Pteridium aquilinum (bracken)

P. aquilinum is a cosmopolitan weed that readily spreads into pasture and marginal areas and is favoured by fire and soil acidity. Its presence reduces land productivity and adversely affects biodiversity. The plant is little affected by animals because of its toxicity. P. aquilinum is difficult to control particularly because of its ability to sprout from an extensive network of underground rhizomes and has large reserves of carbohydrate.

The genus Pteridium (Dennstaedtiaceae) comprises a few relatively stable morphotypes which are at least partially interfertile. Tryon (1941) recognized a single species with two sub-species, containing 12 varieties, and this scheme has been used throughout the world until recent taxonomic advances have made by using both morphometric analysis and DNA fingerprinting. Thomson (2000) reported that morphometric comparisons of frond material grown under standard environmental conditions and DNA fingerprinting by arbitrarily primed PCR were used to assess taxonomic groupings and relationships in the cosmopolitan bracken ferns of 11 of these varieties, excluding var. feei from Central America. This work resolved groupings corresponding to the varieties, var. africanum, var. arachnoideum, var. esculentum, var. latiusculum and var. revolutum from each other. The molecular analysis carried out by Thomson (2000) appeared to elucidate the genetic relationships and some origins of the various varieties studied, concluding that these results were consistent with those from the morphometric analysis, and that the varieties might best be raised in rank to species level. However, apart from some work in Australia in particular, the literature does not reflect the changes suggested by Thomson (2000), and indeed in many instances, such as for publications relating to Africa, the plant is simply referred to as P. aquilinum. Correspondingly, this datasheet employs the all-embracing P. aquilinum concept.

P. aquilinum is a polycarpic geophyte; a perennial fern which reproduces by spores and widely creeping, branching underground stems, sometimes forming colonies. The fronds arise directly from a deep underground rhizome that is much subdivided. The large compound leaves (fronds) are 0.3-1.3 m high, and 15-45 cm long, whereas both fertile and sterile fronds may reach a height of up to 2 m in the UK and can be much smaller under sub-optimal conditions. The leaf stalk, usually mistaken for the stem, actually is attached to the rhizome under the ground. The triangular deciduous leaves turn brown and die after the first autumn frosts in temperate climates, and the new ones arise each spring from the rhizomes. The leaf is divided into numerous segments (leaflets), each of which may be again divided or redivided, with the lowest segments three times compound. The clusters of spore cases densely line the inrolled edges of the underside of the leaves. The sporangium is aggregated into sori on the underside of the frond. Young fronds produce extrafloral nectaries (Grime et al., 1988; Duc et al., 2003).

This group of closely related subspecies probably has the largest world distribution of any plant taxa, occurring in much of the temperate and tropical regions on all continents as well as mainly oceanic islands. The extent of the native range is very broad in the general literature and countries in its native range are listed for Africa, Asia, Europe and North America in USDA-ARS (2003). There are very few reports of P. aquilinum as an introduced species. It can become extremely widespread and cover large parts of the landscape, for example, 8% of the area of Scotland, UK. However, there is much regional variation in the recognized types of cover (continuous versus discontinuous) (Birnie et al., 2000). Whether the species is weedy or invasive in each of the countries when known to occur is debatable, and as such has been left as 'unknown' in the Distribution table. It may, however, be considered invasive in at least some of these countries by using any of the definitions, and as such is classified as an invasive species in the Risk and Impacts table.

Little is known about the potential dispersal of the species but it is unlikely to have been intentionally moved by humans and if it has it would have been restricted to a few regions of the world. It is likely that some movement of the plant (rhizome) has happened via movement of soil. The only report of the species being exotic is from Jamaica and the Bahamas, and it would appear that it is only invasive in the former location (Kairo et al., 2003).

P. aquilinum is cosmopolitan in its distribution and there appears to be only a limited risk of further introduction.

Spores are principally wind-dispersed. There is no known natural dispersal of spores. Soil containing live rhizomes either attached to agricultural machinery or disposed of in landfill sites are possible means of introduction to new areas though no instances have been documented. Intentional introduction is a most unlikely pathway in view of the limited uses and no positive traits of this species.

P. aquilinum affect grasslands, forestry plantations, and some cultivated areas.

Whereas the genus Pteridium is readily distinguished from other fern taxa, separation of the various species/subspecies within the genus is difficult (Thomson, 2000).

