Erica cinerea L., Bell Heather

Account Summary

Growth form and preferred habitats

E. cinerea is a much more scarce plant in Fermanagh than E. tetralix (Cross-leaved Heather), usually occurring only in small patches, upright or sprawling in form, but almost always straggly in appearance, perhaps mainly due to the very slender, semi-prostrate woody branches, but also to the often rather sparse, dark, purple-tinged, needle-leaved evergreen foliage being more or less restricted to the ends of shoots.

On heaths, E. cinerea is associated most closely with Calluna vulgaris (Heather), often accompanied by varying amounts of Erica tetralix (Cross-Leaved Heath), Vaccinium myrtillus (Bilberry), Ulex europaeus (Gorse) and other heath species, including occasionally Empetrum nigrum (Crowberry) and Juniperus communis (Juniper) on the most exposed heaths and cliffs. This subshrub is most frequently found on drier, well-drained, shallow, acid, peaty podsol pastures, including some formed locally over limestone. Bell Heather can also occur in openings in upland woodland and it is frequent on the drier tops of hummocks on otherwise very wet raised and blanket bog surfaces. It is especially frequent on cut-over turf banks, which by their nature are exposed, better drained and considerably drier than their boggy surroundings. A constant feature of all the habitats E. cinerea occupies is their moderate to severe acidity (pH 3.6-6.6) and the consequent nutrient-starved nature of their soils (Grime et al. 1988). This is reflected in the evergreen habit and reduced, peinomorphic or xeromorphic, sclerophyllous, needle-like leaves of the species. (For discussion of this important topic, see the Vaccinium oxycoccus (Cranberry) species account below.)

Fermanagh occurrence

E. cinerea usually forms only small patches of subshrub and it is a much more scarce plant in Fermanagh than E. tetralix (Cross-leaved Heath). E. cinerea has just over one third the number of E. tetralix records and it is represented in 98 tetrads, 18.6% of those in the VC. While it is quite widespread, in quantitative terms it remains concentrated in the peatlands of the Western Plateau. E. tetralix is much more widely distributed across 228 Fermanagh tetrads.

Although E. cinerea is chiefly a plant of open habitats, growing in small quantity, Bell Heather also accompanies Calluna vulgaris (Heather) and Vaccinium myrtillus (Bilberry) in canopy gap situations in upland, damp, mixed deciduous woodland, eg in the Correl Glen NR. Here, Bell Heather is often forced to grow in the shade of other shrubs, often forming a semi-prostrate understorey to the dominant C. vulgaris. Shading produces the straggly, weak-stemmed, adventitiously rooting and horizontally spreading, few-flowered or non-flowering, easily overlooked shoots of E. cinerea that we frequently meet in these situations. Bell Heather is much more tolerant of shade than C. vulgaris, so it penetrates deeper into coniferous or other evergreen or dense deciduous canopy than the latter, thus avoiding competition with this otherwise very dominant subshrub species.

E. cinerea also grows in Fermanagh in crevices on rock outcrops, including the damp sandstone scarps of the Western Plateau and in the more sheltered spots where winter snow might lie on otherwise exposed cliffs and on mountain summits, including, eg on Cuilcagh the highest mountain in the VC.

Calluna vulgaris and E. cinerea also co-exist in Fermanagh on dry, grassy, low-growing, sheep-grazed heath, rooting in shallow, raw humic peat 'ranker soils' formed directly over hard crystalline Carboniferous limestone, eg above Florencecourt and around the Knockmore area. This vegetation and habitat is the local equivalent of the acidified and leached lowland grassland limestone heath or chalk heath of S England, where E. cinerea commonly occurred until the habitat losses of the last 50 years (M.C.F. Proctor, in: Preston et al. 2002).

Under these drier growing conditions, E. cinerea appears better able than other common ericaceous subshrubs at maintaining a favourable water balance. It may even become the dominant species, or co-dominant along with Calluna vulgaris during some of the stages in heathland succession. This is especially so after any form of disturbance that reduces the vigour and dominance of C. vulgaris.

