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Potentially Misidentified Species:
Florida is foster home to another invasive non-native Dioscorea, the
water yam (= winged yam) Dioscorea alata. D. bulbifera and D.
alata are superficially similar in appearance but distinguishing the two is
straightforward. D. alata has a stem that is squarish in cross-section
with wide ridges or wings, whereas the D. bulbifera stem is round in
cross-section. Leaves are opposite in D. alata and alternate in D.
bulbifera. The underground tubers of D. alata are also enormous,
some weighing more than 45 kg, whereas the undergroind tubers D.
bulbifera are small and may even be absent altogether (Langeland and Burks
1998).
Non-native Dioscorea species in Florida may also be mistaken for either of two
native wild yams, D. floridana and D. quarternata. The native plants are only
infrequently encountered in north and west Florida hammocks and floodplains.
Where overlap with the exotic species occurs, the lack of aerial tubers in the
native species can aid in identification (Langeland and Burks 1998).
II. HABITAT AND DISTRIBUTION
Regional Occurrence:
The broad native distribution of Dioscorea bulbifera includes much of Asia
and Africa and the plant has been widely introduced to new tropical and
subtropical areas including the Americas (Coursey 1967, Schultz 1993). In the
U.S., D. bulbifera now
occurs in Florida, Mississippi, Louisiana, Texas and Hawaii. D.
bulbifera also occurs in Puerto Rico. In Florida, air potato now occurs throughout almost the entire peninsula.
IRL Distribution:
Dioscorea bulbifera can be readily found in all six IRL watershed counties.
III. LIFE HISTORY AND POPULATION BIOLOGY
Age, Size, Lifespan:
The stems of Dioscorea bulbifera can attain lengths of 20 m and can grow as
much as 20 cm/day in the summer (www.killerplants.com 2004).
Abundance:
Statewide infestation by Dioscorea bulbifera prompted the Florida Exotic Pest
Plant Council to include the species in its list of the top 10 Most Widespread
Invasive Plants in Florida, i.e., nonindigenous plants with the most county occurrence records.
Reproduction:
Dioscorea bulbifera can propagate through both sexual and asexual means, In
Florida, flowering is rare and sexual reproduction is therefore only a minor
contributor to propagation of the species.
Dioscorea species are dioecious, with male and female flowers occurring in
separate plants. Flowering is uncommon in the Florida populations but the
flowering that does occur takes place in summer. Pollination is dependent on
insects, commonly thrips, and the seeds are wind dispersed (Langeland and Burks
1998, Hammer 1998).
The chief means of reproduction in D. bulbifera is asexual and is
dependant on vegetative growth from underground tubers and above-ground
bulbils. Tubers and bulbils generally sprout in the spring and the new shoots
often climb the dead stems of the previous year to reach the tree canopy. In
the summer (June-July), a large number of new bulbils are produced which fall
to the ground in late August. By the time seasonal stem die-back begins around
October, a single vine may have put out as many as 200 bulbils
(swww.killerplants.com 2004).
Embryology:
In areas where they are produced, Dioscorea bulbifera seeds are wind-dispersed
(Hammer, 1998). Even where flowering occurs more regularly, sexual
reproduction via seed is still likely of secondary importance. Seeds of D.
bulbifera and other members of the genus are believed to undergo an
obligate dormancy period of several months before they germinate. This strategy is probbably an
evolutionary adaptation to ensure the presence of viable seeds in the seed bank
when breaks in forest canopy cover occur. A laboratory germination regime for
viable, non-dormant D. bulbifera seeds indicates that germination occurs
in approximately 21 days at 30°C (Ellis et al. 1985).
Bulbils can last a year or more on the ground and still sprout, and soil
contact in not necessary for sprouting. They also float and may be dispersed
by flood waters and appear to be little impacted by feeding from raccoons, feral
pigs, and other animals (Coursey 1967, Morisawa, 1999).
IV. PHYSICAL TOLERANCES
Temperature:
Cold intolerance appears to be the key factor limiting expansion of the U.S.
introduced range of Dioscorea bulbifera beyond the tropical/subtropical to
warm-temperate areas it now occurs in.
Salinity:
Air potato is not salt tolerant and is an unsuccessful invader of marine and estuarine shorelines (Morisawa 1999).