Species of the genus Pteridium are seriously weedy in marginal land in many parts of the world, such as in the UK where P. aquilinum is particularly a problem in the uplands. In North America, Pteridium spp. occur readily in dry to wet forest margins and openings, peatbogs, logged areas and dry meadows from coastal to sub-alpine zones (Douglas et al., 1991). In the UK, it affects not only rough grasslands but also significantly impacts heather moorland (Birnie et al., 2000) but is also found in woodlands, wastelands, riverbanks and cliffs (Grime et al., 1988). In central Cameroon it is commonly found at the forest savanna boundary and is often associated with the invasive Chromolaena odorata (Youta-Happi, 1998). In much of the tropics, Pteridium spp. is common in some mountain areas and will become more dominant where fire occurs (D'Antonio et al., 2000; Wesche et al., 2000). P. aquilinum is found in a variety of sites in sun to partial shade and on soils that range from deep and rich to hard-packed or sandy. It is one of the first plants to colonize logged or cleared areas though it seldom persists in cultivated areas.

GeneticsThe chromosome number of P. aquilinum var. aquilinum is 2n=104 with a nuclear DNA amount of 12.8 pg (Grime et al., 1988). In his taxonomic re-assessment of the genus Thomson (2000) concluded that the DNA evidence suggested that morphotypes in Pteridium were "determined by specific qualitative and quantitative combinations of a limited number of highly conserved, additively assorted, genomic elements". It exhibits polymorphism for cyanogenesis (Grime et al., 1988). Morphological and morphometric analysis by Thomson (2000) elucidated some genetic relationships between varieties, including the distinguishing of an additional grouping of Atlantic Island (Azores, Madeira) and European brackens as an 'aquilinum complex' including var. aquilinum and a number of morphotypes recognised by C.N. Page and others at various taxonomic levels (near atlanticum, fulvum, pinetorum, osmundaceum). Also, he confirmed that var. yarrabense was a tetraploid hybrid (2n=208) of var. esculentum and var. revolutum, that at least those accessions of var. caudatum examined were tetraploid hybrids (2n=208) involving var. arachnoideum as one progenitor, and that the closest relatives of var. decompositum were var. latiusculum and var. revolutum; and provided evidence of close genomic relationships between var. latiusculum, var. pseudocaudatum and var. pubescens in North America. Thomson (2000) suggested that Tryon's varieties africanum, aquilinum, arachnoideum, decompositum, esculentum, latiusculum and revolutum might best be treated as species; pseudocaudatum and pubescens as varieties within latiusculum; yarrabense and caudatum (at least in part) as hybrids. Thomson and Alonso-Amelot (2002) have dealt with the taxonomic status and relationships of Pteridium caudatum in Central and South America. However, this datasheet does not separate the species as described by Thomson (2000) and covers all varieties within the over-arching P. aquilinum.Physiology and PhenologyIn temperate climates, fronds emerge from late spring onwards and persist until the autumn. Spores ripen in August to September and are shed in August to October in the UK. Under experimental conditions half of the spores take 4 days to germinate and germination is to some extent inhibited by darkness, and the prothallus will grow better in unshaded habitats (Grime et al., 1988). In Western Europe the shoots are copper brown during the dormant season and the fronds will be still standing for much of the winter and only gradually break up. The fronds will grow after the winter season whereas in tropical regions this will tend to occur after fires. Wynn et al. (2000) demonstrated significant differences in response to light and temperature regimes between four genotypes grown under experimental conditions. There is much variation in both the total dry mass per unit area (1.8-5.1 kg/m²) and the total rhizome dry mass (0.24-0.42 kg/m²).Pteridium spp. are to a large extent fire-resistant as the rhizomes send up new shoots after the old ones are burnt. In tropical regions the species is often referred to as a typical postfire successional species (Wesche et al., 2000) and after fire may form so-called 'bracken savannas' (Beard, 1953). However, in Sudan, it is absent from the most fiercely burnt hillsides (Jackson, 1956). In Hawaii, where it is uncommon, it is one of the few seasonal sub-montane native species to increase in burned compared to unburned areas (D'Antonio et al., 2000). Similarly, in much of the temperate zones it is known to be fire-responsive (Cwynar, 1987; Skre et al., 1998). Indigenous peoples such as the Maoris in New Zealand would appear to have been responsible for the increase of the fern through their use of fire (Germann and Holland, 2001).Reproductive BiologyUp to 30 million spores may be produced by a single frond and spore production tends to be greater in open habitats. Spores are wind-dispersed. Viable spores are often found in abundance within a soil profile, and a buried spore bank is suspected. Spores may remain viable for up to 10 years. Natural regeneration from spores may occur in spring and is mainly restricted to areas of disturbed or burnt ground. Expansion of established clones will be chiefly vegetative (Grime et al., 1988).Environmental RequirementsPteridium spp. are commonly found at varying altitudes. In the UK, P. aquilinum is found from sea level to 590 m but is more abundant in the uplands (Grime et al., 1988). In the Imatong Mountains of Sudan its altitudinal range is 1800-2600 m (Jackson, 1956) and up to 3200 m in Colombia (Missouri Botanical Garden, 2003). It prefers acid soil, tolerating soil pH of 3.0-7.6. However, in the UK, it is most frequent and abundant below pH 4.5 and particularly on deep soils. It is found on a range of shaded and unshaded habitats but grows best on productive brown-earths and more open habitats. Young shoots are very sensitive to frost and trampling by large mammals (Grime et al., 1988).AssociationsThe roots bear vesicular-arbuscular mycorrhiza (Grime et al., 1988). The extrafloral nectaries of young fronds provide food for ants and these may rid the plant of insect predators (Tempel, 1983; Grime et al., 1988). In Western Europe herbivory by large mammals such as deer, results in a reduction in many palatable species and with the expansion of P. aquilinum (Lameire et al., 2000; Kirby, 2001). In the UK, vegetational change from arable land to woodland over 100 years resulted in the unexpectedly disappearance of P. aquilinum once the woodland had become established (Harmer et al., 2001). Often there is little vegetation under the canopy of P. aquilinum, just a carpet of its litter. The plant is also considered to be allelopathic (Grime et al., 1988).

