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Species Description:
The growth habit of Asimina tetramera
is as a small tree or large perennial shrub with one or more main
stems. Reaching a height of 1 - 3 m (3.3 - 9.8 feet), it is
a root-sprouting plant with a deep taproot that is sensitive to
transplantation (USFWS 1999). Stems vary in color from reddish-brown
to gray-brown. Leaves are alternate, oblong to oblanceolate,
measure 5 - 10 cm (1.9 - 3.9 inches) in length, and are pale to
medium green on the upper surfaces, gray-green on the lower surfaces.
Leaf margins tend to roll under, and leaf tips are generally blunt.
Maroon flowers, which emit a fetid odor, appear in late spring,
usually occurring singly in the axils of leaves.
There are typically 4 sepals, with 6 petals in 2 sets of 3 (Nelson
1996). Stamens
are spirally arranged on a ball-shaped receptacles having separate
carpels. Fruit is an indehiscent,
berry-like aggregate,
oblong in shape, with a yellow-green color and banana-like aroma
when ripe. Seeds are laterally flattened, dark brown in color,
and shiny (Nelson 1996; USFWS 1999).
Potentially
Misidentified Species:
Asimina
tetramera flowers usually have 4 sepals,
a trait that distinguishes it from other Asimina species.
It is also the tallest of the pawpaws (Nelson
1996).
II. HABITAT AND DISTRIBUTION
Regional Occurrence:
The range of Asimina tetramera is limited and extends from
northern St. Lucie County south through northern Palm Beach County,
Florida. Major populations are found in approximately 20 disjunct
areas: several locations in northern St. Lucie County;
around Jensen Beach, Savannahs State Reserve, and Jonathan Dickinson
State Park in Martin County; and in Palm Beach County from
the Martin County line south to northern sections of Palm Beach
Gardens (USFWS 1998).
IRL Distribution:
Within the IRL, Asimina tetramera has only been documented
from a few locations in St. Luice and Martin Counties. Most
of its required habitat on old Pleistocene dunes has now been developed
for commercial and residential uses.
III. LIFE HISTORY AND
POPULATION BIOLOGY
Age, Size, Lifespan:
Four-petal pawpaws grow to a height of approximately
1 - 3 m (3.3 - 9.8 feet). Leaves measure 5 - 10 cm (1.9 -
3.9 inches) in length.
Asimina
tetramera is a fire-adapted plant that demonstrates optimum
growth in the seasons following a fire that destroys its above-ground
stems. Cox (2000) reported greater vegetative growth and more
flowers appearing on plants that had been burned the previous spring.
Cutting the above-ground stems has also been successfully used as
a substitute for burning.
Asimina
tetramera is long-lived, potentially able to survive hundreds
of years, remaining in a vegetative state until such time as fire
or hurricanes remove canopy plants that overgrow it. New growth
regenerates quickly in the absence of canopy plants (Cox 1998),
but slows once the canopy begins to mature.
Abundance:
The four-petal pawpaw is endemic and rare in Florida. The
Florida Natural Areas Inventory (2001) located 950 total plants
on 17 sites in Martin and Palm Beach Counties, more than half of
which grew on privately owned land (Cox
2001 in: Center for Plant Conservation 2006).
Populations not growing in protected park areas are in rapid decline
due to coastal development, fire suppression, and human disturbance
(USFWS 1998).
Reproduction:
New leaves appear in April,
and continue through summer. Flowers occur only on new growth.
Blooms appear from March through July, with peaks in April and May
(Nelson 1996); though blooming is known to persist well into
fall if the plant had been burned in the spring (Roberts and Cox
2000). Flowers are cream colored at blooming, changing to
a deep maroon color, or occasionally yellow, as blooms mature.
