Heather (Calluna vulgaris) is widespread in Scotland and famed for turning hillsides purple in August, but its abundance is artificially maintained by human management.
Heather occurs widely throughout Europe, from the British Isles and Spain east to the Ural Mountains in Russia, and from northern Scandinavia to Sardinia in the Mediterranean Sea. It also occurs in Iceland, Turkey, the Azores, Madeira and the mountains of northern Morocco. In moorland and heathland areas throughout its range it is often the predominant vegetation, covering extensive tracts of land. Heather has been introduced to other parts of the world, including Canada and parts of the USA, as well as Australia and New Zealand, where it has become a problematic invasive weed, out-competing native plants.
Distribution in Scotland
Heather is one of the commonest and most widely distributed plants in Scotland, occurring in all parts of the country, and in a range of habitats, including open woodland on acid soils (for example the native pinewoods of the Caledonian Forest), dunes, moors, wet heaths and bogs. It grows from sea level to elevations of about 1,000 metres in the mountains, and, as heather moorland, is the most abundant vegetation community in large parts of the upland areas.
However, the prevalence and predominance of heather today is, to a considerable extent, a human-created condition, resulting from past deforestation. Following the removal of the trees, the subsequent management of the land through burning (eg to maintain heather moorland for grouse shooting) or grazing, has prevented the return of the forest. This is known as arrested succession, in which the natural process of ecological succession, whereby one group of plants or vegetation community is naturally replaced by another, is halted by human intervention. Although this has created large extents of heather moorland, it has also kept heather out of some areas, for example in the headwaters of the Affric River watershed. There, intensive grazing has maintained the presence of grasses, and prevented the natural succession to heather and then trees.
Heather, or ling as it is also known, is a small bushy shrub in the heather family, Ericaceae, and is the only species in the genus Calluna . It is woody-stemmed and evergreen, and typically grows up to 60 cm. tall, although in good conditions it can reach 100 cm., or exceptionally 120 cm., in height. Each shrub has multiple branches, which are often intertwined. The branches can take root individually at their bases, and have numerous short side shoots. The leaves are 1 – 3 mm. long, stalk-less and grow in four vertical rows on the branches. They are tightly packed together on the side shoots, but more openly spaced on the main branches. The leaves are scale-like in appearance, with the edges curling in, and are dark green in colour, with reddish-brown tips when they are new.
Heather can live for over 30 years, and its lifecycle is defined in four phases. The first, pioneer phase, which lasts for 5 – 6 years, covers the period from germination to the development of a fully-established bush, when growth takes place vertically and heather still co-exists with a variety of other plants. The second, building phase, continues until the plant is about 15 years old, in which it grows laterally, forming a dense canopy, and patches of heather exclude all other species. The third, mature phase, which lasts until about 25 years, sees the plant continuing to grow, but less vigorously and the centre of the bush becomes more open and mosses grow underneath, aided by the humidity provided by the canopy of branches. In the final, degenerate phase, the growth of new shoots declines and the central branches collapse. The canopy of heather branches becomes more open, enabling other vascular plants and new heather seedlings to become established, as the original plant declines and eventually dies. In most situations in Scotland, young pioneer trees, such as birch (Betula spp.) and rowan (Sorbus aucuparia) would grow at this stage, in the process of natural succession that would see heather moorland replaced by forest.
Heather begins flowering in July, reaching a peak in August, when whole hillsides turn purple from the colour of the blossoms. Some flowers persist into September or even early October. The flowers are bell-shaped, and consist of four petals that are fused at the base, forming a tube-like shape that is about 2 mm in diameter, and grow on leafy spikes or stalks that are 3 – 13 cm. in height. They are pinkish-purple in colour, although plants with white flowers do occur occasionally. Flowering is prolific, with each plant producing several thousand individual blossoms, and this, together with its abundance in moorland, is what enables heather to colour whole landscapes.
Pollination is mainly carried out by insects, with bumblebees (Bombus spp.) and, where they occur, honeybees (Apis mellifera) being the most effective polllinators. Other insects, including butterflies and moths (Lepidoptera) and flies (Diptera), also act as pollinators, and experiments where insects have been excluded from flowering heather have shown that some pollination by wind occurs as well.
After pollination, the petals turn brown and an enclosed capsule forms, which contains the ripening seeds. This is pulled from the downward-pointing position of the flowers into an upright posture by the shriveling of the petals. Heather seeds are tiny, measuring 0.5 by 0.7 mm., and up to 30 occur in each capsule. A mature plant produces over 100,000 seeds per year, and overall seed production for areas dominated by heather has been estimated at over one million seeds per square metre. The seeds can remain viable for up to 100 years, and are dispersed mainly by wind, although animals brushing against the plants also act as carriers for them. The empty seed capsules persist on the plants through winter into the following spring.
One of the reasons credited for heather's success as a species is that it has a mycorrhizal relationship with a fungus (Rhizoscyphus ericae). In this mutually-beneficial relationship, the hyphae of the fungus wrap around and interpenetrate the roots of heather, and an exchange of nutrients takes place. The fungus aids the heather by providing minerals and water it accesses in the soil, while in return the plant provides sugars that are produced through the process of photosynthesis in its leaves.
Because of its abundance, heather forms a food source for a large number of organisms. The majority of these are insects, including beetles, flies and bugs, and other invertebrates such as spiders. The most important invertebrates, in terms of biomass, are the larvae of Lepidoptera – mainly moth species. More than 100 species of moth caterpillars have been recorded feeding on heather in the UK, and some of the most abundant ones include the ling pug (Eupithecia goossensiata), common heath (Ematurga atomaria) and the northern eggar (Lasiocampa quercus callunae). Research has shown that the density of caterpillars on heather is usually 10 – 15 per square metre, but can be as high as 50 per square metre.
Amongst the beetles feeding on heather is a leaf beetle (Altica ericeti) that has been found in Glen Affric, and has, on occasion, caused considerable damage to heather moorland in England. The most notorious beetle associated with heather though is the 6 mm. long heather beetle (Lochmaea suturalis). The larvae and adults of this species both feed on heather, ring-barking the stems and killing the plants. Significant outbreaks can lead to the death of large areas of heather, although in time the beetle populations are reduced by natural predators, which include the hieroglyphic ladybird (Coccinella hieroglyphica), a parasitic wasp (Asecodes mento) and a parasitic fungus (Beauveria bassiana).
A number of different fungi are involved in the decomposition of dead heather, including the horsehair parachute fungus (Marasmius androsaceus) which grows on dead heather stems and leaves. Small cow-wheat (Melampyrum sylvaticum), a UK Biodiversity Action Plan species, is semi-parasitic on a number of plant species, including heather.
Heather is also important as a food source for a variety of bird species, including the red grouse ( Lagopus lagopus scoticus ), which feeds on the shoots, flowers and seeds. Black grouse (Tetrao tetrix) also feed on heather in winter, and their chicks and those of the capercaillie (Tetrao urogallus) feed on moth larvae in the first weeks of their lives that are collected from heather and other plants.
However, the most significant ecological relationship affecting heather now is that with humans, who manage large areas of heather moorland to maintain populations of game birds such as red grouse. As a result of this, heather-clad landscapes will continue to predominate in many parts of the upland areas into the future.