Finally, in addition to almost innumerable vegetation communities of heaths at all levels, boglands, woods and open montane communities, Rodwell et al. (1992) lists E. cinerea occurring in six NVC calcifuge, base-poor grasslands and non-heathy, grazing-sensitive, bracken-infested or relatively unstable substrate montane communities in Britain. It is not a constant species in any of these situations, however, and tends to be of low frequency and cover/presence.

Flowering reproduction

Like Calluna vulgaris, Bell heather is a surprisingly short-lived subshrub. Individuals mature early and flower in their second or third season. The majority of individuals appear capable of surviving for only around 20 summers at most (Bannister 1965).

Between July and September, the small urn-shaped, vivid-purple flowers, borne in elongate, terminal racemes or panicles are very conspicuous and are recognisable even from long distance. The plants flower before C. vulgaris, so their presence amongst or partially beneath the canopy of the latter is most noticeable in July. Plentiful nectar is produced at the base of the ovary in the pendulous, urn-shaped flowers which attracts various bees, butterflies, moths and flies as pollinators. It is probable that unvisited flowers can self-pollinate, if necessary.

Some insect visitors, including some bumblebees, have a proboscis (or 'tongue') that is too short to reach the nectar through the narrow corolla tube. The thwarted insects often bore a small hole near the base of the flower and extract the sugar without disturbing and operating the anthers and stigma at all, thus robbing the flower (Proctor & Yeo 1973, p. 143). Nevertheless, in addition to the nectar energy source, bees do require pollen for their protein and mineral nutrition, and therefore some of them must operate the floral mechanism legitimately and carry out cross-pollination. In B & I, hives of honey bees are often transported in July to upland areas with plenty of C. vulgaris and E. cinerea, so that bee keepers can harvest 'Heather' honey (MacLennan 1995).

Seed production, dispersal and longevity

Fruit capsules examined in Britain by Bannister (1965) contained between 18-34 seeds, about one third of which were abortive. A moderately sized 10 year old plant produced around 1,000 capsules; younger plants produced fewer capsules. The estimated annual seed production based on these figures was c 450,000 potentially viable seed/m². Ripe capsules split open and vast quantities of seed are shed from October onwards (Barclay-Estrup & Gimingham 1994). Dispersal, however, tends to be over very short distances, travel on the wind being greatly hampered by the fact that the withered, urn-shaped corolla persists around the capsule well into the winter months.

Seedlings are most commonly found in the immediate vicinity of the parent plant (Bannister 1965). Despite the heavy seed rain and their known long survival in the soil seed bank (up to at least 33 years (Thompson & Band 1997)), generally seedlings are only observed colonising and establishing new plants in areas of relatively dry, bare mineral soil in vegetation gaps. Seedlings either do not occur, or do not survive for long, in wet, peaty, or litter- or moss-covered soil situations (Mallik et al. 1984).

Competitive ability and vegetation dominance

Temporary dominance in dry parts of heaths or moors may sometimes be achieved by E. cinerea, eg after ground is burnt, since its stumps recover more quickly from the effects of fire than those of the normal dominant species, Calluna vulgaris. While both these heathers have large seed banks buried in the soil and litter layers, that of E. cinerea is greatly stimulated by a brief heat treatment, and its germination is both more rapid and more uniform than that of Calluna, giving the species at least a short-term competitive advantage (Bannister 1965; Mallik et al. 1984; Rodwell 1991(b), p. 383).

A study of threatened lowland heaths in England by Britton et al. (2000) found that while Calluna is highly competitive over much of its life cycle, its competitive ability is reduced during the regeneration phase that follows the death of stands, or which results in response to normally applied management activity including burning and grazing. These workers also discovered that site geographical location had the greatest effect on the pattern of regeneration of any of the four other factors they studied (ie dominant species, management techniques, gap size and seed source availability). One thing becoming clear from this and other studies, is that heath regeneration dynamics may be extremely variable, and that conservation management must be tailored to the environmental conditions found on each individual site (Britton et al. 2000).

