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).
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.
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 disperse 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).
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.
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.
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).
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. Louis, 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.
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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.