Calluna vulgaris (L.) Hull, Heather or Ling
Account Summary
Native, very common, frequently dominant. European boreo-temperate, but also widely naturalised beyond its native range.
1881; Stewart, S.A.; Co Fermanagh.
Throughout the year.
Growth form and preferred habitats
This evergreen subshrub is the characteristic dominant species of the W European Lowland Heath Zone vegetation communities which were delimited by Gimingham (1972, p. 10, Fig. 2). The extent to which Ling is often dominant is illustrated by Webb's (1986) statement that, "the ecology of Calluna is, to a large extent, the ecology of the European heathlands".
Ling is generally associated with rather better drained areas of oligotrophic, strongly acidic, nutrient-poor bogland, heaths, upland woods and moorland. However, it also turns up in much drier, but still acidic, nutrient-starved, canopy gap situations, including locally on some of the wooded islands of Lough Erne and Lough Melvin. The shrub also colonises very shallow raw humus formed directly over hard, crystalline limestone in open situations, eg above Florencecourt and also in the Burren, Co Clare (H9). This latter situation allows C. vulgaris, normally considered a strongly calcifuge ('lime hating') species, to grow intermingled with definite calcicole ('lime loving' or, rather better, 'lime tolerant') species, in an exposed, low-growing community. This fact is indicative of the strong leaching of soils associated with very high rainfall levels in Western Ireland.
The characteristic high atmospheric humidity throughout the whole year associated with a cool, mild, damp Oceanic or Atlantic type of climate is highly significant, permitting Ling to survive on very shallow peaty soils. The one thing that all ericaceous species cannot tolerate, however briefly, is for their roots to completely dry out − and indeed this absolute drought restriction also applies to other ericoid shrub species, including those grown in gardens (Gimingham 1960, 1972, p. 11).
Variation
Considering the wide geographical and ecological range and the abundance of the species in many of its habitats, apart from the normal expected plastic variation in size and performance associated with favourable and unsuitable environments, Calluna vulgaris is incredibly constant in its growth form, to the extent that, apart from over 1,000 selected mutant cultivars of garden importance maintained by the horticultural trade, taxonomists recognise only one named wild variety, a densely downy grey form, well named 'var. hirsuta S.F. Gray' (Gimingham 1960; Griffiths 1994).
Growth and vegetative reproduction
Although C. vulgaris is a shallowly rooting shrub and does not possess a rhizome or stolons, the procumbent lower branches produce adventitious roots and layer themselves on damp soil or peat, enabling a very limited amount of annual lateral spread by vegetative means. Aerial stems are sclerophyllous, possessing evergreen, overlapping scale leaves on long and short shoots, the former of which branch very frequently producing a dome-like growth form in vigorously growing 'building' plants. Leaves persist for a year on long shoots, but up to three years on the dwarf side shoots.
Individual unmodified C. vulgaris shrubs are surprisingly short-lived for an evergreen woody species, normally reaching maturity between 11 and 15 years of age (Marrs 1986). However, shrubs often begin to senesce and gradually lose physiological vigour when they are over 15 years of age, although they can survive for 25, or perhaps very exceptionally for up to 30 years. As they age, plants become increasingly woody and growth greatly slows, the dense crown of the young shrub gradually opening up as the branches diverge and becoming more and more 'leggy', typically forming a central gap which exposes the rather deep layer of litter accumulated beneath the shrub canopy. Watt's (1947) original description of the growth cycle of the heather plant forming four characteristic stages, ie pioneer, building, mature and senescent, and their properties with respect to light, litter, soil moisture, pH, competitive relations with neighbouring species, are summarised by Gimingham (1972) and more concisely by Webb (1986) and by Grime et al. (1988). These references should be consulted for more detail on the morphological and physiological properties of the species during its growth cycle. However, see below for limits on the application of the ecological concept of 'cyclical vegetation succession'.
Flowering reproduction
From July to September, C. vulgaris flowers freely and densely on the current year's long shoots, with zones of short or dwarf shoots produced both below and above them (Gimingham 1960, 1972; Mohamed & Gimingham 1970). Pollination can be achieved by wind, bees and other large insects, or by tiny (1 mm long) slender insects called thrips that live their lives in and on the flowers. Depending on circumstances, the flowers, which offer nectar and perfume to attract visitors, are pollinated by pollen from the same or other flowers, but direct contact between the anthers and stigma of an individual flower is impossible. From a genetic point of view, however, the pollination of C. vulgaris in many cases must be regarded as autogamy, and the vast superabundance of pollen released is easily appreciated by anyone who has walked through a stand of flowering heather and got it coating their trouser legs (Hagerup 1950a; Proctor et al. 1996).