Flowers tend to open before all parts are fully developed, with
maturation occurring from the base of the stem towards the tip (USFWS
1998). Stigmas on flowers become receptive to pollen before
anthers mature to release pollen. Beetles are the primary
pollinators (Cox 1998), though flies and wasps also are known to
visit the odorous flowers. Pollinated flowers typically lose
their petals within one day of fertilization, while unfertilized
flowers usually fall within several days of pollen release.
Carpels mature into fruits soon
after being pollinated, and ripen in approximately 2 - 3 months.
Fruits are oblong and greenish-yellow in color (Nelson 1996;
USFWS 1998).
Embryology:
Fruits are consumed by
a variety of scrub-associated animals including gopher tortoises,
raccoons, and rodents (including beach mice) that usually disburse
seeds around the parent plant. Seed viability is limited in
Asimina tetramera due to the high oil content of the endosperm,
which facilitates germination in new seeds rather than in older
seeds that may have dried out. Seedlings emerge from the soil
1 - 8 months after planting, primarily because the extensive root
system develops first. Peak emergence of seedlings occurs
from approximately September through March (Nelson 1996; USFWS 1998).
IV. PHYSICAL TOLERANCES
Temperature:
Asimina tetramera is generally unaffected by low winter
temperatures, including freezes, due to its root-sprouting growth
habit whereby new growth sprouts from the underground root-crown
(Nelson 1996; USFWS 1998). Sprouting from the root-crown also
enhances its fire-adapted ecology.
V. COMMUNITY ECOLOGY
Competitors:
Asimina tetramera is an understory plant of sand pine and
scrub habitats that is easily outcompeted by species, primarily
evergreen oaks and pines, that overgrow and shade it. It relies
on infrequent fires approximately every 20 to 80 years to periodically
trim the canopy and allow it to regenerate (USFWS 1998).
Associated species:
Four-petal pawpaws occur with sand pines (Pinus clausa),
a variety of oaks (Quercus spp.), rosemary (Ceratiola
sp.), saw palmetto (Serenoa repens), wax myrtle
(Myrica cerifera), blueberries (Vaccinium spp.)
and other scrub plants (Austin and Tatje 1979).
Zebra swallowtail butterflies lay eggs on
leaves of Asimina tetramera, with larvae emerging to consume
both leaves and flowers. A shelf fungus, Phylloporia fruitica,
colonizes stems at sites of injury and develops fruiting bodies.
These appear not to kill plants, since new shoots grow from below
the site of fungal infections. Fungus presence may reduce
growth and flowering (USFWS 1998).
Habitats:
Four-petal pawpaws are generally confined
to scattered open scrub and sand pine scrub habitats on old coastal
dunes inland from the present coastline. Farnsworth
(1988) reported a preference for paola soils.
It is rejuvenated by infrequent fire disturbance
(USFWS 1998). Flowering and fruit set are abundant in years
following a fire, then decrease as oak and pine canopies return.
When fire-suppressed, it is outcompeted by other species.
VI. SPECIAL STATUS
Special Status:
The
International Union for the Conservation of Nature and Natural Resources
(IUCN), which maintains the IUCN Red List of species threatened
with extinction, lists the four-petal pawpaw as Endangered
due to human disturbance
(recreation, tourism, development) and continuing to decline, with
no documented population containing more than 250 individuals.
It
is listed in Florida as Endangered, and has been Federally listed
as Endangered since September 26, 1986.
Major
threats to Asimina tetramera are human-related, with habitat
fragmentation and destruction the principal factors. With
the entire documented species consisting of approximately 950 plants
growing in 17 locations, reduced genetic variability due to severe
habitat fragmentation is a major concern (Cox 2001 in: Center for
Plant Conservation 2006). Further, rapid development of remaining
coastal areas is destroying old Pleistocene dunes in sand pine and
scrub habitats, which are the only habitats Asimina tetramera occurs
in. Other factors threatening the four-petal pawpaw include
fire suppression, lack of recruitment, and possibly, insecticide
use, which may indiscriminately harm the beneficial insect pollinators
of this species (USFWS 1998).
