Pteridium aquilinum (L.) Kuhn

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.

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.

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).

• Dr. John A. Thomson
• Dr. Elizabeth A. Sheffield
• Dr. Paul G. Wolf