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Native Vascular Plants


by
Larry E. Morse
The Nature Conservancy
John T. Kartesz
North Carolina Botanical Garden
Lynn S. Kutner
The Nature Conservancy
Most of the familiar flora of the American landscape, such as trees, shrubs, herbs, vines, grasses, and ferns, are known as vascular plants. These plants have systems for transporting water and photosynthetic products and are differentiated into stems, leaves, and roots. Nonvascular plants--the algae, fungi, and mosses and lichens--are considered in other articles in this volume. Except in Arctic and alpine areas, vascular plants dominate nearly all of North America's natural plant communities. About 17,000 species of vascular plants are native to one or more of the 50 U.S. states, along with several thousand additional native subspecies, varieties, and named natural hybrids (Kartesz 1994).
Human activities have expanded the geographical distributions of many plant species, particularly farm crops, timber trees, garden plants, and weeds. When a non-native plant species is found growing outside cultivation, it is considered an exotic species in that area. About 5,000 exotic species are known outside cultivation in the United States. While many exotic plant species are desirable in some contexts (such as horticulture), hundreds of invasive non-natives have become major management problems when established in places valued as natural areas (McKnight 1991; U.S. Congress 1993). A few particularly troublesome non-natives are regulated under specific federal or state laws as noxious weeds.

Geographic Distribution
Western and southern states have the largest numbers of native vascular plant species in the country. (Fig. 1, revised from Kartesz 1992). California, with more than 5,000 native vascular plant species, has almost one-third of the total number for the entire United States. Texas, with about 4,500 native species, ranks second. Arizona, Florida, Georgia, New Mexico, and Oregon all have over 3,000 native species.

Fig. 1. The number of native vascular plant species in each state.
Hawaii, as a remote oceanic island archipelago, has relatively few native species (Carlquist 1970), but nearly all (89%) of the native Hawaiian flowering (angiosperm) species are endemic to that region (Wagner et al. 1990). A small number of vascular plants, including a species of lycopod (Huperzia haleakalae), are native to both Hawaii and the North American mainland.
In every state, hundreds of plant species are established as exotics. States with coastal areas, major agricultural regions, and large cities generally have the highest numbers of non-native plants. A modest number of native U.S. species, such as the northern catalpa (Catalpa speciosa), have also spread from cultivation beyond their native ranges. Some familiar mainland species, like a wild blackberry (Rubus argutus) and a grass known as broomsedge (Andropogon virginicus), have become problem weeds in Hawaii (Smith 1989).

Rare Species
As of February 1994, 403 native U.S. species, subspecies, or varieties of vascular plants and one nonvascular plant have been formally protected under the provisions of the U.S. Endangered Species Act of 1973 (USFWS 1994). Nearly half the 822 native U.S. federally listed species are plants. The U.S. Fish and Wildlife Service considers an additional 1,953 kinds of plants as candidates for such listing (Federal Register 1993).
The first U.S. national lists of rare plants depended largely on nominations from specialists already familiar with various rare species and omitted many potential candidates. Many state-level rare plant lists were also developed in the 1970's; these generally addressed species considered rare in a particular area regardless of abundance elsewhere.
The Nature Conservancy and the network of Natural Heritage Programs use a consistent methodology to inventory natural diversity and to assess rarity and endangerment for all currently recognized species of vascular plants in North America, Hawaii, and portions of Latin America (Jenkins 1985). By using a five-level scale from 1 (rarest and most vulnerable--typically five or fewer existing occurrences) to 5 (demonstrably widespread, abundant, and secure), a global or rangewide rank (G1 to G5) is determined for each species. With the use of the same five-level scale, conservation priority ranks are assigned for national (N1 to N5) and subnational or state (S1 to S5) status. Ranks are used conservatively throughout the Natural Heritage Network and are assigned after careful review of a species' status. Additional ranks are used to indicate species that occurred historically within a jurisdiction, but which are not presently known. A species is presumed extinct if efforts to relocate it are unsuccessful, if no suitable habitat remains, or if the loss has been well documented. Species are considered "historic" (possibly extinct) if there is reliable evidence from biological surveys that the species occurred within the past few centuries in a given area (Snyder 1993).
The Natural Heritage Network has documented the status of thousands of rare species. At the same time, plant surveys have shown that a comparable number of plants are substantially more common than previously believed. Species status information from all 50 U.S. State Natural Heritage Programs is combined with national and rangewide data in the Natural Heritage Network's Central Scientific Databases maintained by The Nature Conservancy. The inventories and data bases of the Natural Heritage Network continuously gather, organize, and revise information on species rarity and distribution as it becomes available.
The number of species in the United States in each global rank is presented in Fig. 2. For example, more than 4,850 species (about 28%) of the native U.S. vascular plants are considered globally rare (ranked G1, G2, or G3) by The Nature Conservancy and the Natural Heritage Network. Of these, about 960 species are ranked G1 and occur at fewer than five sites globally or are comparably imperiled.

