Pteridium aquilinum (L.) Kuhn, Bracken
Account Summary
Native, common, widespread and locally dominant. Circumpolar temperate.
1881; Stewart, S.A.; Co Fermanagh.
Throughout the year.
Introduction
Bracken is a serious opportunistic, invasive, colony-forming weed almost everywhere in Britain and Ireland, and indeed world-wide on under-used, under-managed or abandoned farmland (Cody & Crompton 1975; Taylor 1990). Current land-use changes in Britain and Northern Ireland induced by socio-economic and political forces, including the policies of 'set-aside' and 'Environmentally Sensitive Area' farmland designation, may very well end up promoting Bracken encroachment. Bracken is such a vigorous invader, persistent and so extremely difficult to eradicate, that its many researchers have formed an International Bracken Group, which regularly holds conferences and issues publications (Smith & Taylor 1986, 1995; Thomson & Smith 1990).
Taxonomy
P. aquilinum is world-wide in its distribution, but it is both genetically and ecologically variable and it is also phytochemically polymorphic (eg with respect to the levels of animal and plant toxins it contains), so that several different schemes of subdivision have been proposed involving subspecies (or species) and up to a dozen varieties (Tryon 1941; Cody & Crompton 1975; Page 1997).
Taken at its taxonomic widest, P. aquilinum has had a couple of hundred varieties and forms described (Hultén 1962, Map 131); but that way madness lies! In Britain and Ireland, what previously was regarded as the solitary species P. aquilinum has been subdivided by Page (1997) into two species, P. aquilinum and P. pinetorum, the former being further subdivided into three subspecies, and the latter (which is confined to scattered sites in Perthshire and Inverness-shire), into two subspecies. The New Flora of the British Isles (1997), on the other hand, takes a very much more conservative view of the variation, downgrading Page's species to subspecies, and regarding his subspecies merely as local ecotypes. We have not differentiated the subspecies of either of these authors in Fermanagh, but almost undoubtedly the Bracken in our area is P. aquilinum subsp. aquilinum, by far the most vigorous and abundant weedy form of the plant throughout the whole of the British Isles.
Growth form, size and timing
The thick black rhizome runs deeply buried, between 10 to 50 cm below the surface in well-drained acid soils (usually less than pH 6.5). The spreading rhizome branches and sends up to the surface the familiar annual branched aerial green fronds. The fronds vary greatly in height, but can be up to 2.5 m tall (a maximum of 2.75 m tall has been recorded (Step & Jackson 1945)), when developed over fertile deep soil. Often fronds are only half that height when the fern is growing on more nutrient-poor substrates.
The fronds begin poking above ground in mid-April, and when the hook-like croziers first appear they are very sensitive to frost and to damage by trampling (Grime et al. 1988). The fronds are slow to develop, requiring time for the great length of stipe and rachis to harden before the physical strain is put on them of the fully spread-out pinnae. In fact the fronds do not stop growing and using up the rhizome energy reserves until the latter half of July. This timing determines the optimum timing for cutting or the use of systemic herbicides for the eradication of Bracken, since the plant's reserves are then almost non-existent. The energy-containing products of photosynthesis are then being translocated down into the underground parts of the plant for storage, and the herbicide will be carried down into the rhizome with this flow of material.
Preferred habitats
Bracken is very frequent on hillsides, sloping, rocky waste ground, woods and roadside embankments. It is especially common on reasonably deep, well drained, acidic upland pastures, where it can produce fronds up to 2.5 m tall and become locally dominant. The plant does not have a high requirement for soil nutrients and the ideal habitat in Scotland was described as ground providing free-draining slopes of brown earth soils on sheltered, lower hill faces of glen sides. It may also occur on similar soils in glen bottoms, but only if the drainage there is unimpeded (MacLeod 1982).
P. aquilinum is much less common on base-rich or shallow substrates where its growth is often stunted. In general, Bracken avoids aquatic sites, shallow soils, poorly drained, deep, acid peatlands and densely shaded conifer plantations. It is also absent from well managed farmland, from frost hollows and from very exposed situations (Biggin 1982; MacLeod 1982).
