British distribution: throughout Britain, but few examples in the predominantly agricultural areas of central, lowland England and now much fragmented elsewhere.
World distribution: lowland Calluna / Erica heaths as discussed here are primarily confined to the British Isles, northern Germany, southern Scandinavia and adjacent, mainly coastal, parts of western Europe, but equivalent vegetation types occur in temperate, montane and subarctic regions elsewhere (summarised in Gimmingham, 1972).
|Erica cinerea / Calluna heath on old mine-workings,|
Goon Gumpas, Cornwall, July 2000.
DEFINITIONS: "WE KNOW WHAT WE MEAN"
The term 'heath' is defined and used in a number of ways. As understood here, it refers to a dwarf shrub community dominated by members of the Ericaceae, the heather family. To a large extent it is expected that Common Heather, Calluna vulgaris, will be a major component of the vegetation, though species of Heath (Erica) will also usually be present, and sometimes dominant. The two widespread species of Erica in Britain are E. tetralix (Cross-leaved Heath) and E. cinerea (Bell Heather). These two species often occur together, but E. tetralix is typical of damper ground and E. cinerea of dryer ground, sometimes very dry indeed.
Calluna and Erica are similar in being evergreen and microphyllous, i.e. possessing minute leaves with adaptations such as sunken stomata to minimise water loss through transpiration - a key factor in the open, often wind-swept conditions in which these plants often grow.
Other (non-microphyllous) members of the Ericaceae may also grow in heath vegetation, notably members of the genus Vaccinium. Thus ecologists may refer to 'bilberry heath', referring to vegetation dominated by Bilberry ('Blaeberry' in Scotland), Vaccinium myrtillus. However, while Vaccinium species do occur in lowland heath, they become dominant primarily in upland and subarctic areas, in vegetation types that may come under the alternative heading of 'tundra'.
A dwarf shrub that may be locally dominant on more calcium-rich ground in subarctic areas is the Mountain Avens, Dryas octopetala, a member of the rose family. Yet ecologists also speak of 'Dryas-heath'. Again, this is a type of tundra vegetation (scarcely developed in the British Isles) and excluded from the heathland concept used here, but it indicates the variable terminology.
This page deals with lowland heath, a concept that ecologists feel they understand, in that there are numerous individual sites and a few more substantial areas that seem to have similar vegetation, physiognomy and ecological characteristics. However, precise definition is still difficult. The UK Biodiversity Action Plan (see links) defines lowland heath as usually being below 300 metres. This places British lowland heaths within areas that more generally have been dominated in the past by forest, as distinct from vegetation above the tree-line or, in northern Europe, the subarctic tundra.
Essentially, then, lowland heathland is a vegetation that has replaced forest, though this is probably not true of every site. Disappearance of forest has usually been a result of human action, so, to a large extent, heathlands are a habitat created and maintained by human activity, though continued activity (or sometimes cessation of it) is also resulting in widespread loss of this habitat.
Heather (Calluna vulgaris) is the characteristic and usually defining species of heathlands, usually dominating lowland heaths and also the drying surfaces of blanket bogs and other acid mires.
Photograph: Glen Clova, Angus, 1977.
Bell Heather (Erica cinerea) is common and locally dominant on dry heaths and on ledges of base-poor rocks. It is usually much more sparse on damper peat surfaces.
Photographs: Goon Gumpas, Cornwall, 2000.
Cross-leaved Heath (Erica tetralix) is characteristic and often locally dominant on damp heathlands and on blanket bogs and other acid mires.
Photograph: Goon Gumpas, Cornwall, 2000.
Crowberry (Empetrum nigrum) is not a member of the family Ericaceae but is an ecologically very similar microphyllous shrub that grows as part of the same plant communities.
Shown here is subsp. hermaphroditum which typically occurs at higher altitudes but is a component of coastal heathland on the north coast of Scotland and in Scandinavia. The lowland plant in most of Britain is subsp. nigrum, a more sprawling plant that differs in leaf-shape and in being dioecious (separate male and female plants).
Photograph: Glen Clova, Angus, 1977.
As so far defined, the concept of 'lowland heath' could include vegetation developed on thick peat. However, such sites, acid mires, tend to be treated differently, with large areas termed 'blanket bog' - the habitat type that develops where rainfall (or precipitation : evapotranspiration ratio) is high enough to cause continued peat development to form a substantial layer over the mineral soil.
Drying surfaces of blanket bogs are typically dominated by Calluna and in the broad sense are undoubtedly examples of 'heathland'. However a distinction can perhaps be drawn between heathlands with a distinct layer of partly humified organic matter as a zonal mor soil and mires with azonal peat soils. Commonly the soil under heathland is a podsol, in which most nutrients have been leached from the A-horizons.
This is not to claim that heath complexes may not be wet or contain active peat growth. 'Valley bog' is a habitat type that readily occurs in heathlands and they are well developed, for example, in the New Forest in Hampshire. A valley bog can be more treacherous to cross than many blanket bogs. Damp and wet heaths are considered further on the next page.
