra Nevada province by troughs 155 m or lower in altitude. The Lower California province is comprised of the northern end of a granitic ridge forming the Baja California Peninsula. In Alaska a belt of mountains forms the South Central Alaska province, leading into the Alaska Peninsula and Aleutian Islands province.

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The following simplified classification provides an overview of the major types of vegetation in North America. Major references on the vegetation of the U.S.A. are Gleason and Cronquist (1964), Küchler (1964), Bailey (1976, 1978), Barbour and Billings (1988) and Barbour and Christensen (1993). Regional accounts for the U.S.A. may be found in Oosting (1956), Braun (1964), Waggoner (1975) and Benson (1979). More detailed accounts of Canada's vegetation may be found in such publications as Macoun and Malte (1917), Halliday (1937) and Taylor and Ludwig (1966). For North America as a whole, see Harshberger (1911), Weaver and Clements (1938) and Barbour, Burk and Pitts (1987).


Tundra forms a broad band across the top of North America beyond the northern limit of trees, including the ice-free parts of the arctic islands and coastal margins of Greenland, and above the timberline on the high mountains south into Mexico. Average temperatures are low, and drying winds are ever present. At treeline tundra is in contact with either boreal or subalpine forest.


Boreal forest, or taiga, is the most widespread and typical vegetation type of Canada. It extends across the continent as a broad band, with its northern boundary running from the Mackenzie delta to the Hudson Bay, and reaches the Atlantic Coast in Newfoundland. From Cook Inlet (Alaska), the southern boundary trends south-eastward to Saskatchewan, eastward to Lake Winnipeg and then into northern New Brunswick. Small pockets occur on the highest mountains of eastern North America, south to the Great Smoky Mountains. Conifers (Abies, Picea and their associates) are dominants, but they are variously mixed with Populus (aspens) and Betula (birches) throughout, and replaced by them in some areas. Although the climate is less severe in this region than in the tundra region, plants here must still tolerate extremes of environment and short growing season.

Pacific coastal coniferous forest occurs from southern Alaska and British Columbia into northern California. It is characterized by high rainfall and low evaporation. Thuja-Tsuga (cedar-hemlock) coniferous forest is most extensive in Washington and British Columbia, while Larix-Pinus (larch-pine) forest is found in the subalpine zone across the mountains of British Columbia and northern Washington. (Similar vegetation occurs on west-facing slopes in Idaho and north-western Montana as far as the Continental Divide). Sequoia sempervirens (redwood) occurs in southern Oregon and northern California.

Subalpine forest (usually at 3000-3500 m) of Abies-Picea (fir-spruce) and associated species occupies the upper slopes of high ranges from southern Alaska and adjacent British Columbia through California into Mexico. There is a short growing season, relatively high precipitation (mostly snow), and wide diurnal and seasonal ranges of temperature. A long winter with high winds is characteristic.

Western montane forest, the most extensive of the western forests, stretches from the eastern slopes of the Rocky Mountains and the mountains of western Texas to the Pacific Coast, where it extends from the mountains of western British Columbia to those of Baja California and mainland Mexico. Dominants are Pinus, Abies and Pseudotsuga menziesii (pine, fir, Douglas fir). The latitudinal range is great, as is the corresponding range of mean annual rainfall (500-1500 mm) and temperature (7-16°C).

Lake forest, with a Pinus-Tsuga (pine-hemlock) association, occurs notably around the Great Lakes. Precipitation is 640-1140 mm annually, and temperatures range from -10° to 41°C over the course of the year. The growing season averages four months.

Deciduous forest is dominant in eastern North America and comprises an array of forest types and woody species, with broad transitions to boreal forest to the north, pine and broadleaved evergreen forests to the south, and grassland to the west. It reaches its northern limits in Minnesota, Ontario, and southern Quebec and alternates or is intermixed with boreal forest in New Brunswick and Nova Scotia. From southern Maine it extends southward to central Georgia, southern Louisiana and eastern Texas. The climate of the deciduous forest region is temperate, with definite summer and winter, a growing season varying over the region from less than 150 to more than 280 days, mean annual temperatures of 8-19°C, and mean annual precipitation of 625-2000 mm; frost to some degree is common to the region. The deciduous forest is the best developed in the Great Smoky Mountains.