Fig. 2. The number of native vascular plant species in the United States in each global rank. GH/GX means species is potentially extinct; G1 to G5 rank the species from rarest (G1) to most common (G5).
Globally rare native species of vascular plants are concentrated in the western and southern states (Fig. 3), with greatest proportions in Arizona, California, Florida, Georgia, Hawaii, Nevada, New Mexico, Texas, and Utah.

Fig. 3. The proportion of globally rare vascular plant species (ranked G1, rarest, to G3, more common) in the native flora of each U.S. state.
In addition to these globally rare species, about 4,500 other species of widespread or more common vascular plants (ranked G4 or G5) are being actively inventoried in at least one state where they are rare.

Loss of Species

The patterns and causes of plant species' losses are often important components of state-level conservation studies. The loss, or suspected loss, of a species from a portion of the landscape is referred to as "extirpation."
A recent study (Kutner and Morse, unpublished report) of the losses of U.S. native vascular plants revealed that about 1,772 (9.8%) of these species have been lost from at least one state. Of these species, 438 (25%) may be lost from the floras of two or more states. The proportion of species potentially extirpated from each state varies dramatically across the nation (Fig. 4), with the largest losses reported from northeastern states and from Hawaii. Delaware has experienced the proportionately highest loss from its flora, with more than 15% of its species potentially extirpated. Many of the northeastern and mid-Atlantic states have lost more than 5% of their native vascular plants. This region of the United States has experienced hundreds of years of human development and includes many of the most densely populated and intensely developed states. Many plants that have been lost from these states may now be similarly threatened in portions of their remaining ranges.

Fig. 4. The proportion of species reported potentially extirpated from the native flora of each U.S. state.
About 28% of the native flora is considered globally rare (ranked G1, G2, or G3) by the Natural Heritage Network, but only 12% of the potentially extirpated species are globally rare. Most potentially extirpated species have been lost from one or two states and are currently globally common (ranked G4 or G5). In the United States, 110 of these globally common species have been lost from three or more states, and more than 35 species have been lost from four or more states. Of the most common species (global rank G5), about 285 have been lost from two or more states. Common species that have been lost from many states may not be as secure from imperilment as previously believed. Additionally, the effect of species' losses on other plants and animals in a community is often unknown. Rangewide analyses could indicate species that would benefit from further research and a better understanding of potential threats, thus helping prevent subsequent losses.
Many species that are endangered, threatened, or formal candidates for federal protection have also lost parts of their ranges. Nearly 6% of listed and proposed endangered species and 20% of listed and proposed threatened species are reported extirpated from at least one state. About 16% of the category 1 candidate species (top candidates for listing as endangered or threatened) and almost 11% of the category 2 candidate species (possibly qualifying for threatened or endangered status, but more information is needed) have been similarly affected.
Some currently rare species had widespread historical distributions. For example, American chaffseed (Schwalbea americana) is a federally listed endangered species with a Natural Heritage rank of G2. The historical range of this species extended from Mississippi to Massachusetts; the plant is currently known from about 20 populations in five states, mostly in South Carolina. The most significant threat to this species is fire suppression, which allows plant succession to proceed to the point where there is not enough light for the plant to compete successfully. Habitat loss has also caused the extirpation of several Schwalbea populations. For rare species such as S. americana, further state-level extirpations could seriously affect the species' survival.

