British habitats |
British distribution: northern and western Britain, especially in north Scotland and in upland areas.
World distribution: oceanic regions with a cool, wet climate, including Britain, western Ireland and adjacent north-west Europe.
Extensive, actively growing blanket bog, showing patterns of old peat cutting, plus erosion due to recent disturbance. Heather (Calluna vulgaris) dominates the dryer hummocks.
Photograph: Isle of Lewis, Outer Hebrides, August 1998.
Wet blanket bog, with Common Cottongrass (Eriophorum angustifolium) a major coloniser of the shallow, open pools, and Bog Asphodel (Narthecium ossifragum) and Cross-leaved Heath (Erica tetralix) frequent in the tussocky vegetation.
Photograph: Clàr-loch Mór, Sutherland, August 2002.
Wet blanket bog, showing active recovery from peat cutting. Hare's-tail Cottongrass (Eriophorum vaginatum), Deergrass (Trichophorum cespitosum subsp. germanicum) and Bog Asphodel (Narthecium ossifragum) are abundant on the formerly cut areas; Heather (Calluna vulgaris) dominates the uncut (or less recently cut) surface beyond.
Photograph: Clàr-loch Mór, Sutherland, August 2001.
DEFINITION and ORIGINS
What is blanket bog?
Blanket bog is an unconfined, ombrotrophic mire type. That is to say the mire is not confined to topographical depressions or small catchment areas, but instead extends over the landscape as a 'blanket' of peat, and even on slopes, nutrition is largely direct from rain. (Ombrotrophic = "rain fed".) Climatic conditions must be cool and wet, with a high number of rain-days being as important as total rainfall. According to Goode and Ratcliffe (1977) a minimum of 160 days with 1mm or more precipitation is necessary for blanket bog development. Topography, i.e. steepness of slope, and average temperatures, i.e. as affect potential evapotranspiration, must also be considered against numbers of rain-days and total rainfall.
Active blanket peat development is now largely confined to the wettest parts of the British Isles, especially north and north-west Scotland and western Ireland. Elsewhere, blanket bog seems often to be persisting in regions where rainfall is lower than when the bogs were first initiated, which has important implications for their potential recovery from damage and for their conservation.
The peat layer is typically 2 to 4 metres in depth, sometimes deeper over underlying depressions and shallower on steeper slopes and outcrops. Goode & Ratcliffe note that in the wettest areas, blanket peat can form on slopes as steep as 30°. Blanket bog is continuous over extensive areas, masking the underlying topography and also cutting the surface vegetation off from minerals in the underlying soil or bedrock. In areas such as Sutherland in northern Scotland or Connemara in western Ireland, the blanket bog extends from mountain slopes down across lowlands until it reaches the sea. The flow country of eastern Sutherland and Caithness is a huge area, covering some 2500 km2 and formerly far more extensive still. Blanket bog also occurs on our upland plateaux, including Dartmoor in SW England, the mountains of central Wales, the Pennines of England, and extensive areas of the Scottish Highlands.
Peat and old Sphagnum moss bind metal cations and even if there is any appreciable water movement within the peat it will not be a source of nutrients. Hydrogen ions are the predominant cation and available bases such as calcium, magnesium and potassium remain very low. Rainfall, especially near the west coast, provides some limited supply of these nutrients, there are some useful data given by Moore & Bellamy (1974), and in very oceanic areas of western Scotland and Ireland it may also provide increased flushing of hydrogen ions. In general, however, blanket bogs have a surface pH of around 3.8-4.0 and they are a hostile environment for all but a few species which are acid tolerant and able to conserve such minerals as they can acquire. Phosphate and nitrate levels are also very low, though the latter may be supplemented by nitrogen fixation by cyanobacteria on the bog surface or by the occurrence of Sweet Gale (a.k.a. Bog Myrtle) (Myrica gale), which has nitrogen-fixing root nodules containing symbiotic actinobacteria. However, Myrica itself is demanding of other nutrients.
Insectivorous plants are able to supplement their phosphate and nitrate, in particular, by trapping small invertebrates. On blanket bogs these are primarily Sundew (Drosera) in Sphagnum carpets and Bladderworts (Utricularia) in the pools and runnels.
