Asimina tetramera, commonly known as the four-petal pawpaw, is a rare species of small tree or perennial shrub endemic to Martin and Palm Beach Counties in the state of Florida. The species is currently listed as endangered under the Endangered Species Act and as endangered by the International Union for Conservation. The four-petal pawpaw is part of the family Annonaceae alongside other Asimina species.
A. tetramera grows to between 1 and 3 meters tall with one or more branches. Mature flowers are maroon with some pink streaks and the fruit is yellow-green. It usually has six petals and four sepals.
It lives exclusively in sand pine scrub habitat on the Atlantic Coastal Ridge. A. tetramera is pollinated primarily by beetles which feed on tissue on the surface of its stamens and on the inner surface of its petals.
A. tetramera is a fire adapted species. The shrub grows best without the presence of a taller plant canopy shading it. A wildfire may remove these plant canopies to allow for the four-petal pawpaw to flourish. With the return of a shading canopy, the plant growth slows. The removal of the above-ground parts of the plant allows it to grow after a disturbance such as wildfire.
The small range of A. tetramera has been threatened by habitat loss and fire suppression for decades. Human development and disruptive activities has removed suitable land where the species can live. Also, the prevention of wildfires in Florida has limited its ability to grow and reproduce. Work is being done to protect and restore the four-petal pawpaw and its habitat. By protecting existing populations, performing controlled burns, and planting new A. tetramera within its native range, conservationists hope to prevent this rare shrub from going extinct.
Description
The four-petal pawpaw (Asimina tetramera) is a large shrub or small tree. Its fruit is aggregate and yellow-green in color. The fruit is banana-scented when ripe. The seeds of the plant are flat, shiny, and dark brown. The four-petal pawpaw is sensitive to transplantation due to its deep taproot.
Size and coloration It is classified as a large shrub or a small tree. Its growth habit ranges from a small tree or a larger perennial shrub. When sprouting, they are about 2 to 3 centimeters tall from the underground taproot. The plant height ranges from 1 to 3 meters tall. This makes it one of the tallest species of pawpaw.
A mature individual has one or more main stems in the plant. The colors of the branched or unbranched stems vary. This includes red, reddish-brown, brown, grayish-brown, and gray colored bark. The plant has pale lenticels.
The flowers of the four-pawpaw are maroon with occasional pink streaks. At the start of blooming, they can be seen with cream-colored flowers. The color of the flower changes as it ages. The fruit that the plant produces is yellow-green and aggregate.
Leaf morphology The leaves of A. tetramera have an oblong shape and alternate in a spiral pattern on the stem. They measure about 5–10 cm in length. The tips of the leaves are usually blunt. The leaf margins of A. tetramera roll under. This means the leaves of the plant have rolled edges. Young leaves are spotted with small reddish hairs. As they age, the leaves turn to a dark green with a smooth surface. The undersides of the leaves are gray-green in color. The undersides are also reticulated as opposed to the smooth top side of the leaf.
Flower structure A. tetramera has six petals split into two sets of three. Usually the petals are three-merous, as said before; however, there is evidence of some four-merous flowers in this species. It has four sepals. A. tetramera has a unique number of sepals since other Asimina species only have three. The stamens are arranged in a spiral pattern on the receptacles. The receptacles of the flower have separate carpels. The outer petals of the flower are about 1.5 to 3 centimeters long. They are oblong oval shaped.
Life history
Development A. tetramera grows a deep taproot. The root crown permits the survival of the plant through droughts, wildfires, and other disturbances. If the aboveground parts of the plant are removed, the plant can resprout from the intact roots.
Growth rate A. tetramera is fire adapted and grows at a faster rate when it is in a recently burned environment. Wildfires burn the taller plant canopy that typically shades the A. tetramera. After burning, the plant regenerates at an increased growth rate. This allows for numerous flowers and fruits to grow. The plant's growth rate declines over time as the taller plants take up canopy space again. A. tetramera relies on natural fires which allow the plant to access sunlight. Fire suppression in its habitat has reduced the number of A. tetramera.
Reproductive cycle The flowers of the four-petal pawpaw open before the parts inside are matured. The stigmas of the flower become receptive to pollen. Following this, the anthers develop to allow pollen release. The four-petal pawpaw is not a self-fertilizing plant. Four-petal pawpaws have cream-colored flowers when they start to bloom. During maturation, the flowers become maroon. Sometimes the flowers are yellow. If the flower is pollinated, the day following fertilization it will lose its petals. If the flower remains unfertilized, the flowers will fall off days after the release of pollen. Pollinated carpels of the flower develop into fruit. The fruits ripen between 2 and 3 months and are greenish-yellow. The number of fruit developed depends on the availability of pollinators.
