Chapter 6

Phytogeography of North America North of Mexico

Robert F. Thorne

BECAUSE of the basically temperate nature of the flora of the United States of America and Canada, the floristic richness of this huge area is considerably less than that of Mexico and several of the other American nations to our south. According to my system of classification, North America north of Mexico has indigenous representatives of 211 flowering plant families. This number can be compared with the number of indigenous angiosperm families known to occur in Mexico (236), Central America including Mexico (245), and South America (273). Other than Mexico, the richest flowering-plant family representations in the New World are found in Peru (227), Brazil (218), Costa Rica (214), Panama (206), West Indies (196), and Bolivia (190).

Comparable figures for regions of the Old World are available for Asia excluding Malesia (289 families), China (260), Africa south of the Sahara (247), Malesia (248), Australia (224), and Madagascar (196). Except for China, most comparable temperate areas of the Old World are considerably less rich in indigenous angiosperm families than North America north of Mexico: the former Soviet Union (153); Europe including Mediterranean Africa (152); islands in the Atlantic Ocean, e.g., Macaronesia (Canary Islands, Madeira, and Azores), Bermuda, and St. Helena (124); and New Zealand (111). These and other indigenous angiosperm floras of the world are listed in table 6.1.

TABLE 6.1. Putatively Indigenous Angiosperm Families and Additional Subfamilies of the World

Dicots Monocots Totals Total Families & Additional Subfamilies
Asia (excl. Malesia) Families 227 62 289
      Add. Subfamilies 125 31 156 445
South America (incl. Trinidad) Families 217 56 273
      Add. Subfamilies 103 36 139 412
Central America (incl. Mexico) Families 198 47 245
      Add. Subfamilies 95 35 130 375
Africa (S of Sahara) Families 192 55 247
      Add. Subfamilies 95 29 124 371
Malesia (Malaya-Fiji) Families 194 54 248
      Add. Subfamilies 81 29 110 358
Australia (incl. Tasmania) Families 164 60 224
      Add. Subfamilies 78 33 111 335
North America (N of Mexico) Families 167 43 210
      Add. Subfamilies 76 26 102 312
West Indies (incl. Bahamas) Families 159 37 194
      Add. Subfamilies 65 28 93 289
Madagascar & Comoros Families 154 42 196
      Add. Subfamilies 67 25 92 288
Pacific Basin (excl. Hawaii) Families 142 33 175
      Add. Subfamilies 60 24 84 259
New Caledonia & Loyalties Families 121 34 155
      Add. Subfamilies 46 16 62 217
Europe (incl. Med. Africa) Families 116 36 152
      Add. Subfamilies 48 14 62 214
Indian Ocean Islands Families 115 32 147
      Add. Subfamilies 49 19 68 215
Atlantic Islands Families 96 28 124
      Add. Subfamilies 32 8 40 164
New Zealand Families 88 23 111
      Add. Subfamilies 19 8 27 138
Hawaiian Islands Families 70 14 84
      Add. Subfamilies 15 5 20 104
Subantartic Islands Families 35 10 45
      Add. Subfamilies 2 3 5 50
Antarctica Families 1 1 2
      Add. Subfamilies 0 0 0 2

Within North America (fig. 6.1), the family representation diminishes with increasing distance from the tropics, as might be expected in a basically tropically oriented class. The richest American flowering plant family floras are found in Florida (183) and Texas (175), again as might be anticipated from their southern latitudes. Somewhat less rich are the more temperate states of Georgia (171), Louisiana (164), California (140), and New York (140). Much more depauperate are the floras of such boreal areas as Manitoba (106), Newfoundland (86), Alaska (75), Labrador (67), and Greenland (52).

Floristic Regions of North America

An excellent discussion of the floristic regions and provinces of North America north of Mexico is included in Floristic Regions of the World, by Armen L. Takhtajan (1986) in collaboration with Arthur Cronquist. Their treatment divides North America north of Mexico into two floristic kingdoms, the Holarctic and Neotropical, the former with two subkingdoms and four regions with ten provinces, one of which, the Sonoran, has three (I have added one more) subprovinces represented north of the border (fig. 6.2). The Neotropical Kingdom is represented only by the West Indian Province of the Caribbean region, which includes the southern third of the Florida peninsula and the adjacent Florida Keys. These phytochorionomic units, or natural floristic units, are discussed briefly below as to their locations, general characteristics, floristic content, primary vegetation types, and degrees of endemism. A.L. Takhtajan's book presents more detailed information about each unit. The floristic regions presented below and on the map legend (fig. 6.2) are numbered in accord with Takhtajan. The missing numbers correspond to regions outside of North America north of Mexico.


(Boreal Subkingdom)


The Boreal Subkingdom of the Holarctic Kingdom has three regions in North America. The first is the vast Circumboreal region, represented in North America by two provinces, the Arctic and Canadian provinces.

  • 1a. Arctic Province

The Arctic Province in North America consists of all the treeless land of the far north, mostly north of the Arctic Circle, i.e., most of coastal Alaska (except the southeastern Panhandle) and northern coastal Canada, and all of the ice-free Canadian Archipelago and Greenland. The flora is depauperate, lacking endemic families and having very few genera largely restricted to circumarctic (and alpine) ranges, e.g., Arctagrostis, Arctous, Braya, Diapensia, Dupontia, Loiseleuria, and Oxyria (R. F. Thorne 1972). Many of the species are circumpolar, and strictly endemic species in each hemisphere are probably fewer than one hundred.

E.Hultén (1963) regarded 352 species as circumpolar or nearly so, plus another 107 species with similar ranges but represented by different races or slightly different species on both sides of the Atlantic Ocean or the Bering Sea. The only complete flora of the Arctic region (N. Polunin 1959) lists 892 vascular plant species in 230 genera of 66 families (or 70 by my interpretation). N.Polunin's (1940) flora of the Canadian eastern Arctic consists of 297 vascular plant species in 38 families. The flora of Greenland (T.W. Böcher et al. 1968), not including adventive and introduced species, contains 496 vascular plant species in 61 families.

The vegetation consists of tundra grading northward into polar desert. Trees are absent, and the flora consists primarily of low shrubs of Salix, Betula, and Ericaceae, and of herbaceous perennials, especially in the Brassicaceae, Caryophyllaceae, Cyperaceae, and Poaceae. Species of Dryas, Oxyria, Pedicularis, Potentilla, and Saxifraga are characteristic. Annuals are virtually absent; lichens and bryophytes are abundant. Because of the extensive Pleistocene glacial denudation of most of this province, the present vegetation can be only a few thousand years old. The floristic elements, however, may be relatively ancient, having survived the glacial advances in refugia both north and south of the ice sheets. With the melting of the glaciers, the arctic species rapidly spread into their present far northern ranges.

  • 1b. Canadian Province

South of the Arctic Province, the Canadian Province forms a wide band across Alaska and Canada north of the Rocky Mountains and North American Atlantic regions. It also includes parts of northern New England, Michigan, and Minnesota. Endemics are few, represented by perhaps two dozen species found primarily in the western and eastern ends of the province. Dominant are the extensive conifer forests in this heavily glaciated land of mostly low relief with many lakes, slow streams, and Sphagnum bogs. The flora is somewhat richer than the Arctic Province but, aside from the trees, the species are likewise mostly widespread. Many are circumboreal. In contrast to the distribution of species in the province, no families and perhaps only 50 genera are primarily circumboreal (and subalpine) in distribution. The dominant trees, Picea glauca, Abies balsamea, Larix laricina, Picea mariana, Pinus banksiana, Betula papyrifera, Populus tremuloides, and P. balsamifera, are distinct from but closely related to Eurasian species. Many of the shrubs, aquatics, and other herbaceous species are circumboreal or closely related to Eurasian species.