Blanket bog formation and human influence
It is a frequent occurrence that tree stumps (especially pine (Pinus sylvestris) or oak (Quercus)) are preserved in the lowest levels of blanket bogs, indicating that the bogs have replaced forest. At one time it was thought that there had been a very sudden climatic change, and indeed some blanket bogs began to form around 5000 BC, which has been taken as the start of the 'Atlantic' period, with a climate substantially wetter than the rather warm and relatively dryer 'Boreal' period that proceeded it. It was postulated that increased rainfall caused leaching of soils and increased waterlogging, resulting in deterioration of the forest cover and invasion of open areas by the peat-forming Sphagnum mosses - blanket bog formation being a natural consequence.
However, climatic changes now appear to have been more gradual, and in any case, many blanket bogs seem to have been initiated at much later dates and quite often over soils that do not seem to have become so impoverished as to no longer be suitable to support forest. There is much evidence of Neolithic Man colonising Britain at this time and sharp declines in elm (Ulmus) and lime (Tilia) pollen in peat stratigraphy can be related to forest management and clearance.
It seems highly plausible, therefore, that a complex of factors was involved, with human activities playing a major rôle. If forest was already under stress from higher rainfall, removal of even a small proportion of the trees would increase the stress on the remainder. Trees can be regarded as pumps, taking up water from the soil and discharging it into the atmosphere by transpiration. Each tree removed would increase the load on the rest, and beyond a fixed tolerance point, the system would crash. Invasion of an area by Sphagnum would result in further water retention, absorption and isolation of minerals and reduced pH, conditions inhibiting decomposition and favouring the Sphagnum mosses themselves. This water retention would allow the Sphagnum to grow beyond the confines of depressions and areas of initially greatest waterlogging. Peat would form below the actively growing surface layer, and eventually it would blanket the landscape. A valuable review of evidence on initiation of blanket mires was given by Moore, Merryfield & Price (1984).
On the assumption that blanket bogs are currently a climax vegetation, but have resulted at least in part from human activity, they can be regarded as a plagioclimax. The natural climax for much of the same area is assumed to be Scots Pine (Pinus sylvestris) forest, though Downy Birch (Betula pubescens) becomes a dominant in surviving forest in the far north.
There are parallels here with development of wet lowland heath and there may not be a clear distinction between, e.g., the (formerly) extensive wet heaths of Galloway, and expanses of blanket bog, in appearance or vegetation. Both may be dominated by Heather (Calluna vulgaris). Heaths are characterised by a relatively thin layer of peat, still shaped by underlying details of topography, and may exist on podsols or on thin, zonal, mor soils directly over gravel or bedrock.
Actively growing bogs will be characterised by hummocks of Sphagnum (see below) and by wetter depressions or scattered bog pools. In the flow country of north Scotland pools may be larger and become aligned in series to form 'patterned blanket mire' or 'dubh loch systems'.
The prime formers of acid mire systems are mosses of the genus Sphagnum. Their cell structure enables them to soak up and hold water, and many grow rapidly to form hummocks, while others are able to grow in the water of bog pools or form flat 'lawns' around the edges of more extensive water bodies or over areas of past drainage attempts. Much peat in the past was formed by the growth of Sphagnum imbricatum, but this is now rare and sensitive to fire and disturbance.
The commonest hummock forming species on blanket bogs are probably S. papillosum and S. capillifolium, the bright red hummocks of the latter often a conspicuous feature. S. magellanicum is a darker-red hummock-former, more frequent in the west, but this species is more characteristic of raised mires (including domed areas within blanket mire complexes, the "intermediate bogs" of Ratcliffe, as on Gleniffer Braes, above Paisley). A common species of the most oligotrophic (nutrient-poor) bog surfaces is S. subnitens, while the most characteristic species of the pools is S. cuspidatum - though S. auriculatum may be important in the pools of patterned mires. Lawns are commonly dominated by S. recurvum var. mucronatum, especially where there has been disturbance or in old drainage ditches. Flat areas between hummocks or exposed bare peat surfaces are often colonised by S. compactum.
Sphagnum capillifolium is a common, hummock-forming Sphagnum of the active bog surfaces. Images are from a direct scan on a flatbed scanner, material from Renfrewshire, March 2000.
