Heath and moorland

The terms “heath” and “moor” apply to extensive areas of land that are uncultivated and virtually treeless, with poor, acid soil—the soil tends to constitute a peat layer overlying sand or gravel. Plants of the heath family (Ericaceae) usually dominate the vegetation. They are found throughout the world usually in arctic, montane, and temperate regions with moderate rainfall and dry summers.

One of the most striking features of this type of environment is the limited number of species on such large tracts of ground. There are exceptions, however, such as the species-rich fynbos heath of the Cape in South Africa. In most areas, few species can adapt themselves to such harsh conditions, exposed to strong winds, great fluctuations in temperature, poor soil, and outbreaks of fire.

Heather (Calluna vulgaris), frequently dominates heaths together with members of the Erica genus. The tough evergreen leaves of these plants have xerophytic qualities, one of which is % that the margins roll down to protect the stomata on the underside. These plants have a special relationship with little insects called thrips (order Thysanoptera) that live in the flowers, where they mature. When the winged females fly out to another flower in search of a mate (the males are wingless), they carry pollen on them that is brushed off onto the second flower, and thus cause cross-pollination. In cold climates where bees are rare, these insects are invaluable as pollinators, although heather does also rely on the wind.

Adaptations to water stress

The winds and the sun tend to cause rapid evaporation of water from the exposed surfaces of the plants. The roots, therefore, have to absorb water from the soil to replace this loss. But the soil may be too dry to allow the water supply to be restored to its correct level. To counteract this problem, the leaves of these plants have developed various adaptations to reduce the rate of transpiration (these plants are known as xerophytes).

The leaves of heath plants are usually narrow and needlelike, as in bell heather (Erica cinerea), or reduced to spines, as in furze (U/ex europaeus). This shape reduces the transpiring surface to a minimum. The stomata, through which most water is lost, are usually confined to the lower surface of the leaves; in some species they are protected by hairs, which surround them and trap water vapor, and in others by inwardly rolled leaf margins. This rolling may be permanent, as in the heathers (Erica spp.), or temporary, as in many grasses, which flatten out their leaf blades when the water supply improves. These adaptations protect the stomata from drying winds and reduce water loss through transpiration.

Other water-conserving adaptations of plants in these habitats include leaves with thick cuticles, often densely crowded together and thickly covered with hairs underneath. The twigs may also be hairy.

Surprisingly, some xerophytic and xero-morphic heath plants grow in peaty, waterlogged conditions where the need for water conservation is not immediately obvious. But roots cannot survive in permanently waterlogged conditions because the soil contains little oxygen. A plant with a poor root system has restricted water uptake and, therefore, needs to control water loss.

Cushion plants and those that form mats on the ground, such as the wild azalea (Loi-seleuria procumbens), can trap warm air among their leaves, which encourages growth. Their shape also reduces water loss and desiccation.

Nutritive adaptations

Heathland soils are poor in plant mineral nutrients, being characteristically “podsolized” (iron and humus in the upper layers are leached out by rain and carried down to the lower layers), it is thought that the sclerophyllous leaves (toughened by sclerenchyma tissue) of some xerophytic heath plants, particularly those in Mediterranean climates, may develop in nutrient-poor habitats as a response to low nutrient availability. They may also be evergreen, with a high ratio of cellulose to nitrogen. By being evergreen, they maximize the total amount of photosynthesis per unit of nitrogen over the whole life of the leaf.

Many of these plants depend on rain for a large part of their mineral nutrient requirements, but also have mycorrhizal fungi associated with their roots, such as ling (Calluna vulgaris) does. In this plant the fungus penetrates the cortical cells of the root and is known as an endotrophic mycorrhiza. Some plants, such as bog myrtle (Myrica gale), obtain nitrogen from symbiont bacteria with which they are associated; these bacteria (for example, Rhizo-bium) fix nitrogen from the atmosphere.

Plants such as common dodder (Cuscuta epithymum) and greater broomrape (Oroban-che rapum-genistae) are parasites that obtain their nutrients directly from a host plant. They have no chlorophyll and so cannot manufacture carbohydrates. Instead they take all their requirements from their hosts. Common dodder is often found attached to the stems of heather, furze, and wild thyme (Thymus sp.).

Another group of heath plants, which in Europe includes eyebright (Euphrasia officinalis) and heath lousewort (Pedicuiaris syivatica), are partial parasites manufacturing their own carbohydrates, but taking mineral nutrient and water from the roots of grasses.

The common dodder (Cuscuta epithymum) on heaths is frequently found attached to furze (Ulex euro-paeus) from which it feeds as a parasite. Dodder has no chlorophyll—instead, rootlike structures (haustoria) penetrate the stem of the host plant and extract mineral nutrients from it. Eventually the host plant dies.

Fire

Because of the dryness of vegetation in summer and the open nature of the environment, fire is a frequent occurrence, but the plants have adapted to withstand its effects. The species of Erica are particularly resilient and grow new shoots soon after fire, from the old stem bases; the seeds, produced in large quantities, germinate freely after a fire. Those plants with underground storage organs, such as ling (with rhizomes), can die back but grow again quickly. For the same reason, bracken, furze, and purple moor grass (Molina caeruiea) are also fire-tolerant.

Mediterranean-type scrub, such as chaparral (A), shares with heaths a dependence on fire. Dominant plants, such as chamise (Aden ostoma fasciculatum), and species of Arctostaphy/os and Caenothus outlive many of the surrounding plants, which, however, have set seeds that lie dormant. These species also inhibit the growth of herbaceous plants by producing phenolic toxins, which seep into the ground around them. Fires occur naturally every 15 to 20 years (B), fueled by the dead branches of the chamise and its resinous leaves. They burn off the toxins and cut back the spreading chamise, which survives as underground stems. Fire also induces germination of dormant seeds of other plants. For a few years an herbaceous layer establishes itself (C), until the sprouting chamise starts to accumulate and exude its toxins once more.