Wetland Species
Although there are fewer than 7,000 native wetland vascular plant species in the United States, plants that occur mostly in wetlands are more likely to be extirpated from at least one state. Based on the USFWS National Wetlands Inventory (Reed 1988), about half of the potentially extirpated species are either obligate (see glossary) or facultative (see glossary) wetland species.
Wetlands and aquatic ecosystems have been severely affected in the United States; approximately 53% of these ecosystems have been destroyed in the 48 contiguous states (Dahl 1990). Aquatic species frequently have specific habitat requirements and can be threatened by both habitat loss and changes in local hydrology. In the mid-Atlantic region, several intertidal vascular plants have been extirpated from the Delaware River system because of habitat alteration (Ferren and Schuyler 1980).

Possibly Extinct Species
About 90 mainland U.S. and 110 Hawaiian vascular plant species may be extinct, according to records of the USFWS and The Nature Conservancy (Russell and Morse 1992). For example, Nuttall's mudwort (Micranthemum micranthemoides) has been recorded from Delaware, the District of Columbia, Maryland, New Jersey, New York, Pennsylvania, and Virginia, but despite searches, it has not definitely been seen since September 1941.
Several species of U.S. plants are extirpated from the wild, but still exist in cultivation. Most familiar of these is the Franklinia (Franklinia alatamaha), a small tree known historically only from the Altamaha River in southeastern Georgia, but which is now widely cultivated as an ornamental in eastern states.
Ongoing fieldwork has resulted in the rediscovery of many species. The running buffalo clover (Trifolium stoloniferum) was rediscovered in West Virginia in 1983 (Bartgis 1985) and has been found subsequently in Indiana, Kentucky, Missouri, and Ohio. In Oregon, a population of Lomatium peckianum was located in 1983 for the first time in more than 50 years. The discovery of additional populations has changed the species' federal status from a category 1 candidate to a former candidate (Kagan and Vrilakas 1993). In Montana, several recent rediscoveries have occurred, including a 1985 rediscovery of Trifolium microcephalum, a species of clover not seen since it was first collected by Meriwether Lewis in 1805 or 1806 (Hoy 1993). Likewise, during the 1991 field season the yellow passionflower (Passiflora lutea) was located at two sites in Delaware for the first time since the early 1800's (Clancy 1993). These examples illustrate the importance of ongoing inventories as well as the dynamic nature of local and regional floras.

Threats to Diversity
Habitat alteration and incompatible land use are the major threats to most rare U.S. plant species. Apart from certain species of cacti, ginseng, and various showy wildflowers, relatively few rare U.S. plants are primarily threatened by overcollecting. Global climate change (Peters and Lovejoy 1992; Morse et al. 1993) and sea-level rise (Reid and Trexler 1991) may pose additional threats to some native U.S. plant species.
Species at higher risk of extinction usually include those having small geographic ranges, narrow habitat requirements, unusual life histories, or vulnerability to exotic pests or diseases. In addition, reduced biodiversity of local floras is of high concern, even if plants lost from a particular geographical region are common and secure elsewhere. Finally, depletion of even widespread species can occur if exploitation or habitat destruction occurs beyond a sustained-yield rate.
Assessment of the causes and patterns of species losses in the United States, combined with ongoing documentation of natural diversity and studies of rarity, endangerment, and threats, will refine conservation priorities by identifying species or areas that will most benefit from further protection and research. Analyses of ongoing inventory and monitoring work could provide early warnings of widespread threats to biological diversity, thereby perhaps improving the protection of both rare and more common plants and allowing the development and implementation of conservation strategies before crises occur.
For further information:
Larry E. Morse
The Nature Conservancy
1815 N. Lynn St.
Arlington, VA 22209
References
Bartgis, R.L. 1985. Rediscovery of Trifolium stoloniferum Muhl. ex A. Eaton. Rhodora 87:425-429.

