Information and Advisory Note Number 127
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NATURAL HERITAGE TRENDS Land Cover 1947-1988: mountain,
moor and heath
1. Blanket Mire
1.1 'Mire' is the generic term for a wetland which
supports vegetation that is capable of peat-formation. Through incomplete decay
and steady accumulation of dead material in a largely waterlogged and anaerobic
environment, a variety of plants can lay down peat: Sphagnum mosses or cotton
and deer grasses in bogs, or sedges, reeds and trees in fens.
1.2 'Bogs' are the most extensive form of mire in Scotland. Receiving nutrients
mainly from precipitation (ombrotrophic), they represent some of the most
nutrient-poor and acidic environments for plant growth. With available nitrogen
and phosphorus in short supply, bog plants are adapted to tight nutrient
cycling, exemplified in the carnivorous sundew, Drosera spp.
1.3 With increasing altitude, latitude and oceanicity, summers remain cool and
there is no seasonal moisture deficit. Where topography permits, conditions
become favourable for the development of a continuous mantle of peat, termed
'blanket mire' in which 'blanket bog' is the main constituent. Blanket mire in
Scotland is extensive, and unusually so on a European scale. Under low-intensity
management of sheep grazing, sport (deer, grouse and salmon) and local
small-scale peat cutting, the conservation interest of blanket mire is of
international importance.
2. Trends
2.1 Over 41 years, between c.1947 and c.1988, the area of blanket mire in
Scotland is estimated to have declined by 21%: a mean annual rate of 117 kmē.
2.2 The rate of decline increased between the 1970s and 1980s: an average of 230
kmē per
year compared with 51 km per year between the 1940s and 1970s.
2.3 Half of the reduction in the blanket mire area (51%) was due to
afforestation. Much of the remainder was drained (44%), and mainly in
preparation for forest planting. As drained blanket mire could no longer be
regarded as actively peat-forming, it was classified as grassland or heather
moorland according to its dominant vegetation cover.
2.4 Some peat stripping occurred, evident from un-vegetated mire being
classified as bare ground.
2.5 Afforestation, drainage and agricultural improvement took place throughout
Scotland, and so blanket mire contracted throughout its range. The scale of mire
modification was most evident in its stronghold areas to the north and west.
2.6 The Western Isles, Orkney Isles and Shetland Isles are not associated with
forestry. There, mire reduction was due to drainage and pasture improvement.
2.7 Overall blanket mire contracted from around 22,700 km2 in the 1940s to
17,900 km2 in the 1980s.
3. Lowland Mire
3.1 Despite a summer moisture deficit in the lowlands, topography and restricted
drainage have given rise to the formation of 'raised bog' in some places.
Distinctive as dome-shaped 'islands' of peat, the appearance of these elements
of wild land is often in stark contrast to their more intensively farmed
surroundings. They provide habitat for many characteristic plants and animals of
Scotland, including invertebrates, amphibians and reptiles, birds and mammals.
Vegetation on raised bogs reflects the position of the water table. Many have
been artificially drained, so enabling the spread of dwarf shrubs and scrub at
the expense of more natural open moss vegetation.
3.2 Lowland raised bogs are scattered across the Central Belt, the Grampian
coastal plain and the Solway shore, with transitional forms which intergrade
into blanket bog. Low-intensity land uses can be sustainable, where grazing,
sporting estate management, carefully managed burning and domestic peat cutting
have only limited and localised effects.
More intensive forms of exploitation, including mechanised drainage, enrichment
with fertilisers, ploughing and afforestation have been more damaging. The
impact of past and present commercial-scale peat extraction in Scotland is
extensive, especially in the Central Belt and the Grampian plain. Less than
one-tenth of the raised bog habitat in Scotland persist in a relatively
unaltered state, yet this still represents one of the richest surviving
concentrations in the European Union.
4. Trends
4.1 Over 41 years, between c.1947 and c.1988, the area of lowland mire in
Scotland is estimated to have declined by 44%: a mean annual rate of 2.5 kmē.
4.2 Where peat stripping occurred, un-vegetated mire was classified as bare
ground (6%).
4.3 Around half (47%) of the reduction in area was due to afforestation. Much of
the remainder was drained, becoming re-classified here as grassland (26%) or
subject to scrub encroachment (13%).
4.4 A range of smaller changes accounted for the rest.
4.5 Lowland mire contracted throughout its range, and especially so in the
south-west. Overall, it is estimated to have contracted from around 230 kmē in
the 1940s to 130 km2 in the 1980s.
5. Heather Moorland
5.1 Dwarf-shrub heaths, which are largely confined to the British Isles and
western seaboard of Europe, are of international conservation importance. Upland
and lowland heaths have been identified as 'priority habitats1 for biodiversity
in the UK. Composed of heather Calluna vulgaris and other dwarf