Comments: Dioscorea bulbifera has been widely distributed through human activities and has become naturalized in many tropical and sub-tropical regions of the world (Martin 1974). In 1905, the U.S. Bureau of Plant Industry sent samples of air potato to horticulturalist Henry Nehrling for experimental cultivation in Gotha, Florida (Nehrling 1944). [This is the earliest record of its introduction to the state that can be substantiated by this author]. The species may now be found from Key West (Doria Gordon, pers. comm. 1992) to at least as far north as Alachua County (Dan Ward, pers. comm. 1992). We have found no written reference of this species invading other states within the continental U.S. However, the species appears to be present in southern Illinois (Randall, pers. comm. 1992 as reported to him by Vicki Nuzzo).
In peninsular Florida, air potato is an aggressive weedy invader in many different mesic habitats including thickets, disturbed areas, fence rows (Bell & Taylor 1982) and hardwood hammocks (Wunderlin 1982). Martin (1974) pointed out that an accumulation of leaves on the ground may be necessary to protect sprouting aerial tubers from dessication. In South Florida, the species is almost always found in tropical hammocks, and invades habitats ranging from xeric uplands to deep solution holes. It is rarely found along disturbed edges of pinelands. Air potato is not salt tolerant and thus is not invasive in marine areas (Rob Line, pers. comm. 1992). Air potato is particularly abundant in Dade County where it is growing in almost every county park with hammock vegetation (Carol Lippincott, pers. comm. 1992). (Some records may confuse presence of D. bulbifera with that of D. alata. In Central and North Florida, D. bulbifera is most common in mesic hammocks and urban lots (Gordon, pers. comm. 1992) and alluvial flood plain forests (Francis E. Putz, pers. comm. 1992). Air potato growth appears most vigorous on hammock edges or where canopy gaps occur (Gordon, pers. comm. 1992, Lippincott, pers. comm. 1992).
Several sources report evidence of small mammals feeding on yam bulbils ([8,77], review by ), but little to no predation of air yam bulbils was reported in Florida (Brinkley and Putz 1992 personal communications cited in ).
Palatability and/or nutritional value: Chemical composition and nutritional value of yams is reported in the following references: [13,45,45]. Although air yams have moderate protein levels, protein quality is generally low .
Dioscorea bulbifera is a perennialvine with broad leaves and two types of storage organs. The plant forms bulbils in the leaf axils of the twining stems, and tubers beneath the ground. These tubers are like small, oblong potatoes. Some varieties are edible and cultivated as a food crop, especially in West Africa. The tubers of edible varieties often have a bitter taste, which can be removed by boiling. They can then be prepared in the same way as other yams, potatoes, and sweet potatoes. The air potato is one of the most widely-consumed yam species. It can grow up to 150 feet tall.
Air potato can grow extremely quickly, roughly 8 inches per day, and eventually reach over 60 feet long. It typically climbs to the tops of trees and has a tendency to take over native plants. New plants develop from bulbils that form on the plant, and these bulbils serve as a means of dispersal. The aerial stems of air potato die back in winter, but resprouting occurs from bulbils and underground tubers. The primary means of spread and reproduction are via bulbils. The smallest bulbils make control of air potato difficult due to their ability to sprout at a very small stage. The vine produces small white flowers, however these are rarely seen when it grows in Florida. The fruits are capsules.
This description covers characteristics that may be relevant to fire ecology and is not meant for identification. Keys for identification are available (e.g., [21,72,89]).