The wet areas with active Sphagnum growth are likely also to have Deergrass (Trichophorum cespitosum), Hare's-tail Cottongrass (Eriophorum vaginatum) and Common Cottongrass (E. angustifolium). All are members of the sedge familiy despite the inclusion of "grass" in their common names. E. angustifolium is often able to colonise open peaty pools and is also able to survive quite severe surface fires, regenerating from deep rhizomes. Purple Moor Grass (Molinia caerulea), a true grass, may dominate areas of true blanket bog, especially in the west, but it often indicates a degree of lateral flushing of nutrients and is more characteristic of other mire types. Bog Asphodel (Narthecium ossifragum) and Cross-leaved Heath (Erica tetralix) are usually present amongst the tussocks of other species. In Sphagnum lawns and on the sides of hummocks, Sundew (Drosera) is often present, most usually Round-leaved Sundew (D. rotundifolia), but sometimes accompanied by Long-leaved Sundew (D. anglica) in the north and west. Drosera species are 'insectivorous' plants, trapping and digesting small invertebrates to supplement the very low nutrient supply.
Dryer surfaces see Heather (Calluna vulgaris) becoming abundant or dominant, often with Eriophorum vaginatum. Lichens, mostly Cladonia species, especially C. portentosa, may form conspicuous white to grey areas amongst the heather, and on the tops of relatively dry hummocks, the moss Hypnum jutlandicum typically occurs under the Calluna.
In the north and north-west, Lamb's-wool Moss (Racomitrium lanuginosum) commonly forms hummocks on the bog surface, especially where there has been past cutting or disturbance. It is better known as a moss of mountain heaths or on boulders at lower altitudes and its ability to grow on northern peat bogs seems to have been little studied.
Heather (Calluna vulgaris) is the characteristic and usually defining species of heathlands, and also dominates the drying surfaces of blanket bogs and other acid mires.
Photograph: Glen Clova, Angus, 1977.
Cross-leaved Heath (Erica tetralix) is characteristic of wetter, more open areas on blanket bogs and other acid mires and on wet heathland.
Photograph: Goon Gumpas, Cornwall, 2000.
Lichens: Cladonia uncialis subsp. biuncialis and C. portentosa on bog surface.
Photograph: Isle of Lewis, August 1998.
Lamb's-wool Moss (Racomitrium lanuginosum) forming hummocks, here associated with old drainage channels but often on open bog surfaces in the far north and north-west.
Photograph: Crask, Sutherland, August 2001.
Following European classifications, the vegetation of blanket mires can be referred mostly to the class Oxycocco-Sphagnetea, though that of pool complexes often belongs to the Scheuchzerio-Caricetea. However there is often a problem that mire complexes are mosaic in nature or that dynamic processes create heterogeneity within what may be regarded as one vegetation type. The National Vegetation Classification (NVC) (Rodwell et al., 1991) does not fully implement internationally recognised classifications but provides a summary that can be used within Britain. The recognised plant communities of ombrotrophic mires are not necessarily exclusive to blanket mire, but some that are applicable are:
- M1 Sphagnum auriculatum bog pool community - in bog pools in oceanic regions of the north and west, including in patterned mire systems (also in valley mires within wet heathland complexes in the south).
- M2 Sphagnum cuspidatum-S. recurvum bog pool community - widespread as the principle pool and lawn community of (mostly) ombrotrophic mires.
- M3 Eriophorum angustifolium bog pool community - widespread in bare areas of ombrotrophic mires following cutting or erosion, including at higher altitudes, characterised by E. angustifolium and with lower bryophyte cover that may include Drepanacladus fluitans as well as Sphagnum spp.
- M15 Scirpus cespitosus-Erica tetralix wet heath - widespread, species poor community with Trichophorum cespitosum ssp. germanicum, Calluna vulgaris, Erica tetralix, etc., on non-actively growing and often thinner peat, including some areas subject to burning (muirburn) for grouse moors.
- M17 Scirpus cespitosus-Eriophorum vaginatum blanket mire - a major community of active, oceanic blanket mire in western Britain, with Trichophorum cespitosum ssp. germanicum, Eriophorum vaginatum, Calluna, Erica tetralix and a well developed Sphagnum layer, especially S. papillosum. Extensive in areas with more than 200 rain-days per year and at altitudes below 500 metres.
- M18 Erica tetralix-Sphagnum papillosum raised and blanket mire - differs from M17 in the greater dominance and growth of Sphagnum species and more typical of raised bogs than of blanket bogs, but noted as occuring within stretches of the latter.
