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First Posted: Aug 6, 2013
Aug 6, 2013

Bracken Fern (Pteridium aquilinum)Poisonous to Horses

Image: Wikipedia/GNUFree Documentation License/Kenraiz
Bracken Fern

Scientific classification:
Kingdom: Plantae
Division: Pteridophyta
Class: Pteridopsida
Order: Dennstaedtiales
Family: Dennstaedtiaceae
Genus: Pteridium

Species:
Pteridium aquilinum
Pteridium caudatum
Pteridium esculentum
Pteridium latiusculum
and about 6-7 other species

Bracken (Pteridium) is a genus of large, coarse ferns in the family Dennstaedtiaceae. Ferns (Pteridophyta) are vascular plants that have alternating generations, large plants that produce spores and small plants that produce sex cells (eggs and sperm). Brackens are noted for their large, highly divided leaves. They are found on all continents except Antarctica and in all environments except deserts, though their typical habitat is moorland. The genus probably has the widest distribution of any fern in the world. In the past, the genus was commonly treated as having only one species, Pteridium aquilinum, but the recent trend is to subdivide it into about ten species. As ferns, brackens do not have seeds or fruits, but the immature fronds, known as fiddleheads, are edible. The word bracken is of Old Norse origin, related to the Swedish word bräken, meaning fern.

Description and Biology

Evolutionarily, bracken may be considered one of the most successful ferns. Bracken has a similar genetic make-up to heather, a plant typically found in moorland environments, and is commonly referred to by local populations in the north of England as 'Moorland Scrub'. It is also one of the oldest ferns, with fossil records over 55 million years old having been found. The plant sends up large, triangular fronds from a wide-creeping underground rootstock, and may form dense thickets. This rootstock may travel a metre or more underground between fronds. The fronds may grow up to 2.5 m (8 ft) long or longer with support, but typically are in the range of 0.6-2 m (2-6 feet) high. In cold environments, bracken is deciduous and, as it requires well-drained soil, is generally found growing on the sides of hills. ...

Distribution

Pteridium aquilinum (bracken or Common bracken) is the most common species with a cosmopolitan distribution, occurring in temperate and subtropical regions throughout much of the world. It is a prolific and abundant plant in the moorlands of Great Britain, where is limited to altitudes of below 600 metres. It does not like poorly drained marshes or fen. It has been observed growing in soils from pH 2.8 to 8.6. Exposure to cold or high pH inhibits its growth. It causes such a problem of invading pastureland that at one time the British government had an eradication programme. Special filters have even been used on some British water supplies to filter out the bracken spores. NBN distribution map for the United Kingdom.

Bracken is a characteristic moorland plant in the UK which over the last decades has increasingly out-competed characteristic ground-cover plants such as moor grasses, cowberry, bilberry and heathers and now covers a considerable part of upland moorland. Once valued and gathered for use as animal bedding, tanning, soap and glass making and as a fertiliser, bracken is now seen as a pernicious, invasive and opportunistic plant, taking over from the plants traditionally associated with open moorland and reducing easy access by humans. It is toxic to cattle, dogs, sheep, pigs and horses and is linked to cancers in humans. It can harbour high levels of sheep ticks, which can pass on Lyme Disease. Grazing provided some control by stock trampling but this has almost ceased since foot and mouth over ten years ago. Global climatic changes have also suited bracken well and contributed to its rapid increase in land coverage.

Bracken is a well-adapted pioneer plant which can colonise land quickly, with the potential to extend its area by as much as 1-3 % per year. This ability to expand rapidly is at the expense of other plants and wildlife, can cause major problems for land users and managers. It colonises ground with an open vegetation structure but is slow to colonise healthy, well managed heather stands.

The biodiversity that depends on these uplands is very special and very rich. Many of the species only occur on upland moorland, tied to features unique to the habitat. The loss and degradation of such areas due to the dominance of bracken has caused many species to become rare and isolated.

Fungi Associations

Woodland fungi can be found growing under the bracken canopy, for example Mycena epipterygia. Both Camarographium stephensii and Typhula quisquiliaris grow primarily from dead bracken stems.

Other Plant Associations

Allelopathy: Bracken fern is known to produce and release allelopathic chemicals, which is an important factor in its ability to dominate other vegetation, particularly in regrowth after fire. Its chemical diffusions, shady canopy and its thick litter inhibit other plant species from establishing themselves - with the occasional exception of plants which support rare butterflies. Herb and tree seedling growth may be inhibited even after bracken fern is removed, apparently because active plant toxins remain in the soil.

Brackens substitute the characteristics of a woodland canopy, and are important for giving shade to European plants such as common bluebell and wood anemone, where the woodland does not exist. These plants are intolerant to stock trampling. Dead bracken provides a warm microclimate for development of the immature stages. Climbing corydalis, wild gladiolus and chickweed wintergreen also seem to benefit from the conditions found under bracken stands. The high humidity helps mosses survive underneath including Campylopus flexuosus, Hypnum cupressiforme, Polytrichum commune, Pseudoscelopodium purum and Rhytidiadelphus squarrosus.

Animal Species That Use Bracken

Brackens of the northern hemisphere are used as food plants by the larvae of some Lepidoptera species including Dark Green Fritillary, Dot Moth, High Brown Fritillary, Gold Swift, Map-winged Swift, Pearl-bordered Fritillary, Orange Swift, Small Angle Shades, Small Pearl-bordered Fritillary. They also form an important ecological partnership with plants such as violet and cow-wheat (Melampyrum pratense) for various Boloria Fritillary species.