Vegetative reproduction

Observed rapid seedling colonisation of burnt bare ground indicates that reproduction of E. cinerea from seed does occur and this is significant both in maintaining genetic evolutionary potential and in transporting the species to fresh sites. However, vegetative spread by rooting of prostrate branches is very common, and shoot recovery by resprouting of rootstocks and older branches after grazing, burning and trampling damage is also prevalent, so that vegetative reproduction is probably very much more frequent than successful establishment from seed, and it is vitally important in maintaining the species in its existing sites.

British and Irish occurrence

Erica cinerea is native and widespread throughout both B & I, but is much less common in C Ireland and the English Midlands and S & E, especially in calcareous areas. Being characteristic of oceanic heathland communities, it is most abundant in the N & W of both islands. E. cinerea is closely associated with the occurrence of well-drained, acid, bare mineral soils of dry heaths and moorland, but has declined in S England due to a general loss of lowland heathlands and changes in chalk heath vegetation associated with a decline of grazing pressure from sheep and rabbits (Bannister 1965; M.C.F. Proctor, in: Preston et al. 2002).

European and world occurrence

Unlike the other widespread B & I heathers Calluna vulgaris and E. tetralix, E. cinerea is very much more restricted to the moist, mild, chiefly lowland, oceanic and southward suboceanic heathland region of the western seaboard of Europe. It stretches from S Portugal to SW Norway and is also present in the Faeroes, Scillies and N Africa. It has been introduced to at least one location in coastal eastern N America (Nantucket Island) (Hultén & Fries 1986, Map 1146). Bannister (1964) has suggested that E. cinerea has only a very limited tolerance of winter and spring reduction in the water content of its tissues compared to E. tetralix. This is true to an even greater extent when it is compared to C. vulgaris which can tolerate large moisture reductions. This property might well determine the very limited continentality shown in the distributional range of E. cinerea.

Variation

There appears to be very little variation in E. cinerea apart from the existence of rare white flowers. A single endemic variety, var. maderensis D.C. from Madeira, is now recognised as a separate species E. maderensis (Benth.) Bornm (Press & Short 1994). Having said this, there is some possibility that a wet peat-tolerant ecotype of the species may exist in Ireland and in similar climatically hyper-oceanic regions of NW Scotland. Plants of E. cinerea over much of the species' range cannot survive prolonged waterlogging and they certainly are much less tolerant of this than either E. tetralix or C. vulgaris. Nevertheless, Bannister (1965) reported E. cinerea growing on wet rankers in vegetation communities of northern affinities. Many of the wet heaths and Molinia-Calluna dominated vegetation types in N & W parts of B & I develop on the slightly better-drained patches of peat banks and hummocks, on peaty podsols, or over shallow peaty rankers. This form of somewhat better-drained, damp to wet vegetation does have northerly, sub-alpine, or even possibly sub-arctic affinities, so that the local tolerance of E. cinerea to wet peat soils may demonstrate wider variation than the species norm on the N & W margins of its overall geographical range.

Limiting effects of temperature and drought

While there is some evidence from oceanic W Ireland and NW Scotland suggesting the existence of a wet-soil ecotype within E. cinerea, or that the species demonstrates a wider ecological amplitude in the west of its range, Bell Heather definitely does not have an arctic tolerance of low temperature. The N and E of the species distribution conforms closely with the limit of the 2°C January isotherm. Also, since its main absorbing roots are shallow and normally form a mat at soil depths around 10 cm, Bell Heather cannot tolerate great summer heat or prolonged drought. Bøcher (1940) found that the S and E limit of its range correlated well with the 22°C June isotherm. Despite considerably higher figures for absolute altitudinal occurrence in various regions of B & I, the upper limit at which E. cinerea forms a regular component of the vegetation in these islands is around 620 m (2,000 ft) (McVean & Ratcliffe 1962), a fact which ties in with the 'temperate-thermic oceanic' classification of the species given it by Bøcher (1940) on the basis of the limiting isotherms quoted above.