Seed dispersal and behaviour
Huge quantities of very small, wind-dispersed seed are produced from September onwards, some of which, given damp, open soil, may germinate in the following two months, or else in the following spring. The deep litter layer beneath heather bushes is not conducive to germination and seedling establishment of C. vulgaris, or indeed of any ericaceous species, but disturbance of the litter layer exposing bare soil or peat and, especially, the disturbance associated with fire which releases essential plant nutrients, greatly assists the process of Ling regeneration from seed. The remainder of the seed produced becomes part of the vast soil seed bank of the species, in which individual seed may persist for over 40 years (Gimingham 1972; Webb 1986, p. 110).
Fermanagh occurrence
Calluna vulgaris is very widespread in Fermanagh, occurring in 303 tetrads, 57.4% of those in the VC. This demonstrates just how much acid, peaty or podsolic ground of poor fertility there is across the county!
British and Irish occurrence
Calluna vulgaris is the most common and widespread member of the Ericaceae in both Ireland and Britain. The New Atlas species map indicates it being recorded in almost 86% of the hectads in these islands and on this rather crude scale of presence measurement, it is well ahead of the second ranking ericoid species, Erica tetralix (Cross-leaved Heath) represented by map symbols in a mere 70% of hectads (Preston et al. 2002).
Habitat loss and species decline
Having said this, there has been a decline in suitable heath, bog and moorland habitats throughout these islands over the last 60 years affecting all the characteristic bogland species, so that the percentage of hectads in the New Atlas with the most recent date class (1987-99) has dropped substantially for all ericoid species. The respective figures for C. vulgaris and E. tetralix are 79% and 62%.
The pressures on these infertile habitats have come from a very wide variety of changes in the countryside and its management. The primary influences come from increasing forestry, intensification of agriculture − including movement between arable and pastoral that depend upon regional climate.
Additionally there has been over-grazing by sheep, abandonment of previously worked land and consequent recolonisation by scrub and trees, and change of land use for building, or for mineral or peat extraction (Tudor & Makey 1995; M.C.F. Proctor, in: Preston et al. 2002). It is estimated that around 18% of heather dominated moorlands in Britain have been lost over the last 70 years, and considerably more than this since the late 18th century (Webb 1986, p. 183; Simmons 2001).
Since about the early 1980s there has been a growing awareness in B & I that C. vulgaris and other ericaceous subshrubs are not only losing ground through the landscape developments mentioned above, but are also suffering reduction in their cover values and competitive dominance, even in areas of extensive heathlands that have been managed for long periods and remain unthreatened by changes in development or land use. Indeed, it is becoming apparent that the contraction of heather species is not a new phenomenon, as the palaeoecological evidence from peat studied suggests that the species losses have been operating over a prolonged period, one measured not in decades, but in centuries (Webb 1986).
In addition to the pressures already mentioned, there are fears that long-term global climatic warming and increasing levels of nitrogen oxides pollution might already be adversely influencing the extent of heather-dominated vegetation directly through changes in the productivity of moorland soils and vegetation, and indirectly through increased opportunities for cropping and the extension of the grazing season. A survey in England and Wales found that there was approximately 44,000 km2 of land with suppressed heather having less than 25% subshrub cover. Suppressed heather is defined as that suffering from, and showing growth forms affected by heavy grazing, neglect or inappropriate management (Bardgett et al. 1995). Although such heaths are damaged, they are potentially capable of recovery. In 1995, the Government introduced a subsidy Moorland Scheme, designed to regenerate moors with more than 25% heather present, by restricting sheep stocking density − in effect an upland equivalent of the Environmentally Sensitive Area scheme Fermanagh farmers enjoyed for some years (Simmons 2001, p. 280).
European and world distribution and recent changes
The range of C. vulgaris covers most of N, W & C Europe stretching from Iceland and N Scandinavia southwards throughout the continent, but reaching only as far as Morocco and the Azores. It is most abundant in the moist, mild, chiefly lowland, heathland region on the western seaboard of the continent, while in the much drier climate of the Mediterranean basin, the species becomes limited to more humid parts of the coasts and mountains of NE Spain, S France, N Italy and W Dalmatia. Corsica is the only Mediterranean island on which it is naturally found. Eastwards the species stretches in a very disjunct manner into C Russia and China and it is also a scattered introduction in E & W North America and in New Zealand (Hultén & Fries 1986, Map 1447).
In Europe, in common with B & I, the species has suffered a decline in habitat area and local abundance during the last five decades or more due to pollution and, particularly, the effect of atmospheric nitrogen deposition, which reaches a critical point of heathland species tolerance at 15 kg nitrogen per hectare. Beyond this threshold, grasses such as Deschampsia flexuosa (Wavy Hair-grass) and Molinia caerulea (Purple Moor-grass) replace ericaceous subshrubs in dry and wet conditions respectively (de Smidt 1995).