Because of the repeated Pleistocene glaciations, the vegetation and present flora of the Canadian Province, like that of the Arctic Province, are very recent, composed largely of immigrant elements from unglaciated territories to the south and west and from such ice-free refugia that might have offered asylum within the present provincial boundaries, as in Alaska-Yukon. Again there is no complete floristic treatment of the Canadian Province, but Frère Marie-Victorin's Flore Laurentienne (1935), largely in the Canadian Province, can serve as a partial example. This work includes 1568 species in the Laurentian area, but Marie-Victorin cited for the whole flora of Quebec, including the Arctic Province and some of the Appalachian Province, 1917 species in 554 genera.

To the north the coniferous forests become sparse and low (taiga) and grade into the arctic tundra, usually without a sharp Arctic treeline. To the south many species of the Canadian Province range into the Appalachian and Rocky Mountain provinces as components of spruce-fir forests. Thus the boundaries of the Canadian Province are nowhere sharp.


The North American Atlantic region stretches across North America from the Atlantic Ocean to the Rocky Mountains and from south central Canada to the Gulf of Mexico. It includes three provinces, the Appalachian, the Atlantic and Gulf Coastal Plain, and the North American Prairies. The flora is relatively rich and characterized by considerable endemism. There are two monotypic families, Hydrastidaceae and Leitneriaceae, and perhaps 100 endemic or near-endemic genera, some with extremely limited ranges. Despite this endemism, the linkage between the floras of eastern North America and eastern Asia is well known and much researched. I have counted (R. F. Thorne 1972) at least 74 genera that are restricted to eastern North America and Asia (most only in eastern and southeastern Asia). Another 68 genera are found in eastern North America and Eurasia and also in western North America and/or the Mexican highlands. An additional 118 genera are wide-ranging in primarily temperate areas; of these, 62 are circum-North Temperate and 56 are represented also in the temperate regions of one or more of the southern continents.

Most of the conspicuous woody genera that are common to eastern North America and eastern Asia usually have species in one region represented closely related to species in the other. The fossil record indicates that in Tertiary times a widespread subtropical or warm temperate flora extended across the Northern Hemisphere, with migration frequent between the two continents. Because the eastern Asiatic flora is much richer and more archaic than the eastern North American flora, possibly there was greater movement from Asia to North America, though certainly North America suffered much more heavily in later Tertiary time from climatic and glacial catastrophes than did Asia.

  • 3a. Appalachian Province

The Appalachian Province includes that part of upland eastern North America that was largely covered by deciduous forest in historic times. It stretches from southernmost Ontario and Quebec to central Georgia and Alabama, includes most of Arkansas and part of eastern Texas, and reaches west through the Ouachita Mountains, Ozark Plateau, eastern Iowa, and southeastern Minnesota. Excluded are the coastal plains, including the Mississippi Embayment, and the treeless prairies and plains. It is bounded on the north by the Canadian Province, on the east and south by the Atlantic and Gulf Coastal Plain Province, and on the west by the North American Prairie Province. As might be expected, much floristic intercourse exists between these adjacent provinces and the Appalachian Province.

No floristic treatment has been done just for this province, but much of its territory (plus some Canadian, Coastal Plain, and Prairie provinces as well) is included in the floras by M.L. Fernald (1950) and H.A. Gleason and A.Cronquist (1963). The former includes 4425 indigenous and 1098 adventive or naturalized species; the latter about 4600 species. Two states entirely within the Appalachian Province are Indiana, with 1838 indigenous and 302 introduced species (C.C. Deam 1940), and West Virginia, with 2155 total species (P.D. Strasbaugh and E.L. Core 1978). If the species of the rich flora of the southeastern portion of the Appalachian Province are added (an area excluded from the works of M.L. Fernald, and H.A. Gleason and A.Cronquist), the total vascular flora for the province might be about 5500 to 6000 species.

In addition to the Eurasian--eastern North American disjuncts of the region as discussed above, mention should be made of the strong floristic relationships between the southern Appalachians and western North America. C.E. Wood Jr. (1971) found that 65% of the approximately 557 genera of seed plants in the southern Appalachians extend to western North America, and 158 of these have one or more taxa with wide disjunctions between the Appalachians and the West.

During the repeated Pleistocene glaciations and coastal plain inundations, the flora of the Appalachian Province found refuge in the Appalachian Highlands on the east and south and in the Ozarks to the southwest. This resulted in some allopatric speciation in the two refuge areas. Specific endemism is very high in this province, with numerous further endemics shared with the Coastal Plain Province. The great deciduous forest that once covered the Appalachian Province has been largely destroyed, or at least greatly altered, by the immigrant Europeans who replaced the Amerinds. In the coves of the southern Appalachians, however, especially the Great Smoky Mountains, there still exists a rich, mixed mesophytic forest of large trees of a score or more species in such genera as Acer, Aesculus, Betula, Carya, Fagus, Liriodendron, Magnolia, Prunus, Quercus, and Tilia. Various plant communities exist within the deciduous forest, as the Fagus grandifolia--Acer saccharum community to the north, Acer saccharum--Tilia americana community to the northwest, and Quercus-Carya communities on the drier fringes to the east, south, and west.

In addition to the dominant deciduous trees of the province, especially to the north and in the Appalachian highlands, are a number of conifers: Pinus strobus, P. resinosa, Tsuga canadensis, T. caroliniana, and Taxus candensis. In the highest elevations of the Appalachians a spruce-fir forest is dominant, with Abies balsamea and Picea glauca both being replaced to the south by the endemic Abies fraseri and Picea rubens. Unfortunately this forest is now under severe stress from acid rain and other aerial pollution, especially on Mt. Mitchell, at 2000 m the highest mountain in eastern North America.

Among the special areas of considerable specific and subspecific endemism in the Appalachian Province are the shale barrens of western Virginia and neighboring states, the granite flatrocks of the Georgia Piedmont and adjacent states, the Paleozoic sedimentaries of the uplifted Cumberland Plateau, and the cedar glades of central Tennessee, Kentucky, and Alabama. Some of the endemics of these special habitats have western affinities, and they presumably reached their present habitats during the warmer, drier, postglacial hypsithermal period.

  • 3b. Atlantic and Gulf Coastal Plain Province

Lying to the east and south of the Appalachian Province, the Atlantic and Gulf Coastal Plain Province ranges from the southern tip of Nova Scotia to eastern Texas. From the much attenuated coastal strip of Cape Cod, the New England islands, and Long Island, the coastal plain widens in New Jersey to a broad plain through the Delmarva Peninsula, Virginia, the Carolinas, much of southern Georgia, and most of Florida. Along the Mississippi River is a deep embayment extending to the southern tip of Illinois, the Mississippi Embayment. Physiographically this province is mostly sharply demarked from the Piedmont of the Appalachian Province by the Fall Line, although the vegetation and flora overlap considerably with those of the neighboring province.

Endemism is high in the Coastal Plain Province, where the flora contains several hundred endemic species, several endemic genera, and one endemic family, the monotypic Leitneriaceae. Because of repeated Pleistocene inundations, it has a young flora recruited largely from the much more ancient Appalachian Province, and to a lesser extent from the West Indian Province to the south and the North American Prairies Province to the west. Although many of the floristic elements are apparently of very recent origin, many presumably archaic genera are represented, such as Ceratiola, Croomia, Dionaea, Franklinia, Gordonia, Illicium, Leitneria, Rhapidophyllum, Sarracenia, Schisandra, Taxodium, Taxus, and Torreya, in addition to the many more widespread Arcto-Tertiary (or Boreo-tropical) genera.