The South-eastern coastal plain pine and broadleaved forests are upland pine forests that include the northern pine barrens and mesic pine communities, upland hardwood forests with trees such as Quercus virginiana, Magnolia grandiflora or Fagus grandifolia prominent in the overstorey, and tropical hardwood hammocks.


Grassland is perhaps the most extensive and varied vegetation formation of the North American continent. Dominating the centre of the continent from south-eastern Alberta, southern Saskatchewan, and south-western Manitoba to Texas and from Indiana to the Rocky Mountains, with significant prairie islands throughout the intermountain west and in California, grassland once covered 50 million ha in Canada (Barbour and Billings 1988) and almost 40% (300 million ha) of the area of the U.S.A. (Küchler 1964). This was upwards of 25% of the landmass north of Mexico. Even today, despite long human activity, grassland still covers more than 125 million ha in the U.S.A. Climate is a major determinant of grassland and grassland type, it being characterized usually by a wet-season/dry-season regime and temperature and precipitation extremes. The three major types of prairies are tall-grass, mixed-grass, and short-grass, the latter being the steppe grasslands of the high plains. All are dominated by grass and grass-like species, but also have a rich assemblage of forbs associated with them. Risser (1985) estimates that North American grasslands contain 7500 plant species. The Great Plains flora contains about 3000 taxa (Great Plains Flora Association 1986).

Grassland comprises several major associations:

  • The tall-grass prairie is the most mesic of the grasslands and is characterized by the greatest north-south species diversity and the largest number of dominants. It extends from south-eastern Manitoba and Minnesota to eastern Oklahoma and Texas and from Indiana to western Nebraska. Most of the tall-grass prairie is now in cultivation.
  • The mixed-grass prairie includes both short and tall grasses and associated forbs and forms an ecotone between the tall-grass prairie to the east and the short-grass prairie to the west. In the northern Great Plains, it is found in the western Dakotas, north-eastern Wyoming, eastern Montana and the southern parts of the central Canadian provinces. It is scattered in Nebraska, being interrupted by the sand hills, and occurs south-west from south-western Nebraska and western Kansas through Oklahoma and central Texas.
  • The short-grass prairie extends from Montana, south-eastern Wyoming, and the Nebraska panhandle southward through eastern Colorado and western Kansas to the high plains of Oklahoma, Texas and New Mexico. Arizona has similar grasslands. Generally less than 50% of the ground is covered by vegetation. Dominants include several species of Bouteloua.
  • Desert grassland extends from the short-grass area in Texas and New Mexico through the south-west into northern Mexico. In pristine times it was dominated by short bunchgrasses but today these have been replaced by desert shrubs.
  • The original dominants of the California grasslands or Pacific prairie, which once covered over 5 million ha, were cool-season, perennial bunchgrasses (Weaver and Clements 1938). The Pacific prairie has been replaced by farmland and urban sprawl, with non-cultivated areas being dominated by weedy annual species of grasses and forbs.
  • The Palouse prairie originally extended throughout south-western Canada, eastern Washington and Oregon, and from south-western Idaho into western Montana. Eighty percent of the flora consisted of cool-season grasses. Excessive grazing and cultivation (as well as fire) have resulted in shrub-steppe grassland.

Woodland and scrub

Pine-juniper woodland (Pinus-Juniperus), a formation composed of small trees, is essentially south-western, xeric and subtropical. It occurs discontinuously from western Texas through northern Mexico to southern California, extending northward into New Mexico and Colorado to south-western Wyoming and westward through Utah and Nevada to northern California.

Chaparral is characterized by a dense vegetation of broadleaved evergreen, sclerophyllous shrubs. Dominants include Adenostoma fasciculatum (chamise), Arctostaphylos (manzanita) and Ceanothus (California lilac). It occurs in the west largely in the southern Rocky Mountains, mountain ranges in Utah and Arizona, and in the Sierra Nevada, California Coast ranges and Cascade Mountains northward to southern Oregon. Climatically, it occupies xeric zones intermediate between grassland and forest.