Origin and history of Calluna vulgaris heaths and bogs: If we consider present day heathland and many areas of bogland in the longer term, measured in thousands of years, they almost certainly originated following disturbance of woodland by early farmers (Mitchell 1986; Simmons 2001). While this is definitely the case for most lowland inland heaths, especially those in S England, a growing body of opinion believes that at least some ancient heaths and bogs may represent 'natural' climatically governed climax vegetation communities, rather than degraded woodland or nutrient-depleted previous Neolithic farmland, an example of the latter being the Céide Fields of West Mayo (H27), an archaeological site in coastal W Ireland.
Palaeoecological evidence in Britain points to the occurrence of Calluna and Empetrum species communities somewhat comparable to those of today (opinions differ as to the degree of similarity), as long ago as 10,000 BP (Birks 1988), and maybe up to 1,000 years earlier than this in W Ireland during the peak in Betula pollen at the end of the Early Post-glacial (Mitchell & Watts 1970; Pilcher & Hall 2001). Having said this, the period of extensive typical blanket bog development in Ireland did not occur until a great deal later: in the Killarney area, Co Kerry (H2), this occurred only around 3,000 years ago (Jessen 1949; Mitchell & Watts 1970).
Different patterns of geological prehistory, topography, changing land use (including grazing pressure and other factors exporting plant nutrients), plus soil and vegetation development process obviously existed around these islands. The type and age of heath or bog vegetation communities that have formed are in response to such factors, as well as to overarching dynamic climatic conditions measured over many centuries. Information on all of this must be collected, examined and integrated in order for us to interpret any particular local instance of current vegetation on the ground. It is therefore unrealistic to generalise too widely on this matter, especially when we are dealing with a species like C. vulgaris which possesses a broad ecological amplitude, and which occupy differing roles in numerous habitats and their plant communities.
This does not prevent us from drawing broad regional distinctions, as has been done for Britain between upland and lowland, and between wet northern and western situations, contrasted with drier eastern and more southerly heather moorlands (Nolan & Robertson 1987; Thompson & Miles 1995). Many wet heaths possibly date back to the early post-glacial and some of them may have natural, rather than anthropogenic origins (Gimingham 1995). On the other hand, fire (naturally occurring or managed by man) and grazing might have tipped the balance from woodland trees towards dwarf shrubs, sedges, heaths and blanket bogs during climatic conditions differing from today.
If the change in vegetation type towards heathland was initiated by man, do we really want to preserve and maintain the status quo? Conservation is necessary for us to appreciate the complex processes of vegetation change, since this should enable the development of practical guidance and intervention skills in order to better manage diversity in related vegetation types in future.
Natural change in heather-dominated vegetation
In terms of our understanding of dynamic vegetation processes, the cyclical succession of species described by Watt (1947, 1955) for heather-dominated vegetation, involving the age of stands and eventual opening of the canopy to permit seedling establishment, probably applies better in wet, more species-rich conditions, compared with drier, more productive heathlands. This type of heather regeneration cycle might not occur at all in some Scottish and Irish moorlands, simply due to a lack of sufficient viable seed (Gimingham 1995; Thompson & Miles 1995; Welch et al. 2000). Heather-dominated ground below the tree-limit, (wherever that might lie in our much modified landscape), would naturally be colonised by sapling trees, especially birch and rowan, but possibly also by oak or by introduced species such as pines and larch (Khoon & Gimingham 1984).
The use of fire to prevent tree invasion and to regenerate heather and other dwarf shrubs and maintain them in the building phase of their age structure has been widely practised as a management tool for heathlands over many generations. The frequency and severity of burns must be carefully related to prevailing grazing pressure and, if unskilfully executed, firing could have contributed to the gradual loss of heather cover across B & I throughout the last 200 to 400 years now observed from maps and from ground, aerial and satellite surveys. Khoon & Gimingham (1984) found birch was virtually excluded by dense C. vulgaris stands in the building and mature phases of its growth cycle maintained by carefully controlled regular burning of heath vegetation. However, occasional fires were followed by slow or patchy heather regeneration and this provided ideal conditions for birch invasion and establishment.
The need is obvious for more research on patterns of heath and moorland change and the relationships between vegetation types, including a re-examination of past results for possible causal factors which could then be field tested to discover which communities are the most stable when left alone, ie the most nearly 'natural' under the conditions of minimal or nil interference. When stable or almost stable unmanaged heathland stands are found, they tend to gradually develop into uneven-aged clones of C. vulgaris forming a mosaic pattern of the different growth stages described by Watt (1947, 1955). Even here, a minimal level of intervention will eventually be required, involving some use of grazing, burning, occasional herbicide treatment, or manual removal of taller woody invaders in order to maintain the vigour of heather species and prevent succession to woodland (Marrs 1986).
Cyclical succession in heathland − does it happen?