Although no floristic treatment for the Coastal Plain Province has been attempted, the richness of the flora is indicated by some local floras completed for various parts of the coastal plain, as southern New Jersey with 1373 vascular plant species (W.Stone 1912), Delmarva Peninsula (mostly Atlantic Coastal Plain) with 2111 species (R.R. Tatnall 1946), southwestern Georgia on the Gulf Coastal Plain with 1750 species (R.F. Thorne 1954b), and central Florida with 2197 species (R.P. Wunderlin 1982).

The vegetation is conspicuously dominated by various species of Pinus, especially P. rigida to the north and P. palustris and P. elliottii to the south. The extensive pine forests are a response to repeated fires and to excessively or poorly drained, sandy soils. When recurring fire is eliminated, the vegetation rapidly becomes dominated by the economically less valuable hardwoods, such as species of Quercus and Carya on drier soils and Fagus, Magnolia, evergreen Quercus, Persea, and other genera on moister, richer soils. Extensive swampy areas, as characterized by the Dismal Swamp of Virginia and the Okefenokee Swamp of Georgia, are dominated by the two subspecies of Taxodium distichum, and species of Nyssa, Fraxinus, Populus, Ulmus, and other broad-leaved hardwoods where the flooding is seasonal.

Coastal marshes are dominated by Juncus roemerianus and various coarse, rhizomatous grasses, especially of the genus Spartina. Floristically very rich are the moist, sandy, acid pinelands with their abundance of species of Cyperaceae, Eriocaulaceae, Poaceae, Xyridaceae, terrestrial orchids, and insectivorous members of Droseraceae, Lentibulariaceae, and Sarracenia. Other habitats of special floristic significance are the bay-heads, with their broad-leaved evergreens of genera such as Gordonia, Magnolia, Persea, Ilex, Cyrilla, and Cliftonia, and the sand hills, especially of Georgia and Florida, with their numerous endemics.

Several areas of extremely high endemism on the Florida Coastal Plain seem to indicate refugia that were flooded during the Pleistocene. Among these floristically significant areas are the Orange Island refugium of central peninsular Florida, the banks of the Apalachicola River, the Marianna Red Hills, and coastal flatwoods in the Florida panhandle. The Apalachicola and Marianna areas have had their floras much enhanced by Appalachian elements carried down the Chattahoochee River (R.F. Thorne 1949).

Many of the genera and species that are conspicuous on the southern coastal plain are also found in the highlands of Mexico and Guatemala. Among these genera are Carpinus, Carya, Clethra, Fraxinus, Illicium, Liquidambar, Magnolia, Nyssa, Ostrya, Pinus, Platanus, and Quercus.

  • 3c. North American Prairies Province

The North American Prairies Province is a huge grassland province of prairies and plains, the latter rather equivalent to steppes, lying between the Appalachian deciduous forest on the east and the Rocky Mountains on the west, the Canadian coniferous forests on the north, and the arid semideserts to the southwest. Only at the Front Ranges of the Rocky Mountains are the boundaries of this province well defined. At least formerly, prairies reached eastward through the Midwest to Ohio or farther east (the "prairie peninsula"), intergrading on the uplands with the deciduous woodlands of the lowlands. This intergradation still obtains throughout Iowa and the eastern Dakotas, southward to northern Oklahoma. The virgin tallgrass prairie remaining today is largely restricted to a few botanic preserves of limited area. There remain, however, some extensive tracts of tallgrass prairie in more or less pristine condition in the Flint Hills of east central Kansas. Much of the region has been grazed, but it has not been severely mismanaged.

Northward the grasslands in Minnesota and central Canada intergrade with Populus tremuloides copses on the border of the Canadian Province, and westward they grade into the coniferous forests of the Rocky Mountain Province. Southwestward some mesquite-grasslands occupy highlands in the Chihuahuan and Sonoran desert subprovinces.

Because of this extensive intergradation on most boundaries and the occupation of drier, open sites in adjacent provinces of members of the grassland flora, endemism is rather limited in the Prairies Province. There are no endemic families, few if any endemic genera, and perhaps fewer than 50 endemic species. Grasses dominate the province although showy perennial forbs, especially of the Asteraceae and Fabaceae, are conspicuous. To the east, the tallgrasses of the genera Andropogon, Sorghastrum, Panicum, and Spartina are dominant. In the more arid west, the short grasses Buchloë dactyloides, Bouteloua gracilis, and Bouteloua hirsuta are dominant. Between the two north-south strips of grassland are midgrasses such as Bouteloua curtipendula.

The mostly recent and adventive flora of this province is somewhat limited because of the general lack of forests and highlands. Including the Black Hills, South Dakota has a vascular plant flora of 1585 species (T.Van Bruggen 1976). The vascular plant flora of the Great Plains in the United States, one-fifth of the area of the conterminous United States of America, apparently consists of only 3067 taxa (Great Plains Flora Association 1977, 1986).

Much of the Prairies Province was glaciated during the Pleistocene glacial incursions; hence the recentness of the flora, especially in the northern portions. In the east, greater moisture and absence of fire have allowed deciduous forests to invade much of the former prairie peninsula of the Midwest. A few patches of grassland and the black prairie loam, among the richest soils in the world, indicate the former distribution of prairie. The grasslands, like the pine forests of the coastal plain, were to a large extent maintained by fire, formerly set by lightning and by the Amerinds. In western Iowa, fire, light grazing, or late harvests of wild hay are necessary to maintain prairie in good health. Otherwise, rhizomatous shrubs, weedy trees, or introduced perennial grasses take over.

Several areas within the Prairie Province are outliers of adjacent provinces, including especially the Black Hills of South Dakota, Pine Hills of Nebraska, and Edwards Plateau of Texas. The former two, though dominated by Cordilleran conifers, have many grassland elements, and the Black Hills have numerous boreal and eastern American elements as well. The Edwards Plateau has grassland and Chihuahuan and Tamaulipan desert elements, but also at least 20 endemic species of great interest, especially in the genera Buddleja, Dasylirion, Forestiera, Styrax, and Yucca.


The high Rocky and Cascade-Sierran mountain systems of western North America, along with the Coastal Ranges south from Kodiak Island of Alaska to the San Francisco Bay Area of California, are included by A.L. Takhtajan (1986) in two provinces of the Rocky Mountain region, the inland Rocky Mountain Province and the coastal Vancouverian Province. This region has no strictly endemic vascular plant family but it has many endemic or near-endemic genera and numerous endemic species. Other than Arnica, Castilleja, Erigeron, and Lomatium, no genus seems to have its major center of diversity in this region; the widespread sedge genus Carex is represented by about 200 species.

The vegetation is dominantly coniferous forest with the greatest diversity of conifers in the New World. Most coniferous species, other than Pinus ponderosa, P. contorta, and Pseudotsuga menziesii, are not dominant and widespread through both provinces. Deciduous trees form a minor part of the vegetation except for the numerous groves of quaking aspen, Populus tremuloides. Above timberline in the highest mountains is often a considerable area of alpine tundra with many arctic species, especially in the more northern alpine areas.