Approximately 50% of all known Asimina tetramera occur
on privately owned land. Under Florida law, endangered plants
on privately held land are considered the property of the owner,
and are thus subject to removal when land is sold for development.
The remainder of the known population occurs in 3 protected parks
in Martin and Palm Beach Counties. In these areas, population
decline is still somewhat evident, but management authorities are
taking steps to ensure the continued survival of the four-petal
pawpaw. Some of these measures include land acquisition to
preserve remaining habitat, maintenance of scrub habitats, continued
monitoring, and prescribed burning. At Jonathan Dickinson
State Park in Martin County, Asimina tetramera areas are
scheduled for prescribed burns every 6 years. In Palm Beach
County, sites are monitored on alternate years, and prescribed burning
has occurred in several locations. It is also possible that
if large tracts of privately owned land are sold for development,
that remaining sites where A. tetramera occurs would be
subject to preservation measures (USFWS 1998).
VII.
REFERENCES
Austin, D.F and B.E. Tatje.
1979. Four-petal pawpaw. pages 5-6 in:
D.B. Ward, ed. Rare and Endangered Biota
of Florida, Vol. 5: Plants.
University Presses of Florida, Gainesville,
FL.
Center for Plant Conservation.
2006. Plant profile: Asimina tetramera.
Accessed online at www.centerforplantconservation.org.
June 26, 2006.
Clark, J.R., V.C. Pence. 2001.
Factors affecting micropropagation of Asimina
tetramera, and endangered Florida
scrub species. Poster presentation
at the 2001 Congress on In Vitro
Biology, St. Loius, MO. In Vitro Cellular
Developmental Biology - Plant 37:41A.
Cox, A.C. 1998. Comparative reproductive
biology of two Florida pawpaws
Asimina reticulata Chapman and A.
tetramera Small. Dissertation Abstracts
International. 59-11, Section B:5662.
Farnsworth, S. 1988. Summary four-petaled
pawpaw report. Unpubl.
report, Florida Natural Areas Inventory,
Tallahassee, FL.
FNAI. 2006. Florida Natural Areas
Inventory. Statewide Tracking list and
Field Guide to the Rare Plants of Florida.
Accessed online at
www.fnai.org. June 26, 2006.
Kral, R. 1983. A report on some
rare, threatened, or endangered
forest-related vascular plants of the South.
Athens, GA, U.S. Dept. of
Agriculture Forest Service. U.S.
Dept. of Agriculture Forest Service
Technical Publication R8-TP2, Vol. 1 and
2.
Nelson, G. 1996. The Shrubs and
Woody Vines of Florida. Pineapple
Press, Inc. Sarasota, FL.
Roberts, R.E. and Cox, A.C. 2000. Sand
Pine Response to Two Burning
and Two Non-burning Treatments. In:
Moser, W.K. and Moser, C.F. (Eds.).
Fire and Forest Ecology: Innovative Silviculture
and Vegetation Management.
Tall Timbers Fire Ecology Conference Proceedings,
No. 21. Tall Timbers
Research Station. Tallahassee, FL.
pp. 114-124.
U.S. Fish and Wildlife Service. 1998.
Recovery plan for three Florida
pawpaws. Atlanta, GA; USFWS. 20 pp.
U.S. Fish and Wildlife Service.
1998. South Florida multi-species recovery
plan. Atlanta, GA; USFWS, Southeast
Region.
Ward, D.B. (ed.). 1979.
Rare and Endangered Biota of Florida, Volume 5:
Plants. University Presses of Florida,
Gainesville, FL.
Wunderlin, R.P. 1982.
Guide to the Vascular Plants of Central Florida.
University Presses of Florida, University
of Florida, Gainesville, FL. 175 pp.
Report by:
K. Hill, Smithsonian Marine Station
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Page last updated: June 26, 2006 |