- M19 Calluna vulgaris-Eriophorum vaginatum blanket mire - northern and higher altitude moorland dominated especially by the two species that give the community its name.
- M20 Eriophorum vaginatum blanket and raised mire - vegetation dominated by E. vaginatum and with few other vascular plants, though E. angustifolium may be important in wetter runnels. An impoverished community developed on ombrotrophic peat where there has been extensive grazing, burning or erosion, associated too with draining and atmospheric pollution and most extensive in the south Pennines where these factors are significant.
Rodwell et al. (op. cit.) should be consulted for extensive details of floristics, diagnosis and ecology of these communities. Note that "Scirpus cespitosus" in community names = Trichophorum cespitosum, probably always as the subspecies germanicum. [The rare T. cespitosum ssp. cespitosum and its independently persisting hybrid with ssp. germanicum are ecologically distinct, requiring wetter sites, e.g. in M1 communities.]
An extensive summary of Scottish mires was given by Ratcliffe (in Burnett, 1964) and there is valuable, though rather scattered, information to be gleaned from the vegetation descriptions in Tansley (1939).
The bleakness, low nutrition and low diversity of blanket mires mean that they are not rich in animal species. Their main importance is perhaps as providing nesting sites for bird species that require large expanses of low vegetation and little disturbance. The Hen Harrier (Circus cyaneus) is an important bird of prey that has been much persecuted for its depredations on grouse moors but which has recovered much in recent years. The Short-eared Owl is another that both nests and hunts on the moors. The Golden Plover and Curlew are very much birds of these vast expanses of bog, and familiar (or locally familiar) coastal waders such as the Dunlin and the Greenshank also come onto the bogs to nest. One blanket bog in the far north is an important nesting site for the Arctic Skua.
The stony lochs and lochans of the north and northwest perhaps cannot be regarded as blanket bog in themselves, except where they are formed specifically as bog pools within the peat, as in the flow country. However, there is no doubt that the northern blanket bog isolates and protects such sites from human interference, providing important nesting sites for birds such as the Red-throated Diver, Black-throated Diver and the Common Scoter.
THREATS and CONSERVATION
From long being regarded as expanses of near useless wasteland, valuable only as a source of peat for domestic fires, the British blanket bogs have assumed international conservation importance. It is a habitat type largely confined to Britain and Ireland and of great scientific value. The patterned blanket bog of the flow country is virtually unique.
However, blanket bogs have suffered a number of types of damage:
- Clearance. Some have disappeared completely, where the peat has been removed to expose underlying mineral soil and creating of poor grazing land. Here, above Paisley, much of the peat cover has been stripped from Gleniffer Braes, leaving only isolated areas of deeper peat, and the same can be said for parts of the Renfrewshire Hills. However, such action has not been viable for larger expanses and where there is little human habitation.
- Peat cutting for fuel has long been a traditional use for blanket bogs, and current management must often take account of local peat cutting rights. On actively growing bogs it can be argued that the impact is low, cutting is typically on a rotational system, allowing regrowth before any particular area is cut again, and it can be argued that this maintains diversity.
- Drainage attempts have been made on many blanket mires and old ditches are often visible, though actively growing systems have readily recovered where ditching has been minor and limited. However, in areas where total rainfall and numbers of rain-days are now marginal for peat growth, drainage has led to drying and erosion.
- Erosion is a major factor where total rainfall does not allow peat to recover from any damage. The protective Sphagnum layer may have gone, bare peat exposed, and small channels can quickly become enlarged by surface run-off on days when it does rain heavily. Creation of peat hags can follow, deep breaks in the peat cover, often two or more metres deep and floored by expanses of very soft, treacherous peat or sometimes exposing underlying tree stumps. Sheep often cause expansion of eroded, hag areas by rubbing themselves against the sides, but they are also sometimes trapped in the soft wet peat that is exposed.
Erosion can be initiated by many factors, drainage attempts, over burning and, increasingly, by damage from "all-terrain" vehicles. A single drive across a bog surface can create permanent damage.