  • 4a. Vancouverian Province

The Vancouverian Province extends as a coastal strip from Kodiak Island, Alaska, southward through the Alaskan panhandle and British Columbia, widening in Washington and Oregon to include the Cascade Mountains plus the Olympic Mountains and other Coastal Ranges, and in California stretching, according to A.L. Takhtajan (1986), through the Klamath and Coastal Ranges to the San Francisco Bay Area and through the Cascades and to the south end of the Sierra Nevada. I prefer this treatment of these areas to the more inclusive Californian Province of P. H. Raven and D. I. Axelrod (1978). On the other hand, I think the Warner Mountains of northeastern California are better included in the Madrean Great Basin Province rather than the Vancouverian Province. The mixture of species from both provinces makes placement of this latter range rather arbitrary.

As with most other floristic regions, there is no floristic treatment of the Vancouverian Province, and the authors, such as C.L. Hitchcock et al. (1955--1969) and C.L. Hitchcock and A. Cronquist (1973), of the regional treatments that most nearly cover the province have not presented a statistical summary for their works. Three local floras within the Vancouverian Province do give some useful data. According to J.A. Calder and R.L. Taylor (1968), the Queen Charlotte Islands of British Columbia have a vascular flora of 593 species and subspecific taxa in 277 genera and 69 families. W.C. Muenscher (1941) found in Whatcom County, the northwesternmost county of Washington, 1008 species in 422 genera and 91 families.

G.N. Jones (1936), for the Olympic Peninsula of Washington, listed 1015 species, with 138 species or 13% adventive, about one-third the flora of the state. He reported 20 species or infraspecific taxa as restricted to the Olympic Peninsula, but 140 species of the Cascade Mountains as missing from the Olympic Mountains. Two regional floras that cover more than the Vancouverian Province are that for British Columbia by R.L. Taylor and B.MacBryde (1977), with 3137 vascular plant species and infraspecific taxa, 2475 or 79% indigenous, in 744 genera and 131 families, and that for Oregon by M.E. Peck (1961), with 3370 species and 604 infraspecific taxa in 749 genera and 117 families. Surely, the flora of the Vancouverian Province must include at least 3000 to 3500 species of indigenous vascular plants.

As to endemism in the Vancouverian Province, C.V. Piper (1906) reported 158 species and 27 subspecies as restricted to the state of Washington, and G.L. Stebbins and J.Major (1965) indicate 406 endemic species for their Vancouverian areas in California and immediately adjacent Oregon. Thus, perhaps 500 to 600 species are restricted, or largely so, to this province. Among the more noteworthy endemics are such plants as Sequoia sempervirens, Sequoiadendron giganteum, Vancouveria, the insectivorous Darlingtonia californica, the mycophytic ericad genera Allotropa, Hemitomes, Pityopus, Pleuricospora, and Sarcodes, as well as Oemlera, Whipplea, and Romanzoffia.

The Klamath region is rich in locally restricted species, some of them apparently archaic relicts, others specialists on serpentine and peridotite substrata. In a study area of about 13 square kilometers around English Peak in the Marble Mountain Wilderness area of Siskiyou County, northwestern California, F.W. Oettinger (1975) found 452 indigenous and 29 introduced vascular plant species in 243 genera of 64 families. Twenty-four of the indigenous species are largely restricted to the Klamath Mountains region. In the nearby Trinity Alps, W.J. Ferlatte (1974) treated 571 species in 266 genera and 73 families. C.E. Wood Jr. (1971) pointed to this region, especially the Siskiyou Mountains, as the "richest conservatory of relict groups of plants in western North America." Perhaps the most interesting of the relicts is Picea breweriana, belonging to a Eurasian section. The flora of this region is transitional to the adjacent Californian region. Other areas of high endemism in the Vancouverian Province are the Olympic and Wenatchee ranges of Washington, the Columbia River Gorge, the southern Cascade Mountains in California, and the Sierra Nevada.

The vegetation of the Vancouverian Province, like the Rocky Mountain Province, is dominated by conifers, but broad-leaved trees are more conspicuous in the Vancouverian Province than in the Rocky Mountain ranges. Among the broad-leaved trees are Acer macrophyllum, Alnus rubra, Cornus nuttallii, Fraxinus latifolia, Populus trichocarpa, Quercus spp., Arbutus menziesii, and Umbellularia californica. The last two trees are evergreen as are many of the common shrubs, such as Mahonia aquifolia (Berberis), Gaultheria shallon, and Rhododendron macrophyllum.

Coastal slopes of British Columbia and Olympic Peninsula of Washington support a very wet and tall temperate rainforest, dominated by huge trees of Picea sitchensis, Thuja plicata, and Tsuga heterophylla, accompanied by smaller broad-leaved Alnus rubra, Acer macrophyllum, Cornus nuttallii, and Arbutus menziesii. To the east and south, summer drought increases fire frequency and thus favors Pseudotsuga menziesii. An attenuated version of the temperate rainforest to the south is the redwood forest, flourishing on seaward slopes that are mostly bathed in fog year-round. The major tree of this forest, Sequoia sempervirens, is one of the tallest trees in the world. The species is constantly under attack by timbermen and is now, except in limited federal and state parks, often being replaced by the faster growing Pseudotsuga menziesii, favored economically by the timber interests.

As in the Rocky Mountains, the Vancouverian ranges, particularly the southern Cascades and Sierra Nevada, show striking vertical zonation in the forest cover. Above the Madrean chaparral and foothill woodland of the western Sierran slopes, the lowest montane zone is mostly an open Pinus ponderosa forest, grading upward into a mixed conifer forest of Pinus jeffreyi, P. lambertiana, Abies lowiana, and, locally, Sequoiadendron giganteum, the world's largest, and among the oldest, of living things.

Above the mixed conifer forest, Abies magnifica and Pinus contorta subsp. murrayana are dominant, often in rather pure stands. The subalpine forest above them, upward to timberline, is dominated by Tsuga mertensiana and several pines, Pinus monticola, P. albicaulis, P. balfouriana, and P. flexilis. Above timberline the alpine tundra harbors many arctic-alpine and circumboreal species as well as local endemics. The Vancouverian Province contains many wide-ranging species, as well as more local species, intruding from adjacent Boreal and Madrean provinces.

  • 4b. Rocky Mountain Province

The Rocky Mountain Province includes the Rocky Mountains and associated ranges from northern British Columbia to the Blue and Wallowa ranges of central Oregon, the Uinta Mountains of northeastern Utah, and the south end of the Rockies in northern New Mexico. Because the Canadian parts of this province have been heavily glaciated, the flora there is very recent, relatively depauperate, and low in endemism. There is much intergradation with the Canadian Province northward, and many of the species are circumpolar or circumboreal, especially in the alpine areas.

Southward in the United States, where glaciation was merely local, the flora is much richer and the degree of endemism considerably higher. According to A.L. Takhtajan (1986), five genera are endemic to this province, and 10 more are shared with the Vancouverian Province. Takhtajan lists about 40 endemic species for such special areas in the province as the sagebrush lands of central Washington, the John Day Valley in north central Oregon, the Palouse country of southeastern Washington and adjacent Idaho, the Snake River Canyon between the mountains of northeastern Oregon and those of west central Idaho, and the sagebrush lands and seleniferous outcrops of southern Wyoming. He also mentions about 50 species as being quite or nearly confined to the southern Rocky Mountains.

Aside from the floristic elements fingering into the Rocky Mountains from adjacent provinces, several geographic elements are of considerable interest. W.A. Weber (1953, 1965, 1987), among others, has noted the following: an eastern deciduous forest element limited to mesic and cool places along the eastern edge of the uplift, circumpolar and boreal elements, relict Madro-Tertiary species, a southwestern desert-steppe element, and a particularly interesting element analogous to one in the Altai Mountains of central Asia in similar semiarid, high elevation areas. Weeds, at least in the Colorado Rockies, are largely natives of southeastern Europe and Asia Minor.