Timing of reproduction The best growth occurs in the seasons following a wildfire. This happens after the destruction of the plants shading A. tetramera as well as its own aboveground parts. The new leaves of the four-petal pawpaw first grow in April and then continue into the summer. The flowers bloom from March through July. The peak blooming occurs in April and May. Flowers only occur on the new growths on the plant.
Offspring quality and quantity A. tetramera grows based on the resources available. The time it takes to flower is correlated with fruit development. More growth and more flowers occur after a disturbance like a wildfire. Best growth occurs when there is a burn the prior spring. Evidence suggests cutting the stem may also promote growth instead of burning. The seeds of A. tetramera have a high oil content in the endosperm. Seed viability is best in new seeds. Old seeds have low viability due to oil loss. After 1–8 months, seedlings appear. The root system develops before the appearance of seedlings. Most seedlings emerge from September through March.
Age of dispersal Seeds are dispersed around A. tetramera. The fruits of the plant are consumed by animals which allows for dispersion. These animals include gopher tortoises, raccoons, and rodents like beach mice.
Annual dormancy Growth stalls in the four-petal pawpaw when it is shaded by other plants. It remains this way until a disturbance removes the plants that outgrow it. The four-petal pawpaw has the ability to remain dormant underground before it resprouts.
Age-specific mortality rates It is very long-lived, probably living well over a century, and able to spend much of its time in a dormant state underground before sprouting again.
Ecology Specialist insect species use A. tetramera to rear larvae. Both Zebra swallowtail butterflies and Asimina webworm moths lay eggs on A. tetramera leaves and the emerging larvae then eat leaves and flowers from the growing plant. Herbivory damages the developing shoots, which stimulates growth in the plant and extends the breeding cycle for the insects.
The shelf fungus Phylloporia frutica attacks A. tetramera at wounded sites near the ground and the mushroom structure emerges from branching points on the plant. The fungus is not fatal to the plant but may decrease flowering and number of produced fruit.
Common fruit-eating animals include gopher tortoises, raccoons, beach mice and other rodents, and other small mammals. The seeds of these fruits may be dispersed and buried as they are eaten by animals, but they do not need to be ingested in order to germinate.
Pollination For Asimina species, pollen is released in aggregates as tetrad structures held together by viscin threads. A. tetramera emits a foul odor as it matures and pollen is released. This foul odor is used to mimic the odor of both fermenting fruit and feces in order to diversify the types of pollinators that visit it.
A. tetramera flowers are primarily pollinated by beetles. The 3 inner petals of the flower form a chamber that beetles occupy between the female and male phases of floral development. Beetles feed on grooved structures covering the stamen, inner petal tissue, and pollen grains as they are released after maturation. Beetles leave the flower after the petals fall with pollen on their elytra (forewings), legs, and mouth parts which they may transfer to another flower.
Flowers are also pollinated by several types of flies and wasps that are attracted to their foul odor.
Cox (1998) surveyed the two largest known populations of A. tetramera for insect interactions. The zebra longhorn beetle was observed visiting A. tetramera flowers the most. Beetles represented 12 out 17 visitor species.
Habitat A. tetramera is found exclusively in coastal dune sand pine scrub habitat. Sand pine scrub is defined by dispersed sand pines and patchy openings. Other associated plant taxa include scrub oaks (Quercus myrtifolia, Q. geminata, Q. chapmanii), saw palmetto, rusty lyonia, and other species of shrub. A. tetramera occurs only along the Atlantic Coastal Ridge complex in the excessively-drained quartz sand of the paola classification. The sand pine scrub ecosystem is mediated by frequent fires that historically created disturbance every 10–50 years. These disturbances enhance the structure and complexity of the system for several years by creating gaps in the canopy for colonization of different vegetation types.
A. tetramera is a shade intolerant understory plant. It occurs in varying levels of canopy, but it thrives in habitat where it has the most access to light. A. tetramera is easily outcompeted by larger sand pine and oak trees and even saw palmetto shrubs which are less likely to be shaded out. The presence of these frequent fires indicates better quality habitat for A. tetramera even if individuals have remained in a vegetative state for many years. Too much fire, however, can reduce the cover vegetation that is home to rodents which help disperse the seeds.
Range Coastal dune sand pine scrub communities along the Atlantic Coastal Ridge have been decimated over the past decades and exist as fragmented islands of habitat for A. tetramera. As a result, the range of the shrub is limited to Martin and Palm Beach Counties in the state of Florida.
Conservation
Past and current geographical distribution The known distribution of A. tetramera has always been small. Past distribution includes only the counties of Martin and Palm Beach in Florida, USA. Today, the distribution remains only in these two counties, but has become more fragmented and sparse over time. Rather than a continuous distribution across the counties, four-petal pawpaw populations exist in small patches within the identified range. According to the U.S. Fish and Wildlife Service's (USFWS) Five-Year review from 2022, the southernmost population of A. tetramera has disappeared. Therefore, the size of the species' distribution has shrunk, and the southern tip of the range has shifted about 7.5 miles north.