Pteridium aquilinum

Pteridium aquilinum (L.) Kuhn
(Latin: aquilinus, of the eagle, eagle-like)

Local names: bracken fern, pasture brake

Terrestrial, often in large colonies; rhizomes deeply buried, long-creeping, with hairs but not scales; leaves monomorphic, deciduous, scattered along the rhizomes, to 2 m long; leaf blades broadly triangular to triangular- lanceolate or triangular-ovate or even pentagonal in outline, usually of 3 main divisions, each division 2-pinnate/pinnate-pinnatifid, the pinnae rigidly herbaceous to subcoriaceous, with glands (extrafloral nectaries) at base of some pinnae, glabrous to pubescent; ultimate leaf segments (= smallest subdivisions of leaf) with margins entire; sori marginal, linear, continuous, covered by a false indusium formed by the recurved margin of the ultimate leaf segments and an obscure inner, delicate, true indusium; 2n = 104 (Thomson et al. 2008). Open woods, wooded slopes, pastures, thickets, often in sandy soils; widespread in e 1⁄3 of TX with populations of a different subspecies in the Trans-Pecos; predominantly n hemisphere, throughout most of the U.S. and s Canada; also Mexico s to Central America as well as Africa, Asia, and Europe. Sporulating Jun–Nov. Treated broadly (e.g., Jacobs & Peck 1993), this variable species, with a number of infraspecific taxa (depending on the source followed), is virtually worldwide in distribution (all continents except Antarctica) and is the most widely distributed fern. It is considered by some to be the most widespread of all vascular plants (with the exception of a few human dispersed annual weeds) (Page 1976). Recent new evidence, based on morphology, cytology, and molecular studies, suggests that it is perhaps better divided into 5 species, with P. aquilinum circumscribed to include only the diploid “predominantly Northern Hemisphere (Laurasian) lineage” (Thomson et al. 2008). Even in this restricted sense it is still extremely widespread, occurring from North and Central America to Africa, Asia, and Europe. It grows aggressively, and its tenacity is shown by regeneration through several meters of volcanic ash on Mt. St. Helens in WA within 1–2 years of the volcanic eruption (Woodland 1997). The species has a deep rhizome, making it well-adapted to fire prone habitats and one of the first species to reappear following a fire (Nelson 2000). In places in the n U.S. “the rhizomatous brackens are the dominant understory, covering thousands of square kilometers. Some clones may be hundreds or even thousands of years old and cover a hectare or more, but because they lack secondary growth, their age is difficult to ascertain” (Wagner & Smith 1993). Bracken fern can sometimes have profound ecological impacts because of its abundance; for example, Pakeman et al. (1996) estimated that in the United Kingdom, Pteridium occupies an area of around 1.7 million hectares, which represents 7% of the total land surface. Bracken fern has a variety of defense mechanisms, some of which have important implications for both animals and humans. For example, in the British Isles it is the cause of “bracken staggers” or “bracken poisoning,” a potentially fatal condition in livestock. Symptoms include a hemorrhagic syndrome, chronic hematuria (“red water”), bone marrow depression, fever, neurological symptoms and staggering, retinal degeneration (“bright blindness”), and cancer. Plant defense chemicals/toxins reported include an enzyme (thiaminase), which can cause fatal thiamine (Vitamin B1) deficiency in livestock, a glycoside (ptaquiloside—mutagenic and carcinogenic), and a cyanide-producing glycoside (prunasin). Ptaquiloside and possibly other carcinogens can be passed to humans via cow’s milk. In Asia, particularly Japan and Korea, people consume the fiddleheads as a cooked vegetable and in bars as a salty snack food (Hodge 1973; Moran 2004). However, the fiddleheads unfortunately have high concentrations of ptaquiloside and have been suggested as a cause of stomach and esophagus cancer. Because it is both carcinogenic and mutagenic, bracken fern should not be eaten. Another chemical defense employed by bracken fern is the production of phytoecdysones, a class of hormone-like compounds that disrupt ecdysis (= molting) in insects—insects eating the plant thus have their developmental sequence altered. Extrafloral nectaries are yet another possible defense mechanism exhibited by bracken fern—these leaf-borne nectar-producing glands attract ants which under some circumstances may protect the plant by deterring herbivores (Power & Skog 1987; Cooper-Driver 1990; Rashbrook 1992). Bracken fern is also known to be allelopathic (= the chemical inhibition of one species by another), with toxins leaching from the tissues adversely affecting surrounding plants—e.g., in the western U.S. the toxins can interfere with tree and shrub regeneration after fires or logging and are also known to inhibit the growth and development of potentially competing herbs. The growth of seedlings may be inhibited even after bracken fern is removed, apparently due to toxins remaining in the soil. In some places bracken fern is such an effective competitor that it can become dominant and disrupt the procession of ecological succession (Gliessman & Muller 1978; Mabberley 1987; Turner & Szczawinski 1991; Moran 1993a; Foster & Caras 1994; Weathers 1998; Bruneton 1999; Smith et al. 2000; Burrows & Tyrl 2001; Frohne & Pfänder 2005; Yatskievych & Windham 2008b; Robinson et al. 2010).