Carlquist, S. 1970. Hawaii: a natural history. Natural History Press, Garden City, NY. 463 pp.

Clancy, K. 1993. The yellow passionflower, Passiflora lutea L., rediscovered in Delaware. Castanea 58:153-155.

Dahl, T.E. 1990. Wetlands losses in the United States 1780s to 1980s. U.S. Fish and Wildlife Service, Washington, DC. 13 pp.

Federal Register. 1993. Plant taxa for listing as endangered or threatened species: notice of review. Federal Register 58:51144-51190.

Ferren, W.R., and A.E. Schuyler. 1980. Intertidal vascular plants of river systems near Philadelphia. Proceedings of the Academy of Natural Sciences of Philadelphia 132:86-120.

Hoy, J. 1993. Rediscovering lost species. Kelseya [Montana Native Plant Society] 6:5.

Jenkins, R.E. 1985. Information methods: why the Heritage programs work. The Nature Conservancy News 35:21-23.

Kagan, J., and S. Vrilakas. 1993. Extinct and extirpated plants from Oregon. Kalmiopsis [Native Plant Society of Oregon] 3:12-16.

Kartesz, J.T. 1992. Preliminary counts for native vascular plant species of U.S. states and Canadian provinces. Biodiversity Network News [The Nature Conservancy] 5:6.

Kartesz, J.T. 1994. A synonymized checklist of the vascular flora of the United States, Canada, and Greenland. 2nd ed. Timber Press, Portland, OR. 622 pp.

 McKnight, B.N., ed. 1991. Biological pollution: the control and impact of invasive exotic species. Indiana Academy of Science, Indianapolis. 261 pp.

Morse, L.E., L.S. Kutner, G.D. Maddox, J.T. Kartesz, L.L. Honey, C.M. Thurman, and S.J. Chaplin. 1993. The potential effects of climate change on the native vascular flora of North America: a preliminary climate-envelopes analysis. Report TR-103330, Electric Power Research Institute, Palo Alto, CA. 120 pp.

Peters, R.L., and T.L. Lovejoy, eds. 1992. Global warming and biological diversity. Yale University Press, New Haven, CT. 386 pp.

Reed, P.B., Jr. 1988. National list of plant species that occur in wetlands: 1988 national summary. U.S. Fish and Wildlife Service, Washington, DC. 244 pp.

Reid, W.V., and M.C. Trexler. 1991. Drowning the national heritage: climate change and U.S. coastal biodiversity. World Resources Institute, Washington, DC. 48 pp.

Russell, C.A., and L.E. Morse. 1992. Plants. Biodiversity Network News [The Nature Conservancy] 5:4.

Smith, Clifford W. 1989. Non-native plants. Pages 60-69 in C.P. Stone and D.B. Stone, eds. Conservation biology in Hawai'i. University of Hawaii Cooperative National Park Resources Study Unit, Honolulu.

Snyder, D.B. 1993. Extinct, extant, extirpated, or historical? Or in defense of historical species. Bartonia 57, Supplement:50-57.

U.S. Congress, Office of Technology Assessment. 1993. Harmful non-indigenous species in the United States. U.S. Government Printing Office OTA-F-565 (September). 391 pp.

USFWS 1994. Box score: listings and recovery plans. U.S. Fish and Wildlife Service Endangered Species Tech. Bull. 19:24.

Wagner, W.L., D.R. Herbst, and S.H. Sohmer. 1990. Manual of the flowering plants of Hawai'i. University of Hawaii Press, Honolulu. 1,853 pp.


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