Reappraisal of the model of cyclical succession involving Calluna vulgaris and other small-scale heathland species proposed by Watt (1947, 1955) suggests it is an oversimplification. The basic assumption of long term heathland stability seems something of a myth. Yet the model has been useful in approaching the interpretation of C. vulgaris dominated heaths with respect to a wide range of environmental influences and pressures and it remains a helpful tool for this purpose, provided we accept its limitations (Gimingham 1988). For example, if we consider grazing pressure, Erica cinerea (Bell Heather) is less productive and more unpalatable to browsing animals than C. vulgaris, but it is more palatable than E. tetralix, so in vegetation containing any mixture of these common heath subshrubs, a browser ranking order exists whenever the most favoured species becomes scarce. This needs to be appreciated and applied in the context of the dynamic instability of the particular heath under review, its history of past management (essentially the pattern of grazing and burning) and the current spectrum of environmental pressures it exists under.
The growth phase patches of the ericaceous shrub species present often comprise a mosaic-like pattern, in which dominance may temporarily, and perhaps very locally, switch between one heather plant clone and another. This will depend upon their relative age, individual history of shoot damage and the different species inherent physiological vigour and morphological powers of recovery. However, other stands may prove to be more even aged, and the older these are, the more vulnerable they become to catastrophic death events, since natural senescence affects the physiological ability of the subshrub to recover after any form of damage. Thus Watt's model of cyclical change in heather-dominated ground may or may not operate, or may do so to an extent which perhaps is geographically (or climatically) determined. Operation of cyclical processes is especially doubtful where larger areas of ericaceous shrub vegetation suffer damage at the same time, eg the catastrophic effects of a severe out-of-season frost, drought, insect attack, disease, major landslip, or an excessively hot fire event. Under this stochastic, less predictable or more randomly occurring event scenario, the more death suffered by C. vulgaris and its subshrub associates, interrupting or cutting short their endogenously governed life-cycles, the greater the acceleration of directional succession leading to their replacement by either birch woodland or acidic grassland (Marrs 1986).
Foliage damage
C. vulgaris growing in wet sites is commonly observed to suffer winter damage known as 'frosting', although in fact it is not directly caused by low temperatures, but rather it is drying damage, due to the low levels of atmospheric humidity the plant encounters during cold windy spells of weather. This 'frosting' effect undoubtedly reduces the productivity of mature evergreen plants, and it may kill off or adversely affect seedlings and young plants growing in bare, unshaded and, therefore, unsheltered patches (Loach 1968; Webb 1986, p. 111). Similar foliage damage, reddening and sometimes killing the plant may occur during summer drought, although this does not happen very often in the mild, damp climate of the west of Ireland.
Names and uses
The genus name 'Calluna' is from the Greek 'kalluno' meaning 'to cleanse', alluding to the fact that the branches of Ling and of numerous less common associated ericaceous subshrubs were often used for making rough sweeping brooms or 'besoms' (Johnson & Smith 1946). Apart from fodder for stock animals, the plant has been used in rural areas until quite recent years to make a springy bed for either man or stock, as roof thatch, besom brushes for the hearth, house and yard, for baskets, as packing material, fuel (especially for kindling), strong rope (strong and suitable for mooring boats), as structural material for wattle-and-daub walls, as a dye plant and, in common with the other ericaceous subshrubs, for herbal medicine (for the latter see the current author's Erica cinerea species account).
Another possibly very important past use of C. vulgaris was as an ingredient of Heather Ale, a mythical Scottish Highland brew, the recipe for which was lost, but which has recently (1990s) been recreated on the basis of supposed archaeological evidence. Basically it simply substitutes flowering heather shoot tips as flavouring instead of hops (Mabey 1996, pp. 158-61). Not least amongst the uses of heather is the wearing of a sprig for luck, thankfully still a widespread practice at appropriate moments in life!
In the light of such extensive use, it is not surprising that many local folk and English names have accumulated for the species, of which Grigson (1987) lists 16, and Britten & Holland (1886) as many as 26! The name 'Heather', together with 'heath', derives from the Anglo-Saxon or Old English 'Hæddre' (Grigson 1974), which Prior (1879) says originates from a word that means the open, untilled and un-forested ground on which heath grows, and its even older root is shared with the word 'home', since open ground provided sites for dwellings, even in very early times, ie the Neolithic. The spelling changed from the 14th century 'hathir' to 'hedder' and, eventually, to 'heather' during the 18th century. The other fairly widespread and well known English common name 'Ling' comes from Old Norse 'lyng' and it is still most frequently found in areas of B & I where Vikings (Danes or Norwegians) settled (Grigson 1974).
Threats
As already mentioned, the species is affected by atmospheric and groundwater pollution which enriches nitrogen levels in particular. It cannot effectively compete with grasses under these conditions, nor when heavily grazed, or after severe or frequent burning or heavy trampling.