Bracken (Pteridium aquilinum) is a conspicuous fern that forms large clonal colonies in a variety of habitats. The large, more or less triangular leaves develop from fiddleheads that develop widely spaced along the branches of an extensive subterranean rhizome that may reach nearly 400 m in length. The taxonomy of the genus remains controversial, but most botanists currently favor a classification involving five or more species. In this sense, Pteridium aquilinum is distributed widely in mostly the northern hemisphere, in both the New and Old Worlds.
Bracken produces a pharmacopeia of toxic compounds, including: thiaminase (which breaks down the amino acid thiamine and results in vitamin B deficiency), ecdysomes (hormones that stimulate uncontrolled early molting in insects), tannins (which bind to proteins and other compounds), and hydrogen cyanide, and also produces carcinogenic compounds. The combination of chemicals renders the plants toxic to most animals, both invertebrates and vertebrates, although some insect specialists ingest bracken tissue to become poisonous to their predators. Humans have long eaten the fiddleheads (emerging young leaves) of bracken, but over-ingestion of fresh or dried fronds has been linked to stomach and esophageal cancers.
Bracken is an aggressive colonizer of disturbed and other successional habitats and has been considered an invader of pastureland. It has been shown to be allelopathic (to produce compounds that inhibit the growth of other plant species) and can for dense monocultures. It is difficult to eradicate or control and, because it is toxic, renders such pasturage unfit for grazing. Pteridium aquilinum is considered a noxious weed, especially in portions of Great Britain and mainland Europe.
The mostly northern hemisphere distribution of Pteridium aquilinum sensu stricto vs. the mostly southern hemisphere distribution of the remaining diploid species in the genus is suggestive of a Laurasian/Gondwanan split, however, there is no fossil evidence to directly confirm the development this biogeographic pattern. Currently, there is a large area of geographic overlap between the two groups in Latin America and portions of southern Asia. Rymer (1973) reviewed the fossil history of Pteridium. The oldest macrofossils attributed to the genus date to the Oligocene of Hungary, and unequivocal leaf compressions of Pteridium have been recorded from the late Miocene of England and late Pliocene of New Zealand (Oliver, 1928). Rymer reviewed the Quaternary history of Pteridium in Great Britain in somewhat more detail, noting that its distribution waxed and waned during successive glacial and interglacial stages, and that generally the current distribution of P. aquilinum is a relatively recent phenomenon affected greatly by prehistoric and historic human-mediated perturbations such as clearing of forests and introduction of livestock.
Pteridium aquilinum is a terrestrial species that occurs in a wide variety of habitats, ranging from sand dunes to peatlands and open meadows to forests. It is especially aggressive in disturbed or successional habitats, including those prone to periodic disturbance by natural processes such as fires. Human-mediated perturbations such as grazing and timber removal tend to stimulate its growth and spread. It occurs from sea level to ca. 3300 m in soils derived from a wide variety of substrates and with various pH levels.
Although the taxonomy of the genus Pteridium is still controversial, four or five species are often accepted in modern accounts, including: P. aquilinum (L.) Kuhn sensu stricto, P. arachnoideum (Kaulf.) Maxon, P. caudatum (L.) Maxon, P. esculentum (G. Forst.) Cockayne, and P. semihastatum (Wall. ex J. Agardh) S.B. Andrews (P. yarrabense (Domin) N.A Wakef.) (Thomson, 2004).
The list of epithets that have been applied as infraspecific taxa within P. aquilinum when the genus was still considered monospecific includes nearly 20 names. A number of these are now applied to segregate species within the genus and the taxonomic status of most others still has not been fully resolved.
The genus Pteridium is a member of the homosporous fern family Dennstaedtiaceae. Generic relationships within the family still are not fully understood, but the genus is morphologically quite distinct. The taxonomy and classification within Pteridium continues to be controversial. Traditionally and as monographed by Tryon (1941), the genus was thought to comprise a single, highly variable and nearly cosmopolitan species, P. aquilinum, which could be divided into two subspecies, with a complex series of eight and four varieties respectively. However, over time, some of the morphological variants became elevated to separate species status, with the result that more than a dozen species epithets have been published within the genus. Long-term research on the systematics and phylogeny of Pteridium, mainly by John Thomson (National Herbarium of New South Wales) and his collaborators (Thomson, 2004) has resulted in the recognition of four or five species: a New and Old World northern hemisphere diploid (P. aquilinum sensu stricto), a pair of closely related New and Old World mostly southern hemisphere diploids (P. arachnoideum, P. esculentum), an uncommon neotropical tetraploid (P. caudatum), and a Malaysian/Australian tetraploid (P. semihastatum, aka. P. yarrabense). Within P. aquilinum sensu stricto, infraspecific classification also continues to be controversial, with ten or more geographically and morphologically defined subspecies possibly recognized.
In the New World, Pteridium aquilinum in the strict sense occurs nearly throughout the United States and southern Canada, with extensions northward to about 55° N Latitude. The species is distributed southward through the mountains of Mexico into Central America as far as Honduras. It is also present in the Caribbean Islands and one subspecies is endemic to Hawaii. In the Old World, P. aquilinum occurs nearly throughout Europe (north to near the Arctic Circle) and Africa (excluding the Saharan portion), is present on a number of islands, including Madagascar, Réunion, the Mascarenes, Mauritius, and Macaronesia. Its Asian distribution extends eastward across Russia and south through China and Indochina into Malaysia. Farther south in both the Old and New Worlds, other species of Pteridium replace P. aquilinum, but there is distributional overlap between P. aquilinum and the other taxa, mainly in Latin America and portions of Asia.
Medium- to large-sized ferns with a usually extensive system of long-creeping, branched, subterranean, hairy rhizomes, often forming large clonal colonies. Leaves mostly 0.5–1.5 m long, rarely to 2.5 m long, glabrous or variously hairy, long-petiolate, the petioles grooved along the upper side. Leaf blades 2 to 3 times pinnately compound and lobed, broadly deltate to deltate-triangular, ovate-triangular, or nearly pentagonal, the ultimate lobes or segments rounded to pointed at the tip, with strongly recurved, often pale margins, these irregular and usually sparsely to densely fringed along the edge, acting as a pseudoindusium to cover the developing sporangia (a true, outward-oriented indusium absent or vestigial). Sporangia in a continuous band along the leaf margins, hidden by the pseudoindusium until maturity. Spores 64 per sporangium, brown, nearly spherical (slightly tetrahedral), with a 3-branched tetrad scar (trilete), 23–40 μm in diameter, the outer surface irregularly granular. Source documents: Tryon (1941), Tryon and Tryon (1982), Kramer (1990), Tryon and Lugardon (1991), Jacobs and Peck (1993), Mickel and Smith (2004).
The species Pteridium aquilinum is considered globally Secure. Conversely, it is considered a noxious weed (officially or unofficially) in portions or North America, Europe, New Zealand, and Australia.