Although P. aquilinum is considered to be heavily defended from mammalian and insect predators (Grime et al., 1988), many insects have been recorded on the plant (Crane, 1990) though their effects on growth and survival are not known. A number of insects attacking P. aquilinum were identified from the UK (Lawton, 1976) New Mexico, USA (Lawton, 1982) and Brazil (Martins et al., 1995), some of which were investigated as potential biological control agents. Lawson (1976) lists 40 species of arthropods feeding on P. aquilinum in the UK and notes seasonal changes in relation to the plant's defense mechanisms against herbivory.

When mature and tough, fronds of P. aquilinum are poisonous to horses and cattle. The rhizomes are five times more toxic than the leaves, but are seldom eaten. Sheep have been poisoned experimentally, but natural poisoning is not common. The poison is cumulative over about 1 month for horses and 1-4 months for cattle before symptoms appear. Horses are usually poisoned by eating large amounts of contaminated hay, containing over 20% P. aquilinum, whereas cattle are poisoned by consuming an amount of green or dried leaves approximately equal to the animal's weight. In the UK, it is a weed of grasslands and forestry and is increasing in the uplands and is difficult to eradicate (Grime et al., 1988). It affects farmers in many parts of the world, with lost opportunity costs associated with the invasion of productive grazing land, veterinary costs associated with poisoning and tumours, ticks and associated disease problems and direct bracken control costs. In forestry plantations, control is often required during the establishment phase (Pakeman et al., 2003).

In terms of conservation, P. aquilinum often has little biodiversity interest and in Britain it has generally replaced habitats of greater importance (Pakeman et al., 2003).

The plant is toxic to livestock and humans (Grime et al., 1988; Crane, 1990). The impact on human health is difficult to quantify, but Pakeman et al. (2003) consider that it could be locally important. There is unproven speculation that drinking water taken from catchments that are predominantly covered with P. aquilinum may contain carcinogens or other toxins that are harmful to human health.

In New Zealand, the Maori ate the starchy, below-ground parts of the fern and used fire as an aid for hunting and to promote regrowth of this edible resource (Germann and Holland, 2001). Faust (2002) has reviewed the ethnobotany of the species in the Yucatan Peninsula. In early spring, the young unrolled leaves and tender leaf stalks may also be cooked as a vegetable, and are consumed in Thailand and Japan, even though they contain carcinogens (USDA-ARS, 2003). In the UK, it used to have some economic importance as a source of fuel, thatch, bedding, compost, food and potash (Grime et al., 1988). It is occasionally used for sheep bedding in Wales to this day, though in small amounts to avoid poisoning of stock from consumption, due to beliefs concerning its possible ethnoveterinary effects. Potential uses of P. aquilinum in organic agriculture in the UK are being considered (Donnelly et al., 2002) and it is being promoted as a component of peat-free growing media (Pitman and Webber, 1998). There are numerous uses in folkloric medicine (USDA-ARS, 2003).