An additional limiting factor is the susceptibility of E. cinerea to late spring or early summer frosts. Plants suffered very severe frost damage on exposure to a ground minimum temperature of -3.5°C in late May and June in NE Scotland in 1964, an event which, in comparison, left C. vulgaris, the much more continental species of the two in its distribution, totally unharmed (Bannister 1965). In terms of seasonality or phenology, Bell Heather, being evergreen, shows little winter growth, followed by a rapid burst of development in late spring and it flowers earlier than C. vulgaris, beginning in July and continuing through until September.

Dynamic succession processes

To a very great extent, the vegetation types in which E. cinerea grows are almost entirely semi-natural and anthropogenic in nature, being produced by mans' destruction of pre-existing forest and subsequent management for grazing or other uses of the heathers present (Bannister 1965; Gimingham 1972). Except in otherwise severe environmental conditions, heathland below the tree limit dominated by Calluna vulgaris and other dwarf or semi-dwarf heathers and subshrubs is not a 'Climatic Climax', nor is it even a particularly stable form of secondary vegetation in these islands. There is always the likelihood of invasion of such ericaceous heath and moorland by species typical of later successional changes, most notably by taller, longer-lived shrubs and trees, amongst which birch, having abundant, very light mobile seed, provides the most obvious threat in many instances (Khoon & Gimingham 1984; Gimingham 1995).

Response to grazing

Sheep are the principal browsers of heaths and moorland in B & I, followed by grouse and cattle, although numerous insects are also listed by Bannister (1965) in his Biological Flora account of the species. Larvae of the Heather beetle, Lochmaea suturalis (Thoms.), can cause local defoliation of the shrub in some years. The foliage appears to be relatively unpalatable, since most herbivores avoid it and show a preference for other available species, including Calluna vulgaris (Bannister 1965).

Experimental work on sheep grazing has shown that the season and period of browsing, as well as the weight of stocking density, are significant in terms of maintenance of heather species. In Europe, as in B & I, the extent of heathland has been very considerably reduced in the last 50 years and especially so in the two most recent decades, due to cessation of previous management for agricultural use, ie a set regime of ± regular burning and grazing designed to maintain the vigour of the heath through keeping the subshrubs in the building phase of their growth cycle. Neglect of active management has led to encroachment by trees and shrubs on many heaths. Other heathlands have been destroyed by overexploitation of the subshrub resource, giving rise instead to nutrient impoverished, species-poor, acid grasslands (Gallet & Roze 2001).

Sheep grazing at a high stocking rate (10-15 sheep/ha/year) on plots of dry and mesophilous heaths in Brittany where E. cinerea was an important component species found that, as expected, heavy grazing led to a decrease in vegetation cover, reduction in canopy height and also in the production of flowers (Gallet & Roze 2001). In this French study, E. cinerea appeared to suffer large decreases in its frequency under several seasonal grazing regimes, but it always recovered to its initial level of cover after the livestock was removed. Following winter grazing, which tends to remove both branch apices and flower buds, thus having a negative effect on flower production, subsequent development and full recovery of vegetative canopy was most probably due to the stimulation after the removal of apical dominance, of the numerous lateral dwarf shoots which are such a characteristic feature of E. cinerea shoots. Grazing in summer removes flowers, but in addition it also appeared to affect the plant physiologically, since the effect was still apparent in the following summer's reduced flower production.

It has been suggested that heavy grazing towards the end of summer causes a reduction in carbohydrate reserves, which could have detrimental effects on the important spring regrowth. It may also cause wilting and death of branches during the winter and, in more general terms, grazing not only has an immediate effect, but it also seems to weaken heathers and other woody plants, decreasing their ability to cope with winter cold (Grant et al. 1978, 1982; Bayfield 1979; Gallet & Roze 2001).