ORIGINS OF HEATHLANDS
As noted above, heaths can be often considered as a habitat that has replaced forests. This is probably not true at least of many coastal heaths, however, and the origins of heaths appear to lie in the interactions between climate, geology, soil impoverishment, developmental history and human action, with the balance between these varying at different sites.
The restriction of lowland heaths to the extreme north-west of Europe gives an immediate indication that climate has a rôle in their development. The climate in this region is 'oceanic', with relatively high rainfall (generally between 60 to 110 cm per year) and more importantly, an even spread of rainfall throughout the year, with a high number of rain days (typically above 115 per year - if a 'rain day' is defined as one in which there is at least 1 mm of precipitation - though varying measurement criteria make these generalisations difficult). Relative humidity remains moderately high, even in the driest months.
Moreover, the proximity of the Atlantic Ocean (including the Gulf Stream influence) also has a buffering effect on temperatures, preventing winters from becoming very cold and summers from becoming very hot. Mild winter temperatures are undoubtedly important for many of the individual plant and animal species that characterise heaths in the south-west and in western Ireland, but an upper limit of 20° C. for the mean temperature of the hottest month appears important for heath development as a whole.
These climatic figures are taken from Gimingham (1972), in which much more thorough discussion is provided.
- geology and soils
A climate in which rain continually seeps into and through soil is one that will promote leaching, the loss in solution of plant nutrients. Where soils are thin or developed over sands, this nutrient impoverishment is likely to be significant, leading to podsol formation. Many of the major heathland complexes in southern England are on geological formations such as the Bagshot Sands or the Lower Greensand, which are typically free-draining and liable to rapid leaching, with soils derived from the Lower Greensand usually very poor in nutrients at the outset. Elsewhere, soils may be developed on rocks that are not only poor as a source of nutrients but which also weather very slowly, as in the cases of granite and gneiss. Nutrients leached from these soils will not be quickly replaced. Even where rocks contain a higher nutrient content, as in the case of serpentine, very slow weathering will result in acid, impoverished soils.
Cool, wet, increasingly impoverished conditions also inhibit complete decomposition of organic material, with accumulations of acid humus further accelerating leaching, particularly of calcium. This may happen even on exposed limestone outcrops, leading to development of 'limestone heath', with normally calcifuge plants such as Calluna colonising the outcrops. (Further comments on this are given on the limestone pavement page.)
It can be argued that established forest will not seriously be subject to such nutrient loss, particularly as mycorrhizal fungi will normally play a significant rôle in the transfer of nutrients from sites of organic breakdown directly back to tree and other plant roots. Forest can persist on impoverished soils with virtually all nutrients contained within the living vegetation. It is possible, however, that slow nutrient loss from a forest ecosystem will eventually begin to lead to natural degeneration even of assumed 'climax' forest.
- disturbance, fire and forest clearance
When forest is damaged or partly felled, then this further removal of nutrients from the system may mean that regeneration potential is limited. Fire or forest clearance together with soil impoverishment may mean that heathland replaces the former forest, either as a transitional stage of a secondary succession, or as a stable plagioclimax (diverted climax) - particularly if grazed or if fire continues to be used in management, as in the rotational burning of grouse moors to prevent scrub development and to ensure a supply of young heather shoots as food for the grouse. Either grazing or fire are usually necessary to prevent invasion by scrub, particularly Betula (Birch), and consequent reversion to woodland, but exposure or impoverishment may prevent this, even in the absence of grazing.
Heathlands occur throughout Britain, though as noted above, climate and geology favour heath development to a greater extent in some areas than they do in others. Some key areas are:
- Breckland (junction of Norfolk, Suffolk and Cambridgeshire, East Anglia) - ancient grasslands and grass-heaths, perhaps never forested, where a layer of blown sand, often deep, overlies chalk and glacial deposits. The origin of the sand has been debated but is now thought to be of ancient maritime origin from the time when the Wash was more extensive. (There is or was a limited equivalent on the north side of the Wash.) The area is the driest in Britain (averaging as low as 55 cm per year) and has a relatively 'continental' climate, often hot in summer but cold in winter and with frosts extending into late spring and early summer. Despite the low rainfall, podsolisation occurs on the deeper sands. Much of this area has been lost to agriculture and forestry, though extensive areas remain in military ranges. The area is important for numerous nationally rare plants and animals.
|Ancient heath grassland in the Breckland of East Anglia.|
Photograph: Foxhole Heath, Suffolk, 1973.
- the New Forest (Hampshire, southern England), which, while it contains important remnants of very ancient woodland, also contains extensive areas of heathland. It is an old 'deer forest', i.e. deriving its name from the time when 'forest' referred to areas where deer were legally protected and had nothing directly to do with trees. In aggregate it is "the largest area of lowland acidic heath remaining in Britain" (Ratcliffe, 1977, vol. 2).
Dry heaths are characterised by dominance of Calluna, Ulex minor (Dwarf Gorse) and Agrostis curtisii (Bristle Bent, a grass).
The area is important also for its wet heaths and valley bogs.
- Ashdown Forest (Sussex, southern England), another area of wet and dry heaths and woodland, that in conservation terms is grouped with the New Forest as a single large unit.