While primarily a plant of woodland shade and open moorland pastures, Bracken is also common on rough grassland on waste ground and roadsides. Although P. aquilinum abhors waterlogging, it frequently occurs near waterways and on the drier margins of lakes and bogs at all altitudes, excepting the most exposed sites. In general, for reasons that are not at all clear, Bracken seems to frequent shade more often in lowland sites than it does in upland areas (Grime et al. 1988). It is also frequently found on cliffs, screes, limestone pavement and other forms of rocky ground. This includes artificial habitats such as old stone quarries and neglected areas of sand and gravel pits.
Soil preferences
As is the case with other primarily or mainly calcifuge plants, Bracken frequently occurs in limestone areas of Co Fermanagh. The explanation of this unexpected behaviour is that soils in the west of Ireland have been so thoroughly leached by prevalent heavy rainfall, acid peat often develops directly on top of calcareous rock. The suggestion has been made that P. aquilinum on limestone soils in places such as Andalucia in southern Spain, may differ to the extent of having only half the normal chromosome number (ie 2n= 52) (Molesworth Allen 1968). Studies on the soil preferences of ferns with respect to acidity carried out in W Europe by Koedam et al. (1992), found that P. aquilinum was 'acidiphilous' (acid-loving or acid-tolerant). In this study, soil samples taken from the fern root mass had a median pH of 4.0. On the other hand, Jonsell et al. (2000) suggest that in Scandinavian countries, Bracken is probably quite indifferent to lime.
Bracken litter ecology
Being a very large colonial, deciduous plant, Bracken has the potential to produce an enormous amount of leaf litter when the aerial shoots die off in late autumn. In communities where P. aquilinum forms virtually pure stands, litter production can be between 8,000 and 14,000 kg/ha/yr (Watt 1976). The rates of both litter accumulation and its decay depend very much on the particular habitat conditions, but in open heathland it can be the actual depth of litter (and the toxic and allelopathic substances it contains), that allows the species to dominate other plants. In deciduous woodland, on the other hand, litter decay is rapid and very little or none remains after just one year (Watt 1976).
An excessive accumulation of frond litter can prove detrimental to Bracken itself, so that it can fall victim to its own success. When this happens, the rhizome degenerates, gaps in Bracken cover occur, and this can allow colonisation by woody species and other plants (Watt 1976). It was his prolonged studies of Bracken and its competition with heathers which led Watt (1947, 1955), to the influential idea of 'cyclical succession' in vegetation, and to his describing the phases of Bracken 'sociology' in terms of a 'pioneer, building, mature and degenerate' phases that form this cycle (Watt 1964).
Between the extremes of litter persistence mentioned above, there are all degrees of dead frond litter accumulation. This phenomenon is often affected directly and indirectly by mans' interference through grazing, trampling, burning and cutting of the live or dead fronds (Watt 1976). From an ecological point of view it is probably wisest to consider Bracken as a pioneer species, that will most likely give way to colonising tree seedlings of species such as birch or oak in succession towards an eventual climatic climax forest vegetation. Within the process of succession, P. aquilinum occupies a niche as a stable, controlled, often very long-lived component species (Page 1986).
Fermanagh occurrence
In Fermanagh, Bracken shows a definite preference for acidic conditions, but it certainly is also frequent in limestone areas. This large, deciduous fern has been recorded in 303 tetrads, 57.4% of those in the VC.
Factors involved in the historic spread of Bracken
Studies in Scotland in particular suggest that colonisation by Bracken has greatly increased from about 1750 onwards, and that until at least the first three or four decades of the 20th century, farmers in areas of upland grazing noticed rapid invasion of their pastures. In this manner, Bracken has become a real menace, and one that continues to spread (Rymer 1976; Page 1997). P. aquilinum was probably originally a plant of open woodland and forest margin, occurring mainly on calcifuge terrain where a degree of shade and competition from other species kept its growth in check. Destruction of the woodland by man's activities, extending from the period of the Neolithic farmers onwards, has removed much of these constraints, allowing this vigorous rhizome-possessing species to form the extensive stands it occupies today on virtually all sorts of sloping rough ground (Rymer 1976; Page 1982(b); 1997, p. 361).
