Very common on wet ground in the Highlands, bog myrtle has a distinctive sweet smell and supports many insects.
Bog myrtle is distributed throughout western and northern Europe, from Portugal, Spain, Britain, Ireland and the Netherlands to Scandinavia. It also occurs in France, Germany, Poland and across the Baltic states of Estonia, Latvia and Lithuania into eastern Russia. It is distributed throughout much of northern North America, including Alaska, all of Canada, Washington, Oregon and from Minnesota eastwards to New England in the USA, as well as the states of Kentucky and Tennessee. A subspecies (Myrica gale ssp. tomentosa) is found in northern parts of Japan, the Korean peninsula, Sakhalin Island and eastern Siberia.
Bog myrtle occurs throughout much of Scotland, particularly in the west, from Dumfries and Galloway in the south, to the far north of the mainland, and including the Inner and Outer Hebrides and some parts of Orkney. It is most abundant in the Highlands, where its habitat requirements of wet, acidic soils are more ubiquitous. In Aberdeenshire and Angus the distribution is sparser, and in the Central Belt and Borders there are a few scattered populations, where the soil conditions are suitable.
Bog myrtle has not been assessed for its conservation status at an international level, so it does not feature on the IUCN Red List of Threatened Species. As a widespread and common species it is unlikely to be at risk, and is not considered to be of conservation concern.
Bog myrtle, which is also known sometimes as sweet gale, is a woody deciduous perennial shrub in the Myricaceae family. It can grow from about 60 to over 150 cm tall, but seldom reaches that height in Scotland today because of its palatability to large herbivores such as red deer (Cervus elaphus). With a spreading, multiple-branching habit and the ability to reproduce by suckering, it occurs in dense clusters or thickets in areas of wet, boggy ground.
The leaves, which emerge from their buds in May each year, are 2-6 cm in length, pale green in colour, and are narrow and tapering at the base, widening towards an oval tip. They are slightly toothed at the outer end and turn a dull yellow in October, before being shed for the winter. The buds for the next year’s catkins are quite visible after the leaves have fallen.
A key feature of bog myrtle is the presence of nitrogen-fixing nodules on its roots. These are formed through a symbiotic partnership with filamentous bacteria in the genus Frankia, and bog myrtle is defined as an actinorhizal plant because of this. Such plants characteristically grow in nitrogen-poor environments – wet, boggy ground in the case of bog myrtle. Through their ability to absorb and fix nitrogen, actinorhizal plants play a crucial role in enriching soils and enabling the process of ecological succession to take place, with more nutrient-demanding species able to grow subsequently. In this, bog myrtle performs a very similar function to the alder tree (Alnus glutinosa), which has nitrogen-fixing nodules from a related bacterium (Frankia alni) on its roots.
Bog myrtle flowers appear just before the new leaves in spring, and the species is usually dioecious, meaning that male and female flowers occur on separate plants. However, in a given population, there are likely to be some monoecious plants (with single individuals having both male and female flowers) and hermaphroditic plants, where the flowers themselves exhibit both male and female characteristics. The male flowers or catkins emerge from upright, orange-brown buds that are about 1 cm in length, and have distinctive bracts or scales in between the pollen-producing stamens. Female flowers are smaller and have several red tufts on them, and look somewhat similar to the female flowers of hazel (Corylus avellana). Typically, many more male flowers occur in a population than females, making bog myrtle unusual in this regard. Pollination is by the wind, and female flowers go on to produce knobbly fruits that are yellowish-green and clustered together on the plant’s stem. Both the leaves and the fruit are covered in yellowish glands that secrete an aromatic oil, giving the plant a characteristic sweet smell when the leaves are rubbed or crushed.
Although bog myrtle produces seeds, which are dispersed by water, its main method of reproduction in Scotland is by suckering. New plants grow up from special roots, called rhizomes, of an existing plant, and this gives rise to thick clumps, with many individuals occurring closely together. The rhizomes are woody and act as nutrient stores for the plant during the winter.
Bog myrtle supports almost 70 species of phytophagous, or leaf-eating, insects, including a large number of moth caterpillars. In some parts of Scotland, the large invertebrate biomass on bog myrtle has been shown to be beneficial to black grouse (Tetrao tetrix) – a rare species that is a priority for conservation in the UK, and whose chicks depend on invertebrates as food for the first three weeks of their lives.
Notable moths whose larvae feed on bog myrtle include the argent and sable (Rheumaptera hastata), Rannoch brindled beauty (Lycia lapponaria), ringed carpet (Cleora cinctaria), great brocade (Eurois occulta) and sweet gale (Acronicta euphorbiae), which is named after the plant. All of these feed on other plants as well, but bog myrtle is an important food source for them.
The leaves of bog myrtle are mined by the caterpillars of two micro-moths (Coleophora lusciniaepennella and Bucculatrix cidarella), and by the larvae of two beetles (Ramphus pulicarius and Rhynchaenus iota). Nymphs (as the immature forms are known) of the common froghopper (Philaenus spumarius) feed on a wide variety of plants, and are frequently seen on bog myrtle in the Highlands. They surround themselves with white foam (known as cuckoospit) created by blowing air through the waste liquid excreted from the sap they suck, and this provides protection from predators and parasites.
Bog myrtle is host to several scale insects, including the nut scale (Eulecanium tiliae) and the bog myrtle aphid (Myzocallis myricae), which sucks the sap of the plant.
Little is known of any mycorrhizal relationships between bog myrtle and fungi, and there are no large fruiting bodies or mushrooms associated with the species. However, several fungi fruit on the leaves and twigs, including twig stunt (Ramularia destructiva), a fungus (Incrucipulum sulphurellum) which produces tiny cream-coloured discs with fringing hairs, and another (Ciboria acerina) that fruits on overwintered catkins as small brown cups.
A number of mammals feed on bog myrtle, including the mountain hare (Lepus timidus), red deer, feral goats (Capra hircus) and domestic sheep (Ovis aries). In North America, the Canadian beaver (Castor canadensis) feeds on the plant and uses cut stems as building material for its lodges and dams. If the European beaver (Castor fiber) is reintroduced to Scotland it is likely it too will use bog myrtle.
In traditional folklore bog myrtle has long been considered a good repellent against insects such as the Highland biting midge (Culicoides impunctatus) and this has recently been confirmed through scientific tests of the oil extracted from the plant.
Alan Watson Featherstone
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