Because many floras lack statistical summaries, it is difficult to obtain statistics on the size of the Rocky Mountain flora. J.M. Coulter and A.Nelson (1909) found 2733 species in the Central Rocky Mountains, and W.A. Weber and R.C. Wittmann (1992) located 3088 vascular plants (including infraspecific taxa) for the whole state of Colorado. P.A. Rydberg (1917) estimated about 4000 species for the Rocky Mountains. C.V. Piper and R.K. Beattie (1901) listed 663 species just for their flora of the Palouse region of eastern Washington and adjacent Idaho. Probably the province contains about 4000 to 4500 species.

The vegetation of the Rocky Mountain Province is conspicuously zoned vertically. Open stands of Pinus ponderosa form the lowest forest zone; they give way above to Pseudotsuga menziesii, which in turn in the uppermost forests is replaced by Abies bifolia and Picea engelmannii. Especially at middle elevations, Populus tremuloides and the fire-dependent pine, Pinus contorta, are often abundant, as is also Picea pungens southward. Scattered small trees of Juniperus scopulorum often are found in the drier foothills, and in southern Colorado and New Mexico scrubby stands of Quercus gambelii often occur below the forest.

(Madrean or Sonoran Subkingdom)


The flora of the American Southwest and adjacent areas of northern and central Mexico is so distinct from the two other Holarctic subkingdoms, the Boreal and the Tethyan (or Mediterranean), that it deserves its high chorionomic rank. The flora is largely locally derived, and the region takes its name from the Sierra Madre Occidental of Mexico. This single Madrean region is represented in North America north of Mexico by three provinces, the Great Basin, Californian, and Sonoran, and the last by four subprovinces, Mojavean, Sonoran, Chihuahuan, and Tamaulipan.

The Madrean flora is exceedingly rich and distinctive. At least three families, Fouquieriaceae, Simmondsiaceae, and Setchellanthaceae, are endemic, and five other wider-ranging families, Crossosomataceae, Garryaceae, Lennoaceae, Limnanthaceae, and Stegnospermataceae, have their principal development within the region. Several larger families have their major centers of diversity in the region, e.g., Onagraceae, Polemoniaceae, and Hydrophyllaceae. Other higher taxa especially well developed there are Agavaceae, Alliaceae, Apiaceae, Arbuteae, Asteraceae, Boraginaceae, Brassicaceae, Cactaceae, Orcuttieae, Papaveraceae, Polygonaceae, Rhamnaceae, and Scrophulariaceae.

According to A.L. Takhtajan (1986), more than 250 genera and probably more than half of the species are endemic. Most of the genera are common to two or more of the Madrean provinces; likewise many of the species. Most of the Madrean taxa are adapted to arid-desert or Mediterranean-type climates and have a long history of such adaptation since the more humid, widespread climates of the late Cretaceous and early Tertiary.

  • 9a. Great Basin Province

The Great Basin Province includes most of the Great Basin between the Rocky Mountains and Cascade--Sierra Nevada axis, including the Warner Mountains of California, the Snake River Plains of southern Idaho, and most of the Colorado Plateau, including the Uinta Basin but not the Uinta Mountains. Provincial boundaries are mostly well defined by the bases of the surrounding ranges. In Arizona the Mogollon Rim is a conspicuous boundary. The physiographic Great Basin consists of many north-south trending mountain ranges separated by often broad, alkaline basins without external drainage. Much of the area lies above 1300 meters and is subject to the continental climate of hot summers and cold winters. Most of the limited precipitation comes as snow during the winters.

The Great Basin flora has been developed largely since Miocene time and has been shaped by drought and by the cold winters, thus restricting migration from the warmer areas to the south. The higher ranges show a strong Rocky Mountain influence, and the lower elevations harbor wide-ranging species of arid lands from many of the adjacent provinces. Thus, endemism is relatively low with but few endemic genera and perhaps 25% specific endemism (A.L. Takhtajan 1986). The three largest genera in the province are Astragalus with 175 species, and Eriogonum and Penstemon with more than 100 species each. Because each genus has 50 to 60 species endemic to the province, the Great Basin can be considered their center of diversity. Other well-developed genera are Cymopterus, Lomatium, subgenus Oreocarya of Cryptantha, Chrysothamnus, Erigeron, Phacelia, Castilleja, and Gilia. All these genera taken together have about 275 endemic species. Many of these and local species of other genera in the province appear to be of rather recent origin. Edaphically distinct habitats such as tuffaceous slopes, hanging gardens on sandstones in the Colorado River drainage, sand dunes, dolomites, or soils rich in gypsum or heavy metals often have their own local species.

The flora of the Great Basin Province is included within the multivolume Intermountain Flora by A.Cronquist et al. (1972+). The boundaries of that flora are essentially the same as our defined province, except that it includes all of Utah. Unfortunately, the work is incomplete, and the authors have not yet produced a floristic analysis. Utah and Nevada comprise most of the Great Basin; hence, two recent floristic studies can give some approximation of the Great Basin flora. In A Flora of Nevada, J.T. Kartesz (1988) mentioned more than 3000 species of vascular plants, and B.J. Albee et al. (1988), in Atlas of the Vascular Plants of Utah, included 2438 mapped and 384 listed taxa for a total for Utah of 2822 vascular plants. Considering that both treatments range beyond the Great Basin and have numerous species in common, one might estimate that the Great Basin flora is somewhat less than 3000 species of vascular plants.

The vegetation is characterized by the wide-ranging Great Basin sagebrush, Artemisia tridentata and its close relatives, on better soils, and by members of the Chenopodiaceae, especially species of Atriplex, on more alkaline soils. Before the severe overgrazing of much of these lands, many grasses accompanied the sagebrush. Above the sagebrush zone on the mesas is often found a woodland of scattered junipers or junipers and pinyons, Juniperus and Pinus species. Often an open Pinus ponderosa forest, or in the south a chaparral dominated by Quercus gambelii, occurs above this zone, where somewhat more moisture is available. At higher montane to alpine elevations, the flora is largely a Rocky Mountain flora with dominance by conifers of the genera Pseudotsuga, Picea, and Abies, and at timberline by species of Pinus. One of these is the bristlecone pine, P. longaeva, members of which are the world's oldest living vascular plants, attaining ages of nearly 5000 years. Broad-leaved tree species are rather few and include a few species of Populus and Acer. The alpine flora consists largely of wide-ranging genera but with few circumboreal arctic-alpine species.

  • 9b. Californian Province

The floristically rich Californian Province occupies most of cismontane California and a portion of adjacent northwestern Baja California. As defined by A.L. Takhtajan (1986), it includes the great Central Valley, inner North Coast ranges, South Coast ranges, Transverse and Peninsular ranges of southern California, and foothills of the Cascade and Sierra Nevada ranges up to the ponderosa pine forests at about 1200 meters in the southern Sierra Nevada. This is a narrower definition than that of P.H. Raven and D.I. Axelrod (1978), but it has the advantage of conforming more exactly to the area subject to the Mediterranean-type climate of moist, cool winters and hot, dry summers.