It is also a favoured haunt of the sheep tick Ixodes ricinus which can carry lyme disease.

Between 27 to 40 invertebrates (including nine moths) in the UK feed on bracken. These include the sawfly, a planthopper (Dytroptis pteridis), the map-winged swift moth caterpillar, brown silver-line moth caterpillar (Petrophora chlorosata) and Paltodora cytisella. The numbers feeding on the bracken increase as the season progresses due to the decreasing levels of toxin, and the production of nectaries in the spring, food for ants which in turn may kill any herbivorous insects in the vicinity.

Where bracken is dominant it excludes most specialist heathland/moorland bird species of conservation concern, although there are a few species that may benefit from a certain proportion of bracken. Deeper bracken stands provide a good food site for many resident or breeding birds, such as threatened ground-nesting species Skylark, Yellowhammer, Curlew and Lapwing. Some British birds such as the Whinchat and the Nightjar use bracken as their preferred habitats. The Nightjar may lay its eggs on the bare ground under the bracken. The Skylark often nests in bracken and uses it for cover. Small stands of bracken provide nesting, feeding and roosting habitat for a variety of smaller birds, including the Willow Warbler (which will also use bracken to construct its nest), the Tree Pipit, the Yellowhammer, the Ring Ouzel, the Woodcock and the Twite.

These stands also give cover, especially during the nesting season, from predators such as birds of prey and crows; and from free-ranging dogs and users straying off the paths which, usually unintentionally, disrupts nesting and can identify the nest site to predators. On heavily used spaces, this may be an important protection. The European adder can be found basking on bracken, the colour of their skin concealing them. ...

Poisoning

The plant is carcinogenic to animals such as mice, rats, horses and cattle when ingested, although they will usually avoid it unless nothing else is available. Young stems are quite commonly used as a vegetable in China, Japan and Korea. However, some researchers suspect a link between consumption and higher stomach cancer rates. The spores have also been implicated as a carcinogen. Danish scientist Lars Holm Rasmussen released a study in 2004 showing that the carcinogenic compound in bracken, ptaquiloside or PTQ, can leach from the plant into the water supply, which may explain an increase in the incidence of gastric and esophageal cancers in bracken-rich areas.

In cattle, bracken poisoning can occur in both an acute and chronic form, acute poisoning being the most common. In pigs and horses bracken poisoning induces vitamin B deficiency. Poisoning usually occurs when there is a shortage of available grasses such as in drought or snowfalls. Along with the DNA damage caused by ptaquiloside it is shown that chemicals in the fern can damage blood cells and destroys thiamine (vitamin B1). This in turn causes beriberi, a disease linked to nutritional deficiency. ...

Control

Some small level of scattered cover can provide beneficial habitats for some wildlife, at least in the UK (as given above). However, on balance, removing bracken encourages primary habitats to re-establish, which are of greater importance for wildlife. Control is a complex question with complex answers, which need to part of a wider approach. Management can be difficult and expensive; plans may need to be about cost-effective, practical limitation and control rather than give an expectation for eradication.

All methods need follow up over time; starting with the advancing areas first. Given the decades elapsed to arrive at the current levels of coverage on many sites, slowing or reversing the process will be also be of necessity be long-term, with consistency and persistence from all parties being key.

Various techniques are recommended by Natural England and the RSPB to control bracken either individually or in combination RSPB Bracken management in the uplands. Cutting - once or twice a year, repeatedly cutting back the fronds for at least 3 years:

  • Crushing/rolling - using rollers, again for at least 3 years
  • Livestock treading - during winter, encouraging livestock to bracken areas with food. They trample the developing plants and allow frost to penetrate the rhizomes. In May and June, temporary close grazing or mob stocking on small areas away from nests, particularly using cattle, horses, pigs or ponies may crush emerging bracken fronds resulting in reduced bracken cover. Sufficient fodder will be required to prevent livestock eating the bracken. This may suit steep areas where human access is difficult and herbicide undesirable.
  • Herbicide - Asulam (also known as Asulox)is selective for ferns; Glyphosate is not; but the latter has the advantage that the effects can be seen soon after application. They are applied when the fronds are fully unfurled to ensure that the chemical is fully absorbed. Rare ferns such as Adder's Tongue, killanery and lemon-scented ferns can also be found in similar habitats and it is important that these are not destroyed in the process of bracken control.
  • Natural England recommends that only Asulam can be sprayed aerially, Glyphosate requires spot-treatment e.g. using a weedwiper or knapsack spray. The toxicity of Asulam is low and has been generally highly cost-effective but its use is now restricted by the EU after 2012; at least until specific registered uses can be defined.
  • Selective sprays like Starane, Access, Metsulfuron 600WG etc. work well but only if sprayed in late autumn so the rhizomes store food for winter and hence absorb the poison.
  • On archaeological sites, chemical control is usually required as mechanical methods may cause damage. Allowing plants to grow in its place, e.g., the establishment of woodland, causes shade that inhibits bracken growth. In the UK, trees, notably rowan, have done well since grazing reduced greatly after the foot and mouth in 2000 but young saplings struggle in high bracken. In decades to come, tree shade cover may increase, if permitted, and so may reduce bracken growth but this is both long-term and in some cases is contentious in the change it would bring to traditionally open heath or moorland; both aesthetically and as a valuable habitat.
  • Burning - useful for removing the litter, but may be counter-productive as bracken is considered to be a fire-adapted species.
  • Ploughing - late in the season followed by sowing seed.

Any bracken control programme must be completed, otherwise bracken will re-establish.


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