Acrostichum aureum, the golden leather fern, is a large understory fern that occurs in mangrove forests and other wetlands. Plants measure approximately 1.2-1.8m (4-6 feet) in height and are as broad as they are tall. Fronds are usually arching around the periphery of the plant, but tend to be more erect near the center. The thick, leathery leaves are compound and large, measuring over 1m (3.3 feet) in length, and 12-50 cm (4.8-19.7 inches) in width. There are 24-30 pairs of alternate leaflets (pinnae) that are non-overlapping, rounded at the tips, and measure approximately 10-34 cm (3.9 - 13.3 inches) in length by 1.3 - 7 cm (0.5 - 2.8 inches) in width. Leaves are shiny and typically dark green above, but paler on the leaf underside. Leaf margins are somewhat uneven and wavy in appearance.
No sori are present as in other ferns. Rather, sporangia are distributed over the entire underside of reproductive pinnae (the most distal five or more pairs), lending a felt-like texture to these leaves. Sporangia are brick red to rust red in color, with spores measuring 37-72 µm in diameter.
Potentially Misidentified Species: A related species, the giant leather fern, Acrostichum danaefolium, is similar in appearance, but grows larger, with fronds generally 2m (6.6 feet) or more in length, and pinnae set closer together. A more definitive characteristic is the presence of reproductive pinnae on fronds. In A. aureum, reproductive pinnae are found only distally, whereas in A. danaefolium, pinnae are reproductive throughout the frond (Lloyd and Greg 1975).
Regional Occurrence: Acrostichum aureum occurs in tropical and subtropical areas worldwide. In the continental United States, it is restricted to wetland areas in south and southwestern Florida.
IRL Distribution: Not present in most IRL counties. It is present but not common in Martin County.
Age, Size, Lifespan: Plants measure approximately 1.2-1.8m (4-6 feet) in height and are as broad as they are tall. Leaves measure approximately 1m (3.3 feet) in length, and 12-50 cm (4.8-19.7 inches) in width. Pinnae measure approximately 10-34 cm (3.9-13.3 inches) in length by 1.3-7 cm (0.5-2.8 inches) in width. The growth rate is moderate (Medina et al. 1990).
Abundance: Though Acrostichum aureum is abundant in some areas of the world, in the continental United States it is confined to southern Florida, where it is listed as Threatened.
Reproduction: Unlike other fern species, Acrostichum species do not form sori on their reproductive leaves. Rather, sporangia are distributed over the entire abaxal surface of reproductive pinnae, lending them a felt-like texture. Additionally, not all pinnae on a frond are reproductive. in A. aureum, only 5-8 pairs of pinnae at the tip of a frond are reproductive. This feature helps distinguish A. aureum from another Florida congener, the giant leather fern, A. danaefolium.
Spore germination in the golden leather fern tends to be higher in non-saline substrates (Lloyd and Buckley 1986).
Acrostichum aureum is halophytic, but generally requires fresh water for it to become established and grow optimally. It does not grow in areas where soil salinity exceeds 50 ppt, nor does it grow on arid coastlines (Medina 1990).
Salinity is the primary environmental stressor and regulator of plant development in mangrove forests (Medina et al. 1990). High salinity levels induce changes in plant height, leaf size, and the osmotic pressure of leaf cell sap. Several authors (Tomlinson 1956; Walter 1973) reported that A. aureum grows optimally on somewhat elevated grounds in mangrove forests that are well protected from frequent tidal influx and have high rainfall, which tends to desalinate upper soil layers.
In Malaysia, two growth forms are recognized. A large form grows to 4m (13.1 feet) in height, and is generally confined to the margins of mangrove forests, where it is frequently inundated with freshwater from heavy rainfall. The other form is stunted, growing less than 1m (3.3 feet) in height, and inhabiting areas where tides inundate the soil 10-28 times per month (Medina et al. 1990).
Other Physical Tolerances: The golden leather fern grows well in nearly all light conditions, from full sun to dense shade (Medina et al. 1990). Maximum rates of productivity, development, and reproductive capacity are observed under full sun exposure; however, Acrostichum aureum is highly shade-tolerant and takes advantage of decreased evaporative rates in shade to reduce salt stress in estuarine environments (Medina et al. 1990).
Habitats: Golden leather ferns inhabit neotropical mangrove swamps, salt marshes, low hammocks, and canal margins. In many parts of the world, they have been considered a vegetative pest that interferes with growth and regeneration of mangrove trees (Medina et al. 1990). However, Chan et al. (1987) showed that Rhizophora mucronata in Malaysia could be regenerated in densely vegetated Acrostichum areas.
Associated Species: The 3 species of Acrostichum all grow in loose association with mangrove vegetation in the tropics (Tomlinson 1986).
Special Status: The golden leather fern is listed as a Threatened species in Florida. It is not listed at the federal level.
Adams, D. and P. Tomlinson. 1979. Acrostichum in Florida. American Fern Journal 69(2): 42-46.Chan, H., N. Husin, and P. Chong. 1987. Is there a need to eradicate Acrostichum speciosum prior to planting Rhizophora mucronata in logged over mangrove forest area? FRIM Occasional paper. Forest Research Institute, Kuala Lumpur, Malaysia. 7 pp.Flora of North America Editorial Committee. 1993. Flora of North America North of Mexico. Oxford University Press, New York, NY.Efloras.org. 2006. Flora of North America, Vol. 2, Acrostichum aureum. Harvard University Herbaria, Harvard University, Cambridge, MA. Available online at www.efloras.org.Gleason, H. and M. Cook. 1926. Plant Ecology of Puerto Rico, Scientific Surveys of Puerto Rico and the Virgin Islands, Vol 7(1). New York Academy of Sciences, New York, NY. 96 pp., plates.Gomez, L. 1983. Acrostichum aureum. Pp. 185 - 187 in: Janzen, D. (ed.). Costa Rican Natural History. University of Chicago Press, Chicago, IL.Harris, J. 1934. The physico-chemical properties of plant saps in relation to phytogeography. University of Minnesota Press, Minneapolis, MN.Lakela, O. and R. Long. 1976. Ferns of Florida. Banyan Books, Inc. Miami, FL. 178 pp.Lloyd, R. and D. Buckley. 1986. Effects of salinity on gametophyte growth of Acrostichum aureum and A. danaefolium. Fern Gazette 13:97-102.Medina, E., E. Cuevas, M. Popp, and A. Lugo. 1990. Soil salinity, sun exposure, and growth of Acrostichum aureum, the mangrove fern. Botanical Gazette 151(1):41-49.Mehltreter, K. and M. Palacios-Rios. 2003. Phenological Studies of Acrostichum danaefolium (Pteridaceae, Pteridophyta) at a mangrove site on the Gulf of Mexico. Journal of Tropical Ecology. 19:155-162.Small, J. 1938. Ferns of the Southeastern United States, Facsimile Ed. 1964. Hafner Publishing Co. New York, NY. 517 pp.Tryon, R. and A. Tryon. 1982. Ferns and Allied Plants, with Special Reference to Tropical America. Springer-Verlag, New York, NY. Pp. 348-351.Waisel, Y. 1956. The Botany of Mangroves. Cambridge Tropical Biology Series. Cambridge University Press, Cambridge.Wunderlin, R. 1982. Guide to the Vascular Plants of Central Florida. University Presses of Florida, University of South Florida, Tampa, FL 472 pp.