The Californian Province, as defined by Raven and Axelrod, was calculated by them to contain 4452 species of vascular plants, with 2125 of those (47.7%) considered endemic to the province. As redefined here, the province may contain fewer than 4000 species with about half of them (50%) endemic. A few of the more than 50 California Province endemic or near-endemic genera are Adenostoma, Bergerocactus, Carpenteria, Cneoridium, Dendromecon, Fremontodendron, Jepsonia, Lyonothamnus, Neostapfia, Odontostomum, Ornithostaphylos, Pickeringia, and Romneya. Among the larger genera and higher taxa that have their main, or at least a principal, center of diversity within the Californian Province are the following: Arctostaphylos, Brodiaeinae of Alliaceae, Calochortus, Caulanthus, Streptanthus, Ceanothus, Cryptantha (subg. Krynitzkia), Downingia, Dudleya, Eritrichieae of Boraginaceae, Eriogonoideae of Polygonaceae, Gileae of Polemoniaceae, Hydrophyllaceae, Limnanthaceae, Lotus (subg. Hosackia), Madiinae of Asteraceae, Mimulus, Onagreae and Epilobieae of Onagraceae, Orcuttieae of Poaceae, and Eschscholzioideae and Platystemonoideae of Papaveraceae. Also, Astragalus and Cupressus have radiated strongly in the province.

In addition to numerous floristic elements intruding into the province from adjacent provinces, there is a strong link between the Mediterranean climatic areas of central Chile and California. Nearly 100 closely related species pairs or identical species are known. Mostly they are self-compatible annuals of open habitats with small disseminules. Their Californian distribution seems to be primary, and their dispersal apparently is recent, repeated, and by migrating birds, most probably by shore birds.

There is also a less obvious linkage with the Mediterranean region of southern Europe and north Africa, ignoring the obvious large contingent of annual grasses and weeds brought from the Mediterranean by early Europeans. Styrax officinalis and other taxa with Mediterranean affinities are discussed in more detail later in this chapter.

The vegetation of the Californian Province is varied and shows some elevational zonation. The Central Valley, which in the recent geologic past was a large inland sea, was in early historical time a vast prairie apparently dominated by bunchgrasses, species of Stipa, Poa, Melica, Elymus, Aristida, Festuca, and Koeleria, accompanied after moister winters by a lush display of showy forbs. Lower areas and shallow basins with an impervious substrate, filled with water in the winter and desiccated through evaporation in the spring and early summer, gave rise to a special vernal-pool or vernal-marsh flora of nearly 200 species (R.F. Holland 1976), many of them endemic to the province. From a total of 128 genera with species found in vernal pools, 16 have their Californian species essentially limited to this habitat (R.F. Thorne 1984). Other areas of the valley with high alkalinity have equally special saltbush scrub communities. Now most of these plant communities in the Central Valley have been obliterated by agricultural, commercial, and residential development.

Except for the riparian gallery-woodlands of Platanus racemosa and species of Salix, Populus, and Quercus, valley grassland was largely treeless. Around the margins above the valley floor, a grassy savanna with scattered trees of several species of Quercus merges upward into open foothill woodland of Quercus douglasii, Pinus sabiniana, and Aesculus californica. On drier foothill and lower montane slopes, sclerophyllous chaparral replaces the oak and oak-pine woodlands.

A mixed evergreen forest occurs in moister canyons and on shaded slopes of the Coast, Transverse, and Peninsular ranges, and includes the conifers Pseudotsuga macrocarpa and Calocedrus decurrens, broad-leaved evergreen species of Quercus and Umbellularia californica, and such deciduous species as Acer macrophyllum and Fraxinus dipetala. The higher montane and subalpine slopes of these ranges support forests similar to those found in the Sierra Nevada, including, with increasing elevation, forested zones dominated by Pinus ponderosa and P. jeffreyi, Abies concolor and Pinus lambertiana, Pinus contorta subsp. murrayana, and at timberline Pinus flexilis, often mixed with P. contorta subsp. murrayana. The few southern Californian peaks with timberline and krummholz have a very limited alpine fell-field cushion community, with a mixture of widespread arctic-alpine and local species.

Occupying drier slopes below the chaparral in interior valleys, and especially along the coast, a low sage scrub of soft (malacophyllous) shrubs of Salvia, Artemisia, Eriogonum, and other genera was formerly very extensive. Other special plant communities of the Californian Province include closed-cone coniferous woodlands dominated by various species of fire-oriented Cupressus and Pinus; open woodlands dominated by Pinus coulteri, Juniperus californica, or Pinus torreyana; serpentine woodland; maritime-desert scrub; and various other maritime communities of coastal bluffs, dunes, salinas, marshes, and submarine habitats.

  • 9c. Sonoran Province

The arid North American Southwest, especially the Mojave, Sonoran, and Chihuahuan deserts, is mostly included within the Sonoran Province of the Madrean region. It covers most of northwestern Mexico and much of the southwestern United States from southern California and southern Nevada to southeastern Texas. North of the border are four readily distinguished subprovinces, the Mojavean, Sonoran, Chihuahuan, and Tamaulipan.

9c.1. Mojavean Subprovince --- Takhtajan prefers to treat the Mojave Desert as a district of the Sonoran Desert, but I consider it as a well-marked subprovince that is rather distinct in elevation, climate, and flora from the more tropical, lower Sonoran Desert. As defined here, this subprovince includes Death Valley and adjacent ranges bordering the Great Basin Province (the Inyo region of P.H. Raven and D.I. Axelrod 1978) south through the "high" desert of San Bernardino County, California, to the base of the Transverse ranges, including the Little San Bernardino and Eagle desert ranges separating this desert from the Colorado portion of the Sonoran Desert. To the east, the Mojave Desert extends through southern Nevada to the southwestern tip of Utah and south into northwestern Arizona. The borders with the Great Basin and Sonoran Desert are not well defined. The Mojave Desert is probably best characterized by Yucca brevifolia, the Joshua-tree, whose distribution is essentially that of the Mojave Desert.

According to Barry Prigge (pers. comm.), the Mojave Desert, defined approximately as above, possesses a total flora of 2085 taxa (including infraspecific entities), 186 of them naturalized. The 1878 species are included in 652 genera and 114 families. With some species counted in more than one group, the flora is apportioned among 1041 perennial herbs, 566 woody plants (trees, shrubs, and half-shrubs), 837 annuals, 81 biennials, and 150 others (parasites, vines, succulents, aquatics, etc.). Our floristic treatment of the higher ranges and Kelso Dunes of the eastern Mojave Desert in California (R.F. Thorne et al. 1981) listed 783 species with 717 of them indigenous.

P.H. Raven and D.I. Axelrod (1978) cited 108 species as being endemic to the Mojave Desert, including their Inyo region, though G.L. Stebbins and J.Major (1965) counted 138 endemics in 70 large- and intermediate-size genera and 78 relict genera for the California Inyo and Mojave regions.

The Mojave Desert is so diverse in topography, geology, and climate that I have listed (R.F. Thorne 1982) 21 plant communities and 9 subcommunities for the Mojavean Subprovince in habitats ranging upward in elevation from alkali sinks to montane white fir--pinyon woodland.

9c.2. Sonoran Subprovince --- North of the Mexican border, the Sonoran Desert ranges from the southern border of the Mojave Desert in California, southern Nevada, and southwestern Utah southward to include much of southern transmontane California, where it is called the Colorado Desert, and much of southwestern Arizona below 1050 meters to the southeastern corner of that state. Much more of the Sonoran Desert is found in the Mexican states of Baja California and Sonora. The total area of the desert north of the border in California and Arizona is about 105,420 km² or 56,57l mi² (F.Shreve 1951). It is lower and hotter than the Mojave Desert, with more summer rainfall and less frost in winter.

