Bracken (Pteridium aqulinum) has expanded its range due to human activities, and is now considered a weed in many places.
Global distribution
Bracken is one of the most widely-distributed plants in the world, and occurs on every continent except Antarctica, as well as isolated oceanic islands such as Hawaii. Throughout its range a number of subspecies have been identified, and in some cases (such as in Australia and New Zealand) these have been proposed as separate species, but there is not general agreement on this. Bracken occurs at elevations from sea level to more than 3,000 metres, with the higher elevations occurring in the tropics, where the temperatures are comparable to those at temperate latitudes. It is absent from desert areas, the highest parts of mountains and the far north (eg Iceland, Faroe Islands, Greenland, northern Scandinavia), where the cold prevents its growth. Within its range, bracken has expanded its extent in many places because of human activities that encourage its spread.
Bracken is also a very ancient plant, with fossil records indicating that it dates back 55 million years, and that by 24 million years ago it had a worldwide distribution.
Distribution in Scotland
Bracken occurs in all parts of Scotland, including the mainland, the Hebridean Isles and Orkney and Shetland. It has been recorded growing at elevations up to 600 metres in the Cairngorms, and occurs in a wide variety of habitats, including woodlands, heaths, grasslands and maritime communities. It grows on well-drained soils, so is often found on slopes, and never occurs on marshes or boggy ground.
Bracken is a fern in the Dennstaedtiaceae family, the members of which are characterised by their large, highly-divided fronds. As a fern, it is a Pteridophyte – the group of plants that have vascular tissues, but which reproduce by means of spores rather than seeds. They also have a two-stage life cycle, alternating between tiny plants (usually less than 1 cm. in size) that produce eggs and sperm for sexual reproduction, and larger plants, which grow after fertilisation has occurred. When they are mature, these larger plants produce the spores, from which a new generation of the small plants grow, and so the cycle continues.
Bracken, as it is commonly seen, is the large plant part of this life cycle, and it consists of individual fronds up to 2.5 metres tall that grow from an underground spreading rootstock, known as rhizomes. It is a perennial deciduous plant, with the fronds dying back every autumn, and new ones growing from the rhizomes in May each year.
The new fronds emerge above ground with their growing tips bent over in a hook-like shape. As each frond grows, the individual leaflets, or pinnules, gradually unfurl from their initial tightly-curled shape that is known as a fiddlehead (after its resemblance to the curved end of a violin) or a crozier. The young growing fronds are covered in reddish-brown hairs, and are relatively soft and vulnerable to frost at this stage. When the fronds are fully unfurled, after a few weeks, the stems harden up and become quite fibrous.
The upper part of each frond is triangular in shape and has several lateral ‘branches’ or blades on each side. Each side branch has further blades branching off it, and those in turn are comprised of a linear pattern of small leaflets or pinnules. On the main stem, at the base of each side branch there is a round green raised area, known as a nectary. These nectaries secrete a sugary liquid, which attracts wood ants (Formica spp.) and other insects such as beetles, wasps and sawflies.
The fronds are green in colour, and they become quite tough and somewhat leathery when they are mature. The edges of each pinnule curl downwards slightly and underneath them is a line of sori, the structures that produce the spores. These mature after 6 to 8 weeks, and are released from the end of July until early October. The spores are microscopic in size and are produced in vast numbers – in one study, a single frond 1.73 metres tall was estimated to produce 300 million spores. Dispersal of the spores is mainly by wind, although some also get transported by water. Once they are in contact with soil, the spores can germinate straight away, but some survive the winter and germinate in the spring. However, very few of the spores that are produced succeed in germinating and growing on to become new plants.
In September, shortening days and colder temperatures trigger the withdrawal of chlorophyll from the fronds, which turn bright yellow and then brown, as they die off for the winter. The dead fronds collapse on the ground, forming a natural mulch that is slow to decay, and may still be present 12 months later, when another year’s fronds accumulate on top of them.
In general, reproduction from spores is relatively uncommon, and bracken’s main method of expansion is vegetative, through lateral spread of its rhizomes. These grow at depths of 50 cm. or occasionally 100 cm. in the soil, and the expansion of bracken by rhizome spread along a front has been recorded at up to 1.27 metres in a year. In well-established patches of bracken, the rhizomes can become so dense that there are more rhizomes than soil in a given area. The rhizomes contain many more buds on them than actually develop into fronds, but if a frond is damaged (eg by frost) or broken, then another bud will grow to become a new frond. This ability enables bracken to regenerate well and persist in areas where it occurs.
Because it has replaced woodland in many areas and provides shade, bracken can support some woodland flowering plants under its canopy. These include bluebells (Hyacinthoides non-scripta), chickweed wintergreen (Trientalis europaea) and common dog violet (Viola riviniana). The combination of bracken and dog violets growing together on sunny, south-facing hillsides is an important habitat for the pearl-bordered fritillary (Boloria euphrosyne), a butterfly that is included on the UK’s Biodiversity Action Plan list, and some other fritlllaries. However, if the bracken becomes denser, the ability of flowering plants to flourish underneath it diminishes and the diversity of species decreases.
Due in part to its rhizomatous method of spread, bracken can invade grassland and abandoned farmland, and will also replace heather in moorland areas, if there is heavy grazing or excessive burning. Once it is established, bracken can reduce the ability of other plants to grow by shading, smothering with its dead fronds and the production of toxic compounds in the soil.
A number of fungi have been recorded on bracken, mainly on the dead fronds, and one species (Camarographium stephensii) that grows on the dead stems is host-specific to bracken, meaning that it occurs on bracken and nowhere else. However, in common with most ferns, bracken is quite resistant to disease, and few fungi grow on the living plants. Despite bracken’s abundance, there is a comparatively small range of insects that feed on it. The caterpillars of a few moth species feed on the fronds, and those of the brown silver-line moth (Petrophora chlorosata) are restricted to bracken. Other caterpillars that feed regularly, but not exclusively, on the fronds include the broom moth (Ceramica pisi) and the small angle shades moth (Euplexia lucipara), while caterpillars of the gold swift moth (Hepialus hecta) and the map-winged swift moth (Hepialus fusconebulosa) feed on the rhizomes.
A short-winged bug (Ditropis pteridis) is a specialist feeder on bracken, and the common froghopper or spittle bug ( Philaenus spumarius ) feeds on bracken as well as many other plants. A fly (Chirosia grossicauda) induces galls on bracken that make the tips of the pinnule curl or roll up, and the little black pudding gall-midge (Dasineura filicina) induces galls on the pinnules that resemble small black sausages. Several species of sawfly have been recorded on bracken, with the most common ones being Strongylogaster multifasciata and Aneugmenus padi.
Bracken fronds are frequently utilised by various insects, including the golden-ringed dragonfly (Cordulegaster boltonii), for either basking or hunting. Bracken is also used by sheep or deer ticks (Ixodes ricinus), as a place to wait for passing animals or people to brush against, thereby enabling them to gain access to a host to suck blood from.
Because its fronds contain toxic compounds, bracken is rarely eaten by mammals such as red deer ( Cervus elaphus ) and sheep, and this is one reason for the expansion of its range. However, wild boar ( Sus scrofa ) will dig up and eat the rhizomes, thereby providing a natural control to bracken’s spread. Uneaten rhizomes that are exposed by the boar’s disturbance of the soil are vulnerable to frost.
Bracken has been used by humans for a long time – for bedding for animals, as a fertiliser, and as food. In Japan the fiddleheads are still eaten, after boiling to remove the toxins. Human activities have enabled this natural part of the forest ecosystem to expand its range and as a result bracken is now considered to be a weed in many places.