Yams are herbaceous, climbing, twining, perennial monocots [13,72,89]. Vines are without tendrils and use dead stems from the previous year's growth to climb into other vegetation [51,89]. Most nonnative yams occurring in the United States grow belowground tubers annually; perennial tubers are generally restricted to the Testudinaria section of the Dioscorea genus [13,46]. None of the 5 nonnative yams belong to the Testudinaria section (see Taxonomy). Most yams also produce aerial tubers or bulbils . Both belowground tubers and bulbils are comprised of stem tissue [45,46]. Regeneration of yams in the United States has been exclusively asexual from tubers or bulbils [51,66,78]. Additional information on asexual regeneration by yams is presented in Regeneration Processes and Vegetative regeneration. Yams are dioecious  and produce very small flowers, if any [21,72]. Sexually produced fruits are 3-winged capsules , but yam fruits are extremely rare in the United States and, if produced, are often sterile [21,24,61]. Even when male and female water yams grew in close proximity, fruits were rare and seeds were typically aborted before reaching maturity . Yam seeds are winged but have not been observed in the United States .
For thousands of years, yams have been a staple food for humans. They have been widely cultivated and domesticated throughout the world. Selection and adaptation have occurred in native and nonnative habitats, producing an abundance of ecotypes. Variable yam forms are possible in the United States, depending on the number of types introduced and escaped. Great variation in the growth and forms of vines, leaves, bulbils, and tubers are reported for air yams and water yams [45,46] and are also likely in other species.
Aboveground descriptions: Air yam (D. bulbifera): Air yam vines twine counterclockwise and may grow to 100 feet (30 m) long [21,89]. Air yam stems are not angled and do not have prickles . Leaves are simple, heart-shaped, and arranged alternately along the stem. Leaves may reach 10 inches (26 cm) wide and long; leaf petioles are generally shorter than the leaf blade [21,51,89,93]. Air yam size and appearance can be variable. Variability in bulbil form and size may be partly due to the different air yam types that exist in Asia and Africa. The Asian type produces relatively smooth, spherical bulbils that may weigh 2 pounds (1 kg). African types produce sharply angled bulbils . Most or all of the air yams in Florida are considered to be the African type . In US floras and other references, air yam bulbils are often described as more than 2 inches (5 cm) in diameter and up to 5 inches (13 cm) long [3,12,21,47]. Flowers, if produced, are widely spaced or reduced to a single flower in simple staminate spikes 4 to 28 inches (11-70 cm) long. Pistillate spikes are generally stiff, up to 9 inches (23 cm) long, and occur in clusters of 2 or more [21,66,89,93].
Chinese yam (D. polystachya): Chinese yam vines are slender, twine clockwise, and may reach 16 feet (5 m) or more in length [24,72]. Chinese yam leaves may grow to 4.3 inches (11 cm) long and wide [21,24]. Leaves are deeply lobed at the base, and upper leaves may have 3-lobed margins [21,78]. Like water yam, the arrangement of Chinese yam leaves is variable. A review by Tu  reports that leaves may be alternate near the top of stems or occassionally found in whorls of 3, and others report that leaves are generally alternate near stem bases and opposite near the end of stems . Chinese yam produces small, rounded, warty bulbils. Bulbils are typically less than 1 inch (3 cm) long and less than 0.8 inch (2 cm) in diameter [21,47,61,69]. Flowers are rare and smell like cinnamon. Male flowers occur in bundles, spikes, or panicles at branch ends , and female inflorescences are few-flowered and generally less than 2 inches (5 cm) long .
Fiveleaf yam (D. pentaphylla): Fiveleaf yam vines are prickly, twine counterclockwise, and may grow to 30 feet (10 m) long [70,89,93]. Leaves are alternate and compound with 3 to 5 leaflets. Leaflets measure 2 to 4 inches (5-10 cm) long and 0.8 to 1.5 inches (2-4 cm) wide. Bulbils are horseshoe shaped and about 1 cm in diameter . Bulbils were rare in Hawaii according to St John . Male flowers, if produced, occur in spikes up to 12 inches (30 cm) long at the branch ends; female flowers also occur in spikes, but these are shorter, 2 to 10 inches (5-25 cm) long .