Apart from woodland destruction, numerous other factors are involved in Bracken spread. Probably most important has been the decline in rural (especially upland) human populations, and the consequent abandonment of areas of farmland previously cultivated or more or less intensively grazed. This would also account for ground where Bracken was previously cut, either for its eradication, or more purposefully for economically significant folk use of the plant (see below).
Subsidiary factors that have encouraged Bracken spread include the general move from cattle to sheep grazing, with a consequent reduction in trampling pressure; a rising rabbit population with its selective grazing pattern avoiding Bracken; poor understanding of heather burning regimes leading to a loss of heather cover; plus a general climatic rise in temperatures occurring over the last 250 years (Rymer 1976).
Colonisation of burnt ground
Bracken is an extremely invasive plant, and it is particularly so on burnt ground. The sporophyte rhizome and the sexual prothallus stage of the species are both very well adapted to rapidly colonise the almost-virgin ground of recently burnt areas in both woods and heaths, while surviving, competing plants have yet to recover from the effects of the fire. While the sporophyte plant shows a definite preference for calcifuge conditions, the prothallus stage rather unexpectedly is base-demanding. The potash and other minerals released by fire clearly provide particularly suitable conditions for rapid spore germination, and the released soil minerals promote the growth of both the Bracken gametophyte prothallus and the juvenile sporophyte arising from it after fertilisation (Conway & Stephens 1957; Page 1997, p. 362).
Grazing at either the pioneering or re-colonisation stage of vegetation development, especially close cropping by sheep, is capable of tipping the competitive balance between available species towards Bracken (Page 1982(b)).
The extent of Bracken in Britain
Estimates of the land area covered by Bracken in the United Kingdom of Britain and Northern Ireland range from 3,000-6,000 km2, with the most serious infestations in upland regions of the west and north (Fowler 1993). Undoubtedly the scale of the Bracken weed problem in Britain is large and the landscape and biological conservation implications are quite frightening. The fern causes problems for agriculture, forestry, conservation, shooting interests, recreation, health and water collection (Pakeman et al. 1994). Several surveys made between 1978 and 1990 estimate that Bracken dominates approximately 3,600 km2 of Britain's land area, representing around 1.5% of the total land cover. It is present, but not necessarily dominant, in around 17,000 km2, or 7.3% of British land cover, and there is a considerable risk of these figures increasing due to changes in both land management and climate (Pakeman et al. 1994).
Bracken continues to spread, and Page (1997) reported this occurring in Britain at a rate of 1-3% per year, which seems an alarmingly high figure. Very probably the move during the last 100 years away from cattle to sheep grazing in upland areas has contributed to the extension of Bracken on such pastures, since apart from the differing grazing pressure, the emerging fronds can more readily survive sheep trampling than that of heavier beasts (Step & Jackson 1945).
Control measures
Efforts to control Bracken largely consist of ploughing it in, regular cutting, crushing, or the use of herbicides, especially Asulam, which is Bracken specific. However, all of these methods of attack are expensive, labour-intensive and require safe access to the land by agricultural machinery. Since Bracken often infests steep or rocky slopes, aerial application of herbicide is often the only current option for control, making it both expensive and problematic, since such widely broadcast spray may well endanger other desirable or protected species. In addition, the problem remains that unless all of the rhizome buds are destroyed, the plant will survive and reappear at a later date when control measures are eased (Taylor 1990).
Rehabilitation of sites is a very important part of conventional Bracken control programmes and this greatly adds to the cost, particularly if fencing is required to prevent access by grazing animals (Fowler 1993). A programme of experiments on biological control using two moth species specific to Bracken imported from S Africa showed definite promise during testing under semi-natural conditions (Fowler 1993). However, at the eleventh hour funding was refused by government for field trials in Great Britain (Taylor 1995). Reviewed in a global context, Bracken is encroaching and not retreating where it occurs, and within the scope of current technology and economics, it is well nigh impossible to reverse, control or eradicate the plant (Taylor 1990).