The flora, in consequence of this warmer climate, is more subtropical, largely Madro-Tertiary in origin, and contains many genera not found in the Mojavean Subprovince. In California alone, about 25 genera do not reach north to the Mojave Desert, among them Ayenia, Bursera, Calliandra, Carnegiea, Castela, Colubrina, Fouquieria, Horsfordia, Koeberlinia, Olneya, Simmondsia, and Ziziphus. There seems to be no available count of the species found in the Sonoran Desert north of the border, although P.H. Raven and D.I. Axelrod (1978) stated that about 100 species of the Colorado Desert are found nowhere else in California. F.Shreve and I.L. Wiggins (1964), in their valuable two-volume Vegetation and Flora of the Sonoran Desert, offered no statistical analysis of the flora. They did, however, treat 138 vascular plant families in the entire subprovince. A count in the two volumes gave 2683 species for the entire Sonoran Desert flora.

The degree of endemism in the Sonoran Desert north of the border has not been calculated, although G.L. Stebbins and J.Major (1965) have 27 endemics in the Colorado Desert of California in the 70 large- and intermediate-size genera, plus 39 relict genera. A.L. Takhtajan (1986) estimated endemism in the whole Sonoran Province as probably more than 25%.

Some of the plant communities of the Mojave Desert do not occur in the Sonoran Desert, among them the white fir--pinyon woodland, the Joshua-tree woodland, the blackbush scrub, the pigmy sagebrush scrub, and some of the alkali scrubs. On the other hand, such low desert woodlands (e.g., microphyll, arborescent cacti, desert oasis, and desert riparian) and stem-succulent (cactus) scrub are abundantly developed (R.F. Thorne 1982; D.E. Brown 1982c). In fact, the Sonoran Desert is best characterized by the microphyll woodlands, dominated by microphyllous trees of the Fabaceae and Rhamnaceae, by the arborescent cacti Carnegiea, Pachycereus, and Stenocereus, and by the great abundance of other succulent-stemmed cacti.

9c.3. Chihuahuan Subprovince --- The Chihuahuan Desert lies southeast of the Sonoran Desert, ranging north of the border from the southern edge of eastern Arizona to trans-Pecos Texas and southward well into Mexico, especially in Chihuahua and Coahuila. Because of its higher average elevation (mainly above 1070 m), the Chihuahuan Desert is cooler than the Sonoran Desert and subject in all parts to frost during the winter. Like the Sonoran Desert, a great variety of life forms are present, although, as in the two other deserts just discussed, great tracts are monotonous open stands of Larrea divaricata subsp. tridentata. Trees are found only along the drainageways, and the commonest tall plants are species of Yucca and cylindropuntias (Opuntia). This desert is especially well endowed in gypsum-rich habitats, and A.L. Takhtajan (1986) listed nearly 50 species largely restricted to gypseous habitats there.

J.Henrickson (pers. comm.) has listed for the entire Chihuahuan Desert 3233 species of vascular plants with 545 of them endemic, or 15.2% endemism. The relatively low level of endemism is probably due to the broad contacts with other drylands that allowed migration of plant taxa. In addition, the southwestern deserts expanded relatively recently in Holocene times (R.F. Thorne 1986). The isolation of the Baja California peninsula and the maritime nature of much of the Sonoran flora there presumably accounts for the greater endemism of the Sonoran Desert, estimated at 20--25%.

9c.4. Tamaulipan Subprovince --- The Tamaulipan Subprovince, because of the greater available moisture, is less desertlike than the other Sonoran subprovinces, and it is more of a thornscrub or mesquite-grassland. In southern Texas, the Tamaulipan Subprovince occupies some 8 million hectares (20 million acres) (D.S. Correll and M.C. Johnston 1970) and is known as the Rio Grande Plains or South Texas Plains. The rolling to level topography, ranging from sea level to 1000 feet, is dissected by streams and arroyos flowing into the Rio Grande or the Gulf of Mexico. The vegetation is largely open prairies, mostly with Prosopis, and brushlands with shrubby, often spiny, species of Celtis, Ziziphus, Karwinskia, Porlieria, Aloysia, Condalia, Castela, Acacia, Leucophyllum, and other genera with Mexican affinities. Much of the area is now rangeland.

This area of Texas and the coastal strip about Brownsville are of special interest to our study, for they accommodate representatives of genera and even families not found indigenous elsewhere in North America north of Mexico. Among these are Phaulothamnus spinescens of Achatocarpaceae, Xylosma flexuosa of Flacourtiaceae, Helietta parvifolia and Esenbeckia berlandieri of Rutaceae, Urvillea ulmacea and Serjania brachycarpa of Sapindaceae, to mention only a few. Along the south Texas coast, the mangroves, marine phanerogams, Batis maritima, Sabal mexicana, and other tropical species add greatly to the diversity of the flora.



  • 23b. West Indian Province

The only portion of North America north of Mexico that is included in the Neotropical Kingdom is the southern third of the Florida peninsula and the Florida Keys. This subtropical area is included in the West Indian Province of the Caribbean region. To botanists trained in temperate North America, this limited area of Florida is a different botanical world, dominated by West Indian genera, tropical families, palms, mangroves, epiphytes, showy tropical cultivated plants, and tropical weeds.

In addition to southern Florida and its Keys, the West Indian Province includes the Bermuda, Bahama, and Greater Antilles and Lesser Antilles archipelagoes. The province as a whole probably contains at least 200 endemic genera and perhaps 50% species endemism (A.L. Takhtajan 1986). There is one endemic family, Goetaeaceae, and the Floridian portion of the province is rather low in endemism because of the Pleistocene and post-Pleistocene immigration of the West Indian biota into Florida. R.W. Long and O.Lakela (1971) listed 1647 species for the three southernmost counties of Florida (Collier, Dade, and Monroe) with about 9% of the species endemic to Florida. They consider 61% of the flora to be tropical in relationship, with 91% of these elements being species occurring in the Caribbean area. Endemism in the tropical element is especially low and surely is very recent. The sandhill endemics, on the other hand, are non-Caribbean in origin, and mostly much older, many probably of Tertiary origin.

Nineteen indigenous families are wholly or largely restricted to southern Florida in the flora of North America north of Mexico: among them are the Avicenniaceae, Canellaceae, Combretaceae, Cymodoceaceae, Goodeniaceae, Meliaceae, Myrsinaceae, Myrtaceae, Olacaceae, Piperaceae, Rhizophoraceae, Surianaceae, Theophrastaceae, and Zamiaceae. Also 13 of the 17 naturalized families in Florida are similarly restricted (R.F. Thorne 1985). They are found primarily in the more tropical plant communities, such as the marine meadows, mangrove swamps, tropical beaches and dunes, and shore and tropical hammocks. This West Indian flora also includes 187 genera, with at least 128 more introduced and presumably naturalized. The West Indian element occurs much farther north than the three counties treated by R.W. Long and O.Lakela (1971) as tropical Florida. Many are found northward to Lake Okeechobee, and the tropical maritime species range well up the coasts to Cedar Keys and even to St. Marys River on the Atlantic Coast.

Origins and Relationships of the Present Flora

The present flora of North America north of Mexico, as mentioned earlier, contains indigenous representatives of 211 angiosperm families and 99 additional subfamilies from the world flora of 443 angiosperm families and 272 additional subfamilies as recognized in my current classification. Twenty-eight pteridophyte and gymnosperm families are also represented. This can be compared with the much richer, more tropical flora of Central America and Mexico, which has indigenous representatives of 245 angiosperm families and 130 additional subfamilies. The richest flora of higher taxa of angiosperms is possessed by Asia (excluding Malesia) with 289 indigenous families and 156 additional subfamilies represented. Table 6.1 allows comparison of the North American flora with that of other continental and oceanic chorological regions.