Water yam (D. alata): Water yam vines twine clockwise and may reach 100 feet (30 m) long [21,46,94]. Vines have ridged stems and are prickly at the base . Water yam leaves are large, elongate, and heart-shaped [46,51]. Leaf blades typically measure 2 to 6 inches (6-16 cm) long, 2 to 5 inches (4-13 cm) wide, and have entire margins. Leaf petioles are generally as long as the leaf blade  Leaf arrangement appears inconsistent. Some report that water yam leaves are alternate at the stem base and opposite near the stem end , while others report that leaves are primarily opposite, but can appear alternate due to leaf abortion . Bulbils are oblong and rough with fleshy protrusions. Bulbils may reach 4 inches (10 cm) long and 1.5 inches (4 cm) in diameter [12,21]. Staminate flowers occur in a zig-zag pattern along a rachis up to 10 inches (25 cm) long; pistillate flowers occur in 4- to 20- flowered inflorescences that may reach 14 inches (35 cm) long . Water yam fruits are capsules that could produce 2 seeds per each of 3 locules, but often if 1 seed is produced it is aborted before maturity. Capsules are rare even when male and female plants are in close proximity .
Zanzibar yam (D. sansibarensis): Zanzibar yam vines twine counterclockwise. Vine stems are about 1 inch (3 cm) thick and may grow to 10 inches (25 cm) or more. Stems can reach support trees prior to producing leaves [21,51,82]. Zanzibar yam leaves and petioles are long. Leaf blades are heart-shaped and can be up to 18 inches (46 cm) long and 23 inches (58 cm) wide. Petioles may be 10 inches (26 cm) long, and leaves often have a tail-like projection at the tip [21,51,82]. Leaves are generally opposite, and the margins of juvenile leaves may be irregularly lobed [21,82,94]. Bulbils are small, smooth, often a purplish color, and can measure 2 inches (6 cm) in diameter [21,51,82]. Male inflorescences typically have 2 to 4 flowers and may be up to 20 inches (50 cm) long; female inflorescences are slightly shorter [21,82].
Belowground descriptions: Most yams produce underground tubers. Yams of the Enantiophyllum section (Chinese yam and water yam) generally produce 1 tuber per season but may produce up to 3 tubers in a single season . Yams generally have "weak rooting systems". Although referred to as roots in the literature and in this review, tubers and belowground structures attached to the root crown are adventitious stem tissue [45,46]. Early in the growing season, the previous year's tubers and sprouting bulbils produce thick, unbranched roots; later in the growing season, thinner, branching, fibrous roots develop (reviews by [3,13]). True roots are only produced by seedlings, and these are short-lived.
Air yam: Most references indicate that air yam generally produces tubers, although they may be small and solitary [3,66,89,93]. However, some suggest that air yam may lack underground tubers [3,21]. Tubers are commonly 2 to 4 inches (5-10 cm) in diameter and weigh less than 2.2 pounds (1 kg) [21,66,89,93]. Tubers from air yams in Florida were usually less than 4 inches (10 cm) in diameter but were up to 8 inches (20 cm) in diameter on vines growing in sandy soil (Gann-Matzen and Line 1992 personal communications cited in ). A particularly large tuber was dug from Alachua County, Florida; it was 9.8 inches (25 cm) in diameter and weighed 11 pounds (5 kg) (personal observation 1992 cited in ). Flora of North America describes air yam tubers as globose and occurring just below the soil surface . However, tuber shape is likely related to ecotype. Air yams from Asia produce spherical tubers, while air yams from Africa produce irregular branching tubers .
Chinese yam: Chinese yams produce 1 to many large, cylindrical tubers [21,93]. Tubers grow vertically from long stalks and typically as deep as 3 feet (1 m) below ground [24,55]. Mature tubers can weigh 8 to 10 pounds (3.6-4.5 kg) .
Fiveleaf yam: Fiveleaf yam typically produces single, irregular to elongated, egg-shaped tubers [70,89,93]. Tubers may occur near the soil surface or more than 3 feet (1 m) under ground [70,89]. Fiveleaf yam tubers dug in Hawaii ranged from 1 to 6.7 inches (2.6-17 cm) long and were about equally wide. The largest tuber weighed 3 pounds (1,370 g) .