Response to trampling

Experimental human trampling of heathers has shown that even occasional trampling severely disrupts all the plant communities studied and causes structural deviations in them. The presence of the ericaceous species in "humid heaths" over dry sandy soils nevertheless enhanced the vegetation resistance, at least to modest intensities of trampling in comparison for instance, to the damage suffered by the more herbaceous floor layer of mesophilous woodland. The woody habit of heathers confers resistance to trampling, but also mitigates against the ability of the plants to recover from trampling damage. In respect to this, there are definite differences between species, eg V. myrtillus has a more limited wear and tear tolerance compared to the other species studied (Roovers et al. (2004).

Another much longer term experimental study on four sites at montane levels in the Scottish Cairngorm mountains involved trampling treatments (ie two men wearing hiking boots!) applied over a short initial period followed by no further disturbance. It examined the responses of heathers and graminoid species in terms of initial damage and subsequent recovery over an eight year study. As expected, damage increased with the level of trampling, but some woody species showed delayed damage; substantial die-back occurred during the following winter, or even later. Again there were major differences between the performances of ericoid species. Calluna vulgaris and Arctostaphylos uva-ursi (Bearberry) proved most susceptible − suffering high initial damage and poor, exceedingly slow recovery. Erica cinerea and E. tetralix were among the species of low susceptibility, showing low or moderate levels of initial damage followed by an increase in relative cover compared to untrampled control plots, although again their recovery process was slow, undoubtedly on account of the altitude. The deciduous species, Vaccinium myrtillus and V. uliginosum (Bog Bilberry), were placed in an intermediate grouping in terms of susceptibility − moderate to high initial damage, followed by fairly good recovery, most of which occurred in the first year − due perhaps to their being able to produce a complete new set of leaves the following spring (Bayfield 1979).

Uses

Apart from the dark brown 'Heather Honey', Bell Heather flowers have been used in Scotland for a yellow-brown dye for wool. Many of the uses listed in the Calluna vulgaris species account apply also to E. cinerea, and all three common and widespread heathers in B & I (ie C. vulgaris, Erica tetralix and E. cinerea) were used interchangeably in herbal medicine for the treatment of a wide range of ailments. These heathers provided almost a panacea, through faith in a relaxing or mildly soporific effect the plants were believed to possess. Heather has been used in the Scottish Highlands to sooth nerves and as a sedative to counter insomnia. Its astringent properties have also been used for stomach upsets and for diarrhoea, and it has some diuretic uses too. In Ireland uses of 'Heather tea' have varied from the treatment of coughs and asthma, and to relieve the pain of rheumatism (Allen & Hatfield 2004).

Names

The genus name 'Erica' is from the Greek 'ereike' meaning a 'heath' or 'heather', probably originally a species of this genus (Johnson & Smith 1946; Gilbert-Carter 1964). The Latin specific epithet 'cinerea' means 'ash-coloured', which is not at all apt with respect to the whole plant, but might be a reference to the colour of the undersurface of the needle-like leaves. The current widespread English common name 'Bell Heather', appears to be a recent transfer from E. tetralix, since Britten & Holland (1886) list it among a total of 19 names for that species on the authority of Jamieson's Dictionary of the Scottish Language (1867 edition). In their later published Appendix (1896), however, Britten & Holland do list E. cinerea as 'Bell-ling', a local Yorkshire name, along with a total of ten alternative names for the species. The name 'Black heath' from Hants the current author feels is quite an appropriate one, in view of the very dark purplish tinge of the typical leaf colour. According to Grigson (1987), the unusual Yorkshire name 'Carlin Heather' is from the Old Norse 'kerling', meaning a witch or hag.

Threats

Like all plants of heath and bogland habitats, there has been a decline in both area and abundance of E. cinerea during the last 60 years due to a combination of changing land use and increasing pollution, both atmospheric and water-borne. Where ground that supported the species has not been reclaimed for agriculture, planted with conifers, built over or otherwise developed, drainage and soil nutrient enrichment has led to a shift in vegetation from woody dwarf shrubs towards grassland. As with Calluna vulgaris, Bell Heather cannot effectively compete with grasses under the changing conditions, nor when the subshrub is heavily grazed, severely or frequently burnt or heavy trampled.

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