subsp. latiusculum (Desv.) Hultén
(Latin: latis, wide, breath, broad, and the diminutive -culum, = somewhat broad),

Local names: eastern bracken fern

Is known from AR (all 75 counties—Peck 2011a) and LA just to the e of the state border and could possibly be found in TX. It can be distinguished from subspecies pseudocaudatum and pubescens by its central pinnae borne at 40º to 60º angles to rachis, its pinnules borne at 40º to 70º to costa, the terminal segments of pinnules only 2–4 times longer than wide, and the segment margins and lower surface of rachises and costae shaggy but the false indusia glabrous. [P. aquilinum subsp. latiusculum (Desv.) W.C. Shieh, P. aquilinum var. latiusculum (Desv.) Underw. ex A. Heller, P. latiusculum (Desv.) Hieron. ex Fries, Pteris latiuscula Desv.] This subspecies is widespread in the e ½ of the U.S. and adjacent Canada as well as n Mexico. Speer et al. 1998[1999] suggested that latiusculum and pseudocaudatum (as varieties) were “geographically overlapping bracken taxa” that “encompass a single uninterrupted gene pool.” Based on DNA studies, Speer (2008) argues that pseudocaudatum and latiusculum “are not two separate taxa, but a single bracken variety” with northern and southern morphotypes. It is thus likely if an individual of the morphology of subsp. Latiusculum is discovered in TX, it will ultimately not be recognized taxonomically. Jim Peck (pers. comm.) notes that “in AR they are morphologically distinct, with subsp. latisusculum more common in the north and subsp. pseudocaudatum more common to the south.”

subsp. pseudocaudatum (Clute) Hultén
(Greek: pseudo, false, and cauda, tail or tail-like appendage, this taxon resembling var. or subsp. caudatum, now often treated as P. caudatum)

Local names: tailed bracken fern, pasture brake, bracken fern

Leaf blades, rachises, and costae with lower surfaces glabrous or with sparse hairs; pinnules with terminal segments much elongated, 6–15 times longer than wide; false indusia glabrous. Open woods, pastures, thickets, often in sandy soils; widespread in Pineywoods and n Gulf Prairies and Marshes w through Post Oak Savannah (e.g., Anderson Co., C.M. Rowell s.n., Sep 1986; Gonzales Co., W.L. McCart 5870, both BRIT), also Red River drainage w to Grayson Co. (S. Crosthwaite, pers. comm.); MA s to FL, w to IN, MO, OK, and TX (more abundant in the se U.S.; Thomson et al. 2008). Sporulating Jun–Nov. [P. aquilinum var. pseudocaudatum (Clute) A. Heller, Pteris aquiline var. pseudocaudata Clute] It has been suggested that the “more skeletonized” leaf blade of this subspecies may reduce evapotranspiration and thus be “of advantage in the semixeric pinelands of the south-eastern coastal plain” (Speer et al. 1998[1999]).

subsp. pubescens (Underw.) J.A. Thomson, Mickel & Mehltreter
(Latin: pubens, downy or softly hairy)

Local names: hairy bracken fern, downy bracken fern, western bracken fern

Leaf blades, rachises and costae typically with lower surfaces with dense covering of contorted, spreading hairs; pinnules with terminal segments not greatly elongated, only ca. 4 times longer than wide; false indusia with hairs at least on margins and sometimes also on surface. Open wooded slopes, near streams, disturbed areas; Jeff Davis (Mt. Livermore, Davis Mts.—Palmer 32010, TEX-LL; Palmer 1927) and Brewster (Rosillos Mts.) (Yarborough & Powell 2002) cos.; widespread in w ½ of the U.S. (predominantly w of the Rocky Mts., but extending to their e slopes) and adjacent Canada with disjunct populations e as far as MI and w TX; also Baja California, Mexico (Thomson et al. 2008). [P. aquilinum var. pubescens Underw., Pteris aquiline var. pubescens (Underw.) Clute] Though rare in TX, this is the commonly found subspecies in much of the w U.S.

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