Other shrub formations: sagebrush (Artemisia) and desert-scrub. The former is found from the Black Hills to southern British Columbia, south-eastern California and northern Arizona. Precipitation (annually 130-500 mm) is lowest in summer and heaviest (as snow) during the four-month winter. Desert scrub, distinguished by its more open structure, is found in the Gila-Sonoran, Colorado, Mohave and Death Valley deserts. It is the most xerophytic type, with annual rainfall of 85-150 mm.

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Due to the basically temperate nature of the flora of North America, the floristic richness of this large area is considerably less than other regions in the Americas, except for the Caribbean. According to Reveal and Pringle's (1993) in-depth history of North American taxonomy and floristics, "the modern history of systematic botany and floristics in North America began when the first Europeans landed and began to collect objects of curiosity". However, native Americans who had arrived millenia earlier had developed their own systems of classification and nomenclature. The origins of the North American flora are diverse. Takhtajan (1986) divides North America north of Mexico into two floristic kingdoms, the Holarctic and Neotropical (see Map 2). The Holarctic is represented by two subkingdoms and four regions with ten provinces. The Neotropical Kingdom is represented by the West Indian province of the Caribbean region, which includes the southern third of the Florida peninsula and the adjacent Florida Keys. For more information on phytogeography see Thorne (1993).

The flora of North America north of Mexico consists of approximately 20,000 vascular plant species in approximately 2350 genera (Morin, unpublished data) and 210 families (Thorne 1993). Two families, Leitneriaceae and Limnanthaceae are entirely endemic to the area; another two, Simmondsiaceae and Fouquieriaceae, also extend into Baja California and Sonora in Mexico. About 900 genera are endemic or nearly endemic to this area (some extend into Mexico) (Takhtajan 1986). It is estimated that approximately 4198 (est. 20%) plant species are endemic to North America (N. Morin 1995, pers. comm.). In the U.S.A., California is the state with the greatest diversity of native plants, with over 5500 species (see Table 12).

Canada has 3269 native vascular plant species (Scoggan 1978-1979). Except for a few relictual areas, most of the flora has reoccupied the land since the Pleistocene glaciations. British Columbia, Ontario and Quebec have the highest number of native vascular plant species (see Table 12).

Greenland has 497 species of vascular plants and only 15 endemic species (Böcher et al. 1978). Most of Greenland is covered with permanent ice. The vegetation of the ice-free coastal strip is composed of arctic/alpine and boreal elements. Patchy mats and herbaceous or shrubby heaths are formed, depending on the environmental conditions.

The flora of North America has been documented in many regional floras (e.g. Small 1933; Correll et al. 1970; Cronquist et al. 1972; Hickman 1993). Recently, two efforts have advanced the knowledge of the North America flora. The Flora of North America Project, a collaboration of botanists in the U.S. and Canada, with its organizational centre at the Missouri Botanical Garden, is producing books and an on-line database on the vascular plants and bryophytes growing outside of cultivation in North America north of Mexico. Two volumes were published in 1993. Kartesz (1994) published a synonymized checklist of the vascular plants of the U.S.A., Canada and Greenland.

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Useful plants

North America, as a temperate region, is relatively impoverished with regard to its gene pools and potential to provide major crops, in contrast to tropical areas, which exhibit great diversity. Native North American crops include plants such as cranberries, blueberries, strawberries, pecans and sunflowers (Myers 1983).

In industrial timber harvest, however, North America exceeds all other continents. Some 687 million m³ were harvested annually (five-sixths softwood) from approximately 700 million ha of forest north of Mexico. About three-quarters of the timber harvest comes from the U.S.A. and a quarter from Canada. Table 7 shows annual wood production in the U.S.A. and Canada from 1989-1991 (World Resources Institute 1994).

Table 8, adapted from Schery (1972), shows the principal trees used for timber in the U.S.A. They are listed in order of volume of timber produced, with Douglas fir being the most exploited. It should be noted that western species occur in significantly greater volume than eastern ones.

Prescott-Allen and Prescott-Allen (1986) list and discuss the many North American plants used for lumber and paper products, medicine, food and industrial products, new domesticates, wild genetic resources, pollution control and pest control. They present numerous tables, listing all of the plants used for these purposes in North America, as well as quantitative information concerning their economic values and a valuable bibliography of the literature on the subject (see also Roecklein and Leung 1987).

For North America, including Greenland and northern Mexico, Prescott-Allen and Prescott-Allen (1986) list the average annual value (in U.S.$ million, for the years 19761980) of terrestrial wild plant resources produced (or imported) by the U.S.A. (see Table 9).

With regard to domestic production of food in the U.S.A. from wild or wild and cultivated sources, Table 10 shows plants which have an average annual value of more than $1 million (Prescott-Allen and Prescott-Allen 1986).

Long before other cultures arrived on the North American continent, American Indians made much use of native plants for food, medicine, dyes, shelter, tools, charms and in the decorative arts. According to Kavasch (1984), almost 80% of all medicinal plants of North America are currently harvested east of the Mississippi River. Numerous publications have been written on economic plant use by native Americans, such as Scully (1970), Moerman (1986) and Duke (1992). Some of the most important indigenous economic plants are listed in Table 11.

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Factors causing loss of biodiversity

The primary cause of loss of plant diversity in North America is the habitat destruction resulting, ultimately, from the continuously increasing human population. Even publicly owned, "protected" areas (federal, state, local) are constantly being subjected to various uses that are detrimental to the conservation of biodiversity. Residential, industrial and agricultural development, with the consequent alteration and loss of habitat, is the primary threat associated with rising population. Additional threats include invasive plant and animal species, fires and ecologically unsound fire suppression, logging, mining, pollution (including pesticides), recreational land use, alteration of of drainages, filling in of wetlands, road building, maintenance of utility rights-of-way, plant collecting for the domestic and international trade (especially cacti, carnivorous plants, ginseng and orchids) and insufficient or faulty management practices.

Impact on North American
vegetation formations

Boreal forests

According to Elliott-Fisk (1988), human disruption of this forest type generally results in occupancy by successional communities similar to those associated with fire. Sometimes disturbances alter the substrate. The water budget is easily altered by mining, hydropower development and destruction of surface vegetation. In turn, this can lead to destruction of permafrost and rapid landscape degradation in northern regions. In certain cases, trees may fail to regenerate following logging, and a more or less permanent tundra subclimax may result.

Inuit and Eskimos use plants of the tundra to some extent. Some areas are now being affected by livestock grazing. Exploitation of mineral resources, especially oil, may negatively impact the tundra (Barbour and Christensen 1993).

Rocky mountain forests
As stated by Peet (1988), nearly all these forests are in some stage of recovery from significant prior disturbance by fire, wind, insect outbreaks, extreme weather, volcanism, ungulate browsing and events associated directly with people. Fire has historically been the most significant natural disturbance. In prehistoric times, the activities of the native Americans involved periodic burning, but the early Europeans caused a major increase in the frequency of fires. Fire suppression practices have reduced fire destruction significantly in this century, but the ecological consequences of this have not always been positive; long-term, unnecessary fire control may result in loss of biodiversity.

Pacific Northwest forests
Clear-cutting is the most common disturbance in these forests, with consequent erosion and nutrient loss as well as forest destruction. Also, rates and paths of succession are altered by planting and the elimination of non-arboreal species (Franklin 1988). Less than 10% of old-growth rain forests survive, scattered in small fragments throughout the Pacific Northwest.

California upland forest and woodlands
Air pollutants, primarily those from automobiles, play a significant role here. The ozone, hydrocarbons, nitrogen oxides and sulphur dioxide are major phytotoxicants, the last two also being components of acid rain (Barbour 1988).

Intermountain deserts, shrub steppes and woodlands
As stated by West (1988), livestock grazing over the last 100 years has been a significant cause of vegetation change in the Great Basin sagebrush, as well as in other vegetation types. One result is loss of dominance by perennial grasses. Undesirable plants, especially introduced annuals, have increased in abundance. Two examples of these are Halogeton glomeratus and Ceratocephala testiculata, both poisonous to livestock. The conversion from perennial to annual dominants seems to be a permanent trend, and succession is leading to tree dominance in many areas. Also, in recent times, erosion has become more severe because of reduced soil and litter cover.

Warm deserts
Activities associated with increasing urban development have been affecting the desert vegetation, often with long-term deleterious effects. Cattle grazing and recreation, especially involving off-road vehicles, also have serious consequences for the vegetation (altering its composition by, for example, reducing the diversity of annuals) and the soil. Among the desert plants being destroyed are such especially valuable plants as Simmondsia chinensis (jojoba; its oil can substitute for whale oil) and Parthenium argentatum (guayule; a rubber substitute can be produced from it) (McKell 1988).

The grasslands that once dominated central North America have been greatly reduced through extensive overgrazing, agricultural development and fire suppression. Fire has played a significant role in the development and maintenance of grasslands by, for example, suppressing the encroachment of trees and shrubs and by reducing competition from invaders such as Poa pratensis and Bromus inermis. It also improves the palatability and nutritional value of forage. Although they harbour relatively few endemics and lack high species diversity, the grasslands are very important for their value as watershed, forage and habitat for large numbers of domestic and wild animals and are the home of species of realized, or potential food crops (Sims 1988). Grasslands are the principal agricultural lands of North America; only 1% of the original prairie ecosystem is still intact (Barbour and Christensen 1993).

Deciduous forests
Greller (1988) states that, in general, the distribution of tree species in the east has not changed since pre-colonial times, but tree stature was significantly larger in the past. Much of the forest that once occupied 95% of the land has been cut over one or more times or has been completely destroyed by agriculture, urbanization and many other types of development and exploitation. Human use and activity continue to pose many threats to the present-day forests. The Carolinian forest in Ontario, Canada has been largely eliminated (McAllister 1994).

South-eastern coastal plain
Prehistorically, Indians are believed to have occupied this land around 12,000 BP. From 8-10,000 BP, land management by the Archaic Indian culture in the Coastal Plain was intensive, involving the use of fire. The first European colonists in temperate North America settled in the Coastal Plain. Their impacts varied with their location and cultural traditions, but all quickly learned to use fire to manipulate the landscape (Christensen 1988). Present-day threats are much the same as everywhere else in North America - the consequences of relentless development.

Wetlands are not a specific vegetation formation but they are of considerable conservation concern. Only half of the original wetlands in the U.S.A. remain, and 1300 km² of wetlands are being lost through agriculture and construction every year (N. Morin 1995, pers. comm.). Draining of wetlands is one of the most severe threats to biodiversity on the continent, endangering species and threatening the existence of some vegetation types.

Rare plant species

About 10% of the species in the flora of the U.S.A. (Elias 1977) and 31% of the flora of Canada (Argus and Pryer 1990) are considered to be endangered or rare. About 90 plant species became extinct in North America between 1800 and 1950 (Yatskievych and Spellenberg 1993). A survey conducted by the Center for Plant Conservation suggests that as many as an additional 475 continental U.S.A. taxa may become extinct by the year 1998. This is five times as many extinctions in one-third the time compared with the 1800-1950 period.

Figures on the number of native vascular plants and number of rare (G1-G3) species per state have been calculated by the Network of Natural Heritage Programs and Conservation Data Centers and The Nature Conservancy (Table 12). Data on numbers of native plant species were obtained from a phytogeographical summary in preparation by Kartesz (1995, pers. comm.). The states with the highest number of globally threatened and rare species are California (1762); Arizona (616); Utah (516); Nevada (495); Texas (439); New Mexico (399); and Oregon (367). In Canada, British Columbia (138); Quebec (78) and the Yukon (78) are the areas with the largest number of globally threatened species.

In 1990 Argus and Pryer published a study on the rare vascular plants in Canada. Since nearly 70% of the Canadian population is located in a narrow band along the border with the contiguous U.s.a., the natural vegetation has been drastically reduced. This border area cuts through the northern edge of four major floristic provinces (Argus and Pryer 1990). As a consequence, a large number of plant species that have a small range in Canada are regarded as nationally rare. The concept of rarity is based on a phytogeographical concept, not a threat concept. The purpose of the list was not only to promote the conservation of individual species, but to emphasize the need to preserve what is left of their habitats and ecosystems. Over 1000 taxa, or 31% of the native flora, are considered rare in Canada (G. Argus 1995, pers. comm.). Table 13 gives an estimate of the number of rare species in each Canadian province/territory. British Columbia and Ontario have the highest number of rare plants, with 426 and 355 respectively. In British Columbia, many of the rare species occur on the Queen Charlotte Islands (CPD Site NA9). In Ontario, the Carolinian Forest of the Eastern Deciduous Forest province is a floristically diverse vegetation that not only has a limited distribution in Canada, but is severely threatened by agriculture and urbanization.

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Federal, state, and provincial legislation exists to protect plant diversity and endangered species to some degree in both Canada and the U.S.A. Of these two countries, the U.S.A. has the best developed national legislation for plant protection. The federal Endangered Species Act, passed in 1973, provides legal protection for listed taxa on federal lands. Approximately one-third of the land (2,994,688 km²) in the U.S.A. is owned and administered, with at least nominal protection, by federal government agencies. The vast majority of this land lies in the western third of the nation (IUCN 1992).

Federal agencies concerned with conservation and land management in the U.S.A. include the United States Fish and Wildlife Service (responsible for enforcing the Endangered Species Act and for habitat conservation, especially of wetlands), the United States Forest Service, the Bureau of Land Management and the National Park Service. There is no comprehensive inventory of U.S.A. native plant species or community diversity on which to base management and land-use decisions. Therefore, in 1993, the National Biological Survey was established to undertake research aimed at biological and ecosystem monitoring.

At the state level, conservation activities are undertaken by a network of state Natural Heritage Programs and by various conservation, natural resource, fish and game, forestry or other departments, and by native plant societies. While somewhat more than half of the states have rare plant laws, virtually all have rare plant lists. Some major private organizations in the U.S.A. concerned with conservation are The Nature Conservancy, Natural Resources Defense Council and the Center for Plant Conservation.

In 1988, Natural Heritage inventories were initiated in Canada (starting in Quebec) by The Nature Conservancy of Canada, as had previously been done in the U.S.A. Today, conservation data centres exist in Quebec, Ontario, Manitoba, Saskatchewan and British Columbia. The Canadian government has jurisdiction in the territories and provides information and coordination for the country as a whole, but the provinces initiate and have the final responsibility for their own floras. Manitoba, Ontario, Quebec and New Brunswick have Endangered Species Acts. The provinces of Alberta, British Columbia, New Brunswick, Saskatchewan and Quebec have Ecological Reserves or Wilderness Areas, with conservation as one of their purposes, but protection is limited. The provinces have provincial parks or Crown Lands that provide some species and habitat protection. The federal government is establishing ecological reserves in the Northwest Territories. Under the federal Green Plan, Parks Canada has taken a more active role in the conservation of biodiversity in its holdings and has set up policies to this effect (Argus 1977a, b; see also Maini and Carlisle 1974; Argus and Pryer 1990).

In 1992, Canada's federal, provincial and territorial ministers of the environment, parks, wildlife and forestry signed "A Statement of Commitment to Complete Canada's Network of Protected Areas" (Canadian Museum of Nature 1994). This plan proposes to expand the National Park system with the ultimate goal of preserving about 2.8% of Canada's lands.

Greenland has no national legislation specifically designed to protect plants. The country, however, contains the largest National Park in the world, North East Greenland National Park, encompassing 700,000 km² (IUCN 1992), and by its size alone protects significant portions of the country's biological diversity.

A review of the protected areas system and conservation legislation throughout the region is given in IUCN (1992, 1994). Table 14 provides a summary of protected area coverage.

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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. Click on the above title for a list of and information concerning the North America centres.

References for the North America regional overview

This overview was written by Shirley L. Maina and Jane Villa-Lobos (Smithsonian Institution, Department of Botany NHB 166, Washington, DC 20560, U.S.A.)

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