Water yam: Tubers produced by water yams are described as massive [21,41]. Water yam may begin producing tubers 14 to 40 days after planting (review by ), and tubers grow rapidly near the end of the growing season . Although single tubers are most common, several are possible [21,46]. Tubers are branched, grow vertically, and may be deeply buried [21,46,93]. Water yam tubers weigh 22 pounds (10 kg) to more than 110 pounds (50 kg) [3,51]. A 180-pound (81 kg) water yam tuber was harvested from a garden in Trinidad .
Zanzibar yam: Irregular, rounded lobes are common on the generally globose tubers produced by Zanzibar yams. Generally tubers are shallowly buried (up to 6 inches (15 cm) deep) and may reach 16 inches (40 cm) in diameter [21,82].
Yams often occur in mesic habitats in the United States. In Hawaii, air yam is common in the moist regions of shaded valleys and disturbed forests, and fiveleaf yam is common in windward mesic valleys . Nonnative yams in Florida often occur on disturbed sites . Water yam occurs near ponds, marshes, and drainage canals in Florida and Georgia . Air yam and Zanzibar yam grow in hammocks and swamps in Florida , but the more common air yam also occupies thickets, disturbed woodlands, fence rows, and urban lots (review by ). Although Chinese yam occupies a much larger range than the other yams, its habitats are similar. Disturbed sites, old homesteads, roadsides, fence rows, thickets, alluvial woodlands, stream banks, and canals all provide Chinese yam habitat throughout its range [21,43,50,54,61].
Climate: For all but Chinese yam, sub-tropical to tropical climates are preferred, and growth is limited at temperatures below 68 °F (20 °C). Optimal growing conditions for tropical yams are temperatures of about 86 °F (30 °C) and growing season precipitation of about 59 inches (1,500 mm). Although high temperatures and drought conditions are tolerated by established yams, in the early growth stages, these conditions can cause mortality (review by ). Based on its US distribution, growth of air yams is best where average annual minimum temperatures do not fall below 10 °F (-12.2 °C). Freezing temperatures kill air yam bulbils . Water yams require a 7- to 8-month growing period (review by ) and seldom persist in areas where cool temperatures and/or dry periods are common during the growing season. Long rainy periods in the growing season produce the best water yam growth, but "excessive" moisture is not tolerated .
Chinese yam is more frost tolerant than the other nonnative yams and occurs in climates ranging from tropical to northern temperate . Chinese yam is hardy at least as far north as New York . In southern Illinois, Chinese yam populations occur where the growing season averages 187 days, annual precipitation averages 43 inches (1,110 mm) , and summer temperatures average 76.5 °F (24.7 °C) .
Elevation: The widespread Chinese yam is common at elevations from sea level to 1,600 feet (500 m) in the United States , but in Great Smoky Mountains National Park, it is reported from elevations of 950 to 3,300 feet (290-1,000 m) . Air yam occurs at about sea level in Florida  and from 20 to 2,200 feet (5-670 m) in Hawaii. Fiveleaf yam occurs from 375 to 1,070 feet (115-325 m) in Hawaii .
Soils: Yams grow best in "loose, deep, free-draining, fertile soil" (review by ). Water yam grows well in permeable clays and does poorly in sandy soils that lack moisture and nutrients . In its native habitats, air yam occurs in loams or loose clays with good drainage (review by ). Increased organic matter promotes vine and tuber growth. High tuber yields require high potassium levels .
Chinese yam often occurs on silty loams in alluvial habitats (review by ). Although common in rich alluvial soils, Chinese yam also occurs in semi-xeric habitats with rocky soils . Soil characteristics were described for Chinese yam populations in alluvial habitats in southern Illinois. Chinese yam occurred most often in silty loams. In 7 of the 9 populations evaluated, soil moisture was high, averaging 52%. Soils were low in potassium, high in nitrogen, phosphorus, and calcium, and very high in magnesium. Soil pH averaged 6.2 .