- South Dorset heaths (southern England) - a now much fragmented area but still containing remnants of great importance for their flora and fauna, including all of the British reptile species. The heaths are developed over the Bagshot Sands and other base-poor strata and also include dune heath in the Studland NNR. The area also contains the Hartland Moor NNR. Erica ciliaris (Dorset Heath) is a nationally rare species centred on this area and all three British Ulex (Gorse) species are locally important components of the heaths.
- the Lizard (Cornwall, south-west England) - a remarkable area on a peninsula in the extreme south-west, where a major rock is serpentine, leading to soils rich in calcium, and with a very 'oceanic' climate, in particular mild winters. Many nationally rare species are entirely confined, in Britain, to this area and include Erica vagans (Cornish Heath), which is locally dominant. Also in Cornwall are well developed coastal heaths on the cliff-tops along the northern coast of the county.
- Sands of Forvie (Aberdeenshire, East Scotland) - sand dunes, with large areas of dune heath with Calluna and Empetrum nigrum (Crowberry).
- Invernaver (Sutherland, north Scotland) - an exposed peninsula on the north coast that receives much blown shell-sand and is consequently lime rich, though more leached areas support fragmentary heathland. Floristically the site is upland in nature (or better considered as 'tundra'), with Empetrum nigrum subsp. hermaphroditum, the normally montane subspecies of Crowberry, abundant here in coastal heathland.
As defined here, lowland heaths are dominated by evergreen, microphyllous ericoids, i.e. Calluna and/or Erica species. However, Empetrum nigrum (Crowberry), while not a member of the Ericaceae, seems to fit into this group ecologically, not only in being evergreen and microphyllous, but also in possessing mycorrhizas (symbiotic, i.e. 'mutualistic', associations of plant roots with specialised fungi) that are structurally very similar to those of Calluna and Erica.
Heathlands are mostly species-poor and dominance by one or very few species is characteristic. Consequently, classification and naming of heathland vegetation is substantially based on the dominants. However, other species may be very important in establishing a vegetation taxonomy and phytosociological knowledge has moved on from lumping the various plant communities dominated by Calluna into a single, heterogeneous concept of "Callunetum".
In the National Vegetation Classification (NVC) (Rodwell et al., 1991) it is considered that the more oceanic nature of British dry heaths means that they do not fit neatly into the established classifications on the continent, though similarities and relationships are discussed in the account. Some of the communities recognised are:
- H1 Calluna vulgaris-Festuca ovina heath - on dry, base-poor sands in Breckland and other areas of East Anglia and south-eastern England, very species-poor and lacking those of oceanic heaths.
- H2 Calluna vulgaris-Ulex minor heath - in southern England, primarily the dry heaths of the New Forest, Ashdown Forest and Surrey.
- H3 Ulex minor-Agrostis curtisii heath - the main heather-gorse-grass dry heaths of the New Forest and eastern Dorset.
- H4 Ulex gallii-Agrostis curtisii heath - the more oceanic, south-western counterpart of H3, in which Ulex minor (Dwarf Gorse) is replaced by U. gallii (Western Gorse).
- H7 Calluna vulgaris-Scilla verna heath - the predominant dry heath vegetation on the western and northern coasts of Britain, with Scilla verna (Spring Squill), a small blue-flowered bulb, a key (but not exclusive) component.
- H8 Calluna vulgaris-Ulex gallii heath - a rather broad or diverse concept, mainly in Wales and western England but reaching the coast of East Anglia, i.e. where Ulex gallii itself has a few eastern populations. The concept includes western 'limestone heaths'.
- H9 Calluna vulgaris-Deschampsia flexuosa heath - lowland to rather more upland heather-grass vegetation in northern England, e.g. the North York Moors, on acid soils including those derived from the Millstone Grit - a major geological sandstone formation. Includes some of the grouse moors of the area.
- H10 Calluna vulgaris-Erica cinerea heath - lowland to moderately upland on acid soils and rocky ground in much of western and central Scotland. Includes some of the grouse moors of the area.
- H11 Calluna vulgaris-Carex arenaria heath - the major heathland type on acid dune systems in the north, i.e. 'dune heath', Carex arenaria being the Sand Sedge, a pioneer of loose sand that persists into the heathland phase of succession.
- M16 Ericetum tetralicis (or Erica tetralix-Sphagnum compactum wet heath) - lowland damp to wet heaths throughout much of Britain and adjacent Europe.
- M21 Narthecio-Sphagnetum (or Narthecium ossifragum-Sphagnum papillosum valley mire) - listed here as this is the major plant community of the valley mires that lie within southern heathland complexes, notably in the New Forest.
Note that those communities with an 'M' rather than 'H' prefix are described in the NVC as 'mires' rather than 'heaths'.
A comprehensive descriptive account of Scottish lowland dwarf-shrub heaths was given by Gimmingham (in Burnett, 1964) and there is also an extensive compilation of descriptive information on British heaths, albeit somewhat outdated, in Tansley (1939).
Further notes on heathland types, their animal life and their conservation, together with links and references, continue on the next page.