Reproduction
Under dense woodland canopy Bracken fronds are quite often sterile. However, spore production is enormous in unshaded habitats, where a single frond is capable of producing up to 30 million spores (Conway 1957). Having said this, Bracken spore production is sporadic, development being affected by plant and frond age, degree of shading, exposure, weather conditions, perhaps soil characteristics, and by the genetic make-up of the individual. In most years spore output is generally poor, at least in Britain and Ireland, and some populations appear to be consistently sterile, even when others nearby spore copiously (Dyer 1990).
Despite the potentially enormous spore production, established stands of Bracken often reproduce exclusively by vegetative means due to the amount of frond litter they produce smothering the surface of the ground and preventing spore germination. Thus spores are probably only important in the colonisation of new sites on burnt or otherwise disturbed ground (eg, animal burrows, damp hollows and lime-rich cavities in rocks, old walls and rubble (Grime et al. 1988; Dyer 1990). More work is urgently required on the colonising potential of Bracken spores and the significance of Bracken spore banks in the soil.
Bracken nectaries
As long ago as 1877 Francis Darwin first observed that Bracken plants secrete sugars through numerous foliar nectaries found all over the plant, but especially on the under-surface at the junctions between pinnae and the rachis, and also in smaller amounts at the junctions between pinnules and the pinna midrib (Page 1982(c)). The size and prominence of the nectaries varies with the particular habitat occupied, being larger on plants growing in more open sites (Page 1982(c)).
The function of sugar glands like these in Bracken and other plants (including, worldwide, a few other unrelated ferns), has been a topic of debate since the early years of the 20th century. Some biologists suppose them to be purely excretory, while others believe them to attract pugnacious ants into a mutualistic relationship with the plant, whereby in return for a food reward they protect it from herbivore attack (Tempel 1983; Page 1997). The real question here is whether or not possession of such nectaries provides the species with an ecologically significant advantage over plants without such structures? Numerous studies carried out have found that it is not easy to answer this apparently simple question.
Bracken and ants
Experimental work in New Jersey by Tempel (1983) confirmed that Bracken nectaries are most active in the young expanding frond, and that they did attract ants. However, she also showed that mature plants continue to secrete small quantities of sugar, even though levels of ant activity sharply fell away on fully expanded fern fronds. Despite indications that Bracken is adapted to some form of mutualistic relationship with ants, Tempel concluded that no such interaction actually existed in her particular geographical region, since the ants in her study were non-aggressive and they did not protect the fern from herbivore damage. This still leaves a number of open questions relating to the significance of foliar nectaries on Bracken in Britain and Ireland, amongst which must be, is it a topic worthy of further study?
Toxicity
All parts of the plant, including its airborne spores, contain carcinogens as well as various other poisons. Some of these remain toxic after the plant has been cut and dried, so that it can be a danger to both man and livestock. The carcinogenic and immuno-suppressive effects of Bracken are an active area of medical research (Cooper & Johnson 1998). As with other plant toxins, the role of poisons in Bracken is to deter herbivores and inhibit the growth of neighbouring plants (ie it is allelopathic to potential competitors). The chemical armoury of Bracken is generally extremely effective in these respects. Allelopathic toxins are contained primarily in Bracken roots, rhizome and litter, and they are released into the soil environment to suppress the growth of associated plants (Gliessman 1976).
However, not all Bracken populations possess the full complement of animal deterrent toxins, and these plants may sometimes be detected by livestock and become heavily grazed. Sheep usually avoid Bracken, but if starving they will graze it and they can then become addicted to it. The same situation applies with horses. Another Bracken constituent is known to cause thiamine deficiency in non-ruminant animals such as horses and pigs (Cooper & Johnson 1998).
Other ill effects of Bracken in pastures include the shelter it provides for the sheep ticks that transmit Louping ill virus to both grouse chicks and sheep. Sheep ticks are also implicated in the transmission of Lyme disease to a range of animals including man (Fowler 1993).
Uses
Before the health risks inherent in handling Bracken were known, Bracken was collected and used in farms for animal fodder, bedding, kindling, thatch, compost, fertiliser (on account of its potash content) and as packing material for fruit. It was even used as human food: the rhizome contains a lot of starch, although in reality it tastes very astringent (Grieve 1931; Step & Jackson 1945; Rymer 1976). Young frond croziers were previously eaten in Japan like asparagus, while in Siberia and Norway expanded fronds were used in the past along with malt to brew some dreadful form of beer (Grieve 1931). Rhizomes were dried and powdered to make flour from which bread was baked either directly, or after mixing with wheat flour, a practice which was found in native cultures as far apart as New Zealand and Normandy (Rymer 1976).
The astringent properties of the rhizome also saw it being used to dress and prepare kid and chamois leather, but although this has been reported many times from Lightfoot (1777) onwards, we do not know where or when this was ever the case (Rymer 1976). The ash of Bracken contained enough potash to recommend its use in glass making, and it was also boiled with tallow and used as soap in parts of the East.
Bracken is a light and quick-burning fuel (making it a severe fire risk when it is abundant in or near recreational areas), and it produces a very violent heat (Rymer 1976). In many parts of the British Isles, it has been used in the past for burning limestone, for heating ovens used in baking and brewing, and for firing bricks. The cutting of more or less dead Bracken fronds for fuel, thatch, bedding, packing material or other purposes, had the effect of removing some, but not all, of the fern's frost-protective litter layer. Since it was gathered late in the season, however, this will have had relatively little ecological consequence on the performance or survival of the species.
A most informative, thoroughly researched review of the ethnobotany of Bracken has been published by Rymer (1976) and it is highly recommended reading.
Names
The origins of the plant's botanical names are quite fairly described as 'obscure'. The genus name 'Pteridium' is derived from the Greek diminutive of 'pteris', fern, from 'pteron', meaning 'a wing', 'winged', (ie 'little wing'), or 'a feather', an allusion suggesting that some fern fronds resemble a bird's wing (Stearn 1992). This notion, except with respect to size, fits the expanded Bracken frond really rather well. The English term 'fern' is similarly derived from the Anglo-Saxon 'fepern', meaning 'a feather' (Grieve 1931).
The Latin specific epithet 'aquilinum' means 'eagle-like', a notion reputedly suggested to the Swedish botanist, Carl Linnaeus, by the pattern of vascular bundles that is observed when the lower stipe is cut across obliquely, which supposedly resembles a spread eagle (Grieve 1931; Gilbert-Carter 1964). For those interested, this and other similar notions of the name origins are given by Step & Jackson (1945).
Folklore and folk medicine traditions are given full accounts by Grieve (1931, p. 305) and Vickery (1985, pp. 44-45). The most interesting and widespread folklore tradition is that Bracken, or its spores, confers invisiblity (Rymer 1976).
The English name 'Bracken' (sometimes 'Brecken') is the plural of 'Brake', and apparently is derived from the Old English 'bracu', possibly referring to something broken. This would be appropriate to the dead fern in winter, forming as it does a dense tangle of broken stems (Grigson 1974). It should be noted that the name 'Brake' or 'Bracken' was used in pre-scientific days for large ferns generally, and also more particularly applied to Pteridium aquilinum (Britten & Holland 1886). Another possible origin of 'Brake', 'Brakes' or 'Bracken' is suggested by Prior (1879), who derives it from the German 'Brache' or 'Brach-feld', meaning uncultivated land or land that is breakable, or open to tillage after a term of years, ie land that is not preserved as forest. This particular etymology suggests that Bracken has long been known as an active coloniser of abandoned arable land (Rymer 1976).
Threats
Reviewed in a global context, where Bracken already occurs it is encroaching and not retreating. Within the scope of current technology and economics, it is well nigh impossible to reverse the spread in these areas, or to control or eradicate the plant from them (Taylor 1990).