Associated Species:
Dioscorea bulbifera occurs in a variety of natural and disturbed habitats. It
is an aggressive invader that will overgrow understory community associates and
also canopy species if canopy gaps allow it to gain a foothold (Hammer 1998,
Horvitz et al. 1998).
VI. INVASION INFORMATION
Invasion History:
The plant was first introduced to the New World during the slave trade, and
subsequently transported to Florida in 1905 as a USDA sample sent for
horticultural examination (Coursey 1967, Schultz 1993). The Florida
horticulturist who received the samples quickly saw the potential danger this
aggressive potential invader posed for the state.
Vigorous asexual reproduction via bulbils has facilitated the continued spread
of this invasive vine throughout most of the state. Martinez (1993) reported
that Dioscorea bulbifera had become "rampant on undeveloped land" around
Tampa. By 1996, the plant had been reported from natural areas in 23 Florida
counties (Langeland and Burks 1998). As of 2007, the Florida Exotic Pest Plant Council Early Detection and
Mapping System indicates D. bulbifera now occurs in at least 46 (of
67) Florida counties.
In 1999, D. bulbifera was added to the Florida Department of Agriculture and Consumer Services Noxious Weed List,
banning introcuction, possession, transport, or release of the species in
Florida without a permit (Langeland 2001).
Potential to Compete With Natives:
Long Dioscorea bulbifera vines can blanket forest canopies, shading out
understory species and eventually overgrowing and choking canopy species as
well. The resulting loss of native plant diversity, combined with the relative
unpalatability of the tubers and bulbils typically diminishes the habitat value
for faunal community associates.
The species is adapted to take advantage of stochastic opportunities ranging
from treefalls that open up forest canopy to hurricanes that readily disperse
the small, buoyant bulbils. Horvitz et al. (1998) report that air potato
quickly became a dominant species in the south Florida hammock community canopy
in the wake of Hurricane Andrew.
Possible Economic Consequences of Invasion:
Dioscorea bulbifera is listed as a Category I invasive exotic plant in Florida,
indicating that the species is currently altering native plant communities by
displacing native species and changing community structures or ecological
functions. Asexual propagation by means of bulbils that drop off of parent
vines to the ground in abundance ensure that eradication of the plant is
exceedingly difficult once it has invaded an area.
In many parts of the world the tubers and bulbils of D. bulbifera are
used as food and the plant is cultivated as an agricultural crop. The plant
has also long been used as a folk medicine including use as an analgesic,
aphrodisiac, diuretic, and a rejuvenative tonic. The steroid diosgenin, an
active component of birth control pills, is extracted from the plant as is the
anti-fungal compound dihydrorodioscorine (Fox undated).
VII.
REFERENCES
Coursey, D.G. 1967. Yams: an account of the nature, origins, cultivation, and
utilization of the useful members of Dioscoraceae. London: Longmans, Green and
Co. Ltd. 230 pp.
Ellis R.H, Hong T.D., and E.H. Roberts. 1985. Handbook of seed technology for
genebanks - Volume II. Compendium of specific germination information and test
recommendations. International Board for Plant Genetic Resources, Rome.
Available online.
Hammer, R.L. 1998. Diagnosis: Dioscorea. Wildland Weeds:8-10.
Horvitz C.C., Pascarella J.B., McMann S., Freedman A., and R.H. Hofstetter. 1998.
Functional roles of invasive non-indigenous plants in hurricane-affected
subtropical hardwood forests. Ecological Applications. 8:947-974.
Langeland K.A. 2001. Natural Area Weeds: Air Potato (Dioscorea bulbifera).
UF/IFAS Document SS AGR 164. 4 pp. Available online.
Langeland K.A. and K. Craddock Burks. 1998. Identification and Biology of
Non-Native Plants in Florida's Natural Areas, IFAS Publication SP 257.
University of Florida, Gainesville. 165 pp.
Morisawa T.L. 1999. Weed Notes: Dioscorea bulbifera, D. alata, D.
sansibarensis. The Nature Conservancy. Wildland Invasive Species Program.
Morton J.F. 1976. Pestiferous Spread of Many Ornamental and Fruit Species in
South Florida. Proceedings of the Florida State Horticulture Society 89:348-353.
Report by:
J. Masterson, Smithsonian Marine Station
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Page last updated: October 4, 2007 |
