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DistributionMore info for the terms: bulbil, natural, vines
North American distributions: The five yam species covered in this review are not native to North America. Of these species, Chinese yam is the most widely distributed in the United States. It occurs from Texas north to Kansas and Illinois and in all eastern states as far north as Vermont and as far south as Florida [29,50,69,78]. Water yam and air yam are generally restricted to the US Gulf Coast region, although both also occur in Hawaii . Water yam is reported in the Florida peninsula and panhandle  and is considered "escaped locally" in central and southern Florida . It is reported in Georgia, but may also occur in other Gulf Coast states . Air yam is frequent in Florida and occurs in Mississippi, southern Louisiana, and eastern Texas [3,94]. Zanzibar yam occurs in Florida and, as of 2003, was reported from 4 locations in Collier and Miami-Dade counties [30,94]. According to a 2008 report, Zanzibar yam may have been successfully eradicated from Florida (Pemberton cited in ). Air yam, water yam, and fiveleaf yam occur in Hawaii, and air yam is the most widespread . Fiveleaf yam is also considered widespread in Hawaii, occurring on Kauai, Oahu, Molokai, Maui, and Hawaii islands [70,89]. Flora of North America and Plants Database provide maps of the distribution of all 5 yam species.
Native origins: Yams not native to North America originated in tropical regions of Asia or Africa. Due to widespread early cultivation and transport of yams, exact origins for some species are unknown [14,51,89]. Chinese yam is native to eastern Asia . Fiveleaf yam is native to tropical Asia or eastern Polynesia . Zanzibar yam is native to Africa [21,51,94]. Water yam has been reported as native to southeast Asia [14,51], but Coursey  indicates that water yam is "unknown in the wild state anywhere in the world" but was first cultivated in Assam or Burma. Water yam was likely transported to the eastern coast of Africa 2,000 years before present (review by ). Air yam is known from both Asia and Africa [13,21], but it is unclear if air yam is native to both continents or was introduced from one to the other . Indigenous air yam populations were also reported on Australia's northern coastline (review by ).
Methods and timing of North American introductions: There is both speculation and evidence regarding the method and timing of yam introductions in the United States. Once yams were introduced, cultivation was encouraged because of their food value and ability to quickly develop into attractive shading vines.
Yams in Hawaii: As early as the 4th century, Polynesians brought air yam, water yam, and/or fiveleaf yam to Hawaii . Fiveleaf yam was brought at least once by southern Polynesians as recently as 800 years ago. Depending on the number of successful introductions, yam populations in Hawaii may represent one or many clones .
Air yam: Natural air yam populations occur in both Asia and Africa; these types have several morphological differences (see Botanical description). Most if not all air yams in Florida are the African type . In 1777, an early American botanist reported air yam growing in a garden in Mobile, Alabama (Bartram 1998 cited in ). In 1905, Nehrling  grew air yam in his garden. Nehrling was known for the introduction, promotion, and dissemination of plants for the Florida nursery trade . Later Nehrling sent air yam bulbils to Bureau of Plant Industry officials who were evaluating the plant's medicinal uses. Nehrling's comments on the substantial growth and vigor exhibited by air yam are presented in Impacts.
Chinese yam: Chinese yam was likely planted as an ornamental or edible crop in the 1800s in the United States [21,69]. In the Bearcamp Creek watershed in the western Carolinas, Chinese yam was common at homesteads abandoned around 1900 . In Illinois, Chinese yam was reported as escaped from cultivation by 1986 .
Water yam: Water yam may have arrived on slave ships from Africa as early as the 17th century (reviews by [13,14]). By 1897, water yam was available for sale from Florida's Royal Palm Nursery (Pemberton 1997 personal communication cited in ). After water yam's introduction, it was widely cultivated.
Zanzibar yam: The method by which Zanzibar yam was introduced to Florida is unknown , but Hammer  speculates that Zanzibar yam, which is considered to have magical properties in parts of Africa, may have been brought to the United States for use in religious rituals by members of Santería, an Afro-Cuban religion.
Current and changing local US distributions: Population increases and expanded ranges are described in the southern United States for Chinese yam, water yam, and air yam. Increased ranges are also reported for Chinese yam in Illinois. In 1977, water yam was reported as cultivated and "sparingly escaped" in Florida's Escambia, Leon, Alachua, Lee and Dade counties (Ward 1977 cited in ). By 1996, water yam was disrupting native plant communities, especially coastal hammocks in southern Florida (EPPC 1996 cited in ).
In Florida in 1977, air yam was described as "becoming extensively naturalized". By 1978, air yam was well established in Dade and Broward counties, and by 1996 occurred in 23 Florida counties (review by ). In 2006, air yam occupied 15.2% of conservation areas and 25% of habitats surveyed in southern Florida (Gann and others 2006 cited in ).
Chinese yam has increased its range in northern as well as southern US habitats. In 2008, an estimated 24,110 acres (9,760 ha) of southern forests were invaded by air yam or Chinese yam . Chinese yam was not reported in an Oklahoma flora printed in 1969, but as of 1996, populations occurred along fence rows and in thickets and woodlands in McCurtain, Grady, and Cherokee counties . In Great Smoky Mountains National Park, a survey conducted in the late 1980s estimated that 1% of the Park contained Chinese yam and reported that the species was "rapidly expanding and severely impacting various habitats" (Clement unpublished data cited in ). In 1987 and 1988, Chinese yam occupied an estimated 161,000 feet² (15,000 m²) of Park area on 125 surveyed sites. When just 45 of the 125 original survey sites were visited in 1994 and 1995, researchers estimated that Chinese yam occupied 380,000 feet² (35,000 m²) . A previous resident of the Park reported that in the past Chinese yam did not "grow all over the place like it is now" (Oble 1995 personal communication cited in ). In Illinois, Chinese yam was not known outside of cultivation in 1970, but by 1986 was reported in disturbed areas of Jackson County. In 2001, escaped populations were reported from 21 Illinois counties (review by ). For more information on the expansion of Chinese yam populations in southern Illinois and in Great Smoky Mountains National Park, see Bulbil dispersal and Spread.