- Afforestation. For those who viewed the blanket bogs as wasteland, the introduction of forestry trees such as Sitka Spruce (Picea sitchensis), a tree that can grow in wet peat, seemed like the perfect answer. While vast areas of viable commercial forest have indeed been created, it has become clear that in areas such as the flow country of Caithness and Sutherland, there has been extensive damage to unique ecological systems without even any significant commercial return. A system of tax incentives encouraged piecemeal private afforestation, often simply as forms of investment by high earners in sports and show business. Greater environmental awareness at policy making levels and, no doubt more significantly, economic limitations of the forestry industry, have slowed this damage, but afforestation remains a substantial cause of habitat loss.
- Grazing evidently has more impact on some blanket bogs than others. Sheep grazing is significant on many upland sites and, as noted above, NVC community M20 is regarded as a floristically impoverished community, with high dominance of Hare's-tail Cottongrass (Eriophorum vaginatum) a frequent consequence of high grazing pressure. The Dwarf Birch (Betula nana) a very local, low-growing shrub of upland sites may suffer in particular, though there may well be climatic reasons why it never seems to show the same health and vigour that it achieves in Scandinavia.
- Burning. Grouse moors are in many (but not all) cases established over blanket peat. A controlled rotation of burning over several years is used to remove old, "leggy" heather and so encourage the young shoots that are a principle food of the Red Grouse (Lagopus lagopus scoticus). Clearly it is not the aim to burn off all surface vegetation and in skilled hands, muirburn can be seen as enhancing diversity, though certain species, such as Sphagnum imbricatum may be adversely affected. Unplanned fires, or burns that become out of control can be far more damaging, killing nearly all surface vegetation and exposing bare peat that will then be subject to erosion. They are most serious on blanket bogs with little active growth and low cover of Sphagnum.
- Pollution. The southern Pennines are adjacent to highly industrialised towns and cities, such as Manchester, and acid deposition has been seen as a significant problem affecting the growth of Sphagnum and other plants. Commercial afforestation has been limited for this reason. With recent substantial reductions in sulphur dioxide release, acid deposition problems are no doubt diminishing. The insidious and uncertain effects of nitrogen oxides, principally from car exhausts, may also be a factor, though most blanket bogs are remote from heavy traffic.
- Wind farms. The latest serious concern also relates to energy generation, this time with the reported possible threat of severe scenic impact and direct destruction of internationally important sites.
Recently, the conservation value of our peatlands has been more widely recognised and various restoration and recovery programmes are in effect. In the flow country, the RSPB reserve at Forsinard, Sutherland, is one in which planted forest is now being removed again, with the hope that the active peat growth will reinstate the original habitat. The majority of our important blanket bogs are now designated Sites of Special Scientific Interest (SSSIs) or specifically managed as National Nature Reserves, but blanket bogs remain a high priority under the UK Biodiversity Action Plan. It is to be hoped that the UKBAP itself continues to be rated a high priority in land-use conflicts.
|• ||Burnett, J.H., (1964). The vegetation of Scotland, Oliver & Boyd, Edinburgh.|
|• ||Goode, D.A., & Ratcliffe, D.A., (1977). Peatlands, in Ratcliffe, D.A. (ed.), A nature conservation review, vol. 1, chapt. 8, Cambridge Univ. Press, Cambridge.|
|• ||Moore, P.D., & Bellamy, D.J., (1974). Peatlands, Elek Science, London.|
|• ||Moore, P.D., Merryfield, D.L., & Price, M.D.R., (1984). The vegetation and development of blanket mires, in Moore, P.D., (ed.), European mires, chapt. 6, Academic Press, London.|
|• ||Rodwell, J.S., et al. (1991). British plant communities, 2: Mires and heaths, Cambridge Univ. Press, Cambridge.|
|• ||Tansley, A.G., (1939). The British Islands and their vegetation, Cambridge Univ. Press, Cambridge.|
|• ||UK Biodiversity Action Plans - Blanket bog|
| ||The 'official' current British statement on the conservation status of blanket bog as a priority habitat, and required actions.|
|• ||The Life Peatlands Project 2001-2005 - Restoring Active Blanket Bog of European Importance in North Scotland|
| ||A valuable account of current peatbog conservation and restoration in northern Scotland. Includes a nice page on blanket bogs (good aerial photographs) - follow the "All about Peatlands" link from the contents list.|
|• ||Action for Scotland's biodiversity (Scottish Executive)|
| ||The formal statement of Scottish action on maintenance of biodiversity. Blanket bog is only briefly considered under part 7, but there is valuable general discussion on factors such as muirburn.|