Only three families are restricted to North America north of Mexico: Hydrastidaceae, Limnanthaceae, and Leitneriaceae, each monogeneric. Five more families and four subfamilies, however, are shared with Mexico or extend farther into Central America and the West Indies: Crossosomataceae, Fouquieriaceae, Garryaceae, Simmondsiaceae, Stegnospermataceae, the papaveraceous Eschscholzioideae and Platyspermoideae, dracaenaceous Nolinoideae, and agavaceous Yuccoideae. The parasitic Lennoaceae reach slightly farther south into Colombia in northwestern South America. The relatively poor representation of endemic families and subfamilies in North America is a result of the long involvement of the continent with Eurasia and the more recent Pliocene linkup with South America through the Central American isthmus. North America thus lacks the isolation that results in the high family and generic endemism found in Australia, South America, Africa south of the Sahara, Madagascar, and New Caledonia.

North American Loss of Floristic Richness

The North American flora was much richer in the past. For example, the present flora of California contains indigenous species of 140 angiosperm families, but fossils record at least 40 more families no longer found in the state (R.F. Thorne 1986). Several families, such as Bombacaceae, Chloranthaceae, Dilleniaceae, Gunneraceae, Icacinaceae, Proteaceae, Sabiaceae, and Winteraceae, occur in the New World but are not now found north of Mexico. Others, such as Alangiaceae, Cercidiphyllaceae, Eupomatiaceae, and Trapaceae, no longer occur as natives in the Western Hemisphere. Many genera represented in Tertiary or Cretaceous strata of North America have also vanished as natives from the New World, e.g., Caldesia, Cunninghamia, Cyclocarya, Engelhardia, Eucommia, Euodia, Exbucklandia, Glyptostrobus, Mastixia, Metasequoia, Nypa, Paliurus, Paulownia, Phellodendron, Platycarya, Pterocarya, Rhoiptelea, Sciadopitys, and Zelkova (B.H. Tiffney l985b, among others). This depauperization of our flora is well documented by J.A. Wolfe and T.Tanai (1987) for Acer and by S.R. Manchester (1987) for the Juglandaceae. An extensive review is provided by A. Graham (chap. 3).

Recent Additions to the North American Flora

The present subtropical West Indian flora of southern Florida must be a very recent addition, for much of the peninsula was under seawater until Pleistocene time. Some of the 19 indigenous families that are wholly or largely restricted to southern Florida are listed above in the discussion of the West Indian Province, along with the habitats in which most of them are found. Also mentioned above, 187 indigenous genera of West Indian origin and 128 naturalized genera are found now in southern Florida. The frequent hurricanes that sweep over southern Florida from the adjacent Bahamas, Cuba, and other West Indies can account for most of these Caribbean floristic elements now indigenous in southern Florida. The warm Gulf sea currents and migrating birds are presumably responsible for the other indigens, and the intentional or accidental activities of humankind for the more recent adventives.

Origins of the North American Flora

The origins of the North American flora are diverse. In addition to the numerous autochthonous elements, as in the Madro-Tertiary geoflora in the Southwest and the Arcto-Tertiary and Boreotropical geofloras throughout the continent, immigration from the other continents has enriched the flora.


Another pattern of distribution concerns the plants of deserts and bordering semiarid areas. These are now concentrated south of the Mexican border, where warm deserts are larger than to the north. These plants seem clearly to have originated as the arid areas of North America expanded through the Tertiary---for the past 65 million years, and especially in the last 15 million years. They include western North American representatives of Achatocarpaceae, Agavaceae, Bignoniaceae, Buddlejaceae, Burseraceae, many Cactaceae, Cochlospermaceae, Dracaenaceae, Ebenaceae, Flacourtiaceae, Fouquieriaceae, Lennoaceae, Malpighiaceae, Martyniaceae, Menispermaceae, Nyctaginaceae, Passifloraceae, Rafflesiaceae, Sapotaceae, Simaroubaceae, and Sterculiaceae among others.


Other immigrants may well have come through Mexico and Central America but have originated in South America. The dominant plant of our southwestern deserts, Larrea divaricata subsp. tridentata, seems to be conspecific with the Patagonian population of L. divaricata subsp. divaricata. Likewise, Frankenia salina is disjunct between Chile and the Californias, as is Koeberlinia spinosa between Bolivia and our Southwest, and Capparis atamisquea between Argentina-Chile and Mexico-Arizona. Also with strong links to temperate South America are the southwestern species of Menodora, Prosopis, Lycium, and Nicotiana. All of these except Prosopis are represented also in South Africa. Lacking from South America now but with relatives in Africa are Thamnosma of the Rutaceae and Selinocarpus of the Nyctaginaceae, the former being found in Namibia, Somalia, and Arabia and the latter only in Somalia. A good many of the herbaceous species common to temperate South America and western North America seem to have emigrated southward, rather than northward from South America, as discussed earlier.


Many of the circumboreal and circumarctic species found in Canada and Alaska, and some in higher mountains south of the Canadian border, whether originated in North America or Eurasia, obviously migrated readily and perhaps frequently between the two continents via the North Atlantic and Beringian land bridges when they were operative. Less commonly disseminules may have been blown across ice-bound straits or frozen seas. Where there has been evolutionary diversification, as in Diapensia lapponica, Armeria maritima, and Scheuchzeria palustris, it is sometimes possible to make out the probable land bridge used.

Some of the circumboreal species or genera are bipolar, presumably having been carried to Fuegia by migrating birds. Good examples of such are Empetrum, Hippuris, and Primula. The Old World distribution of other species offers the clue to the origins of our representatives. Thus our American species of Echinopanax, Lysichiton, Paeonia, and Phyllospadix, found only in the West, have most likely migrated over Beringia. The boreal muscatel, Adoxa moschatellina, is closely related to two recently described genera in China, and it may well have originated there. It could have reached North America over either land bridge.

Two archaic genera of the southeast, Illicium and Schisandra, have numerous relatives in eastern and southeastern Asia and surely are of Asiatic origin. Common or dominant western American species, such as Artemisia tridentata, Ceratoides lanatum, and Datisca glomerata, likewise have rather certain Asiatic origin. Species of Atriplex, Kochia, Mahonia, Monolepis, Prunus subg. Amygdalus, Stipa, and Suaeda have strong links with the arid areas of central Asia. They presumably immigrated via Beringia.


There is a considerable Mediterranean element in western North America, which includes such taxa as Fagonia, Peganum mexicanum, Oligomeris linifolia, Plantago ovata, Senecio mohavensis, and Styrax officinalis. Other southwestern genera with strong relationships to the Mediterranean region, including North Africa, are Antirrhinum, Cupressus, Juniperus, Pistacia, and possibly Astragalus, Lupinus, and Trifolium.


In any discussion of the North American flora, one cannot disregard the past and present heavy addition of those species brought into the continent by humankind, either by accident or by intention (see R.L. Stuckey and T.M. Barkley, chap. 8). The Californian flora of perhaps 5500 indigenous species has been enriched by perhaps 1500 naturalized plants brought in by immigrants, from the Spanish conquistadores to our present peripatetic citizens. These naturalized taxa may comprise one-sixth or more of our present North American flora, which has been estimated to consist of perhaps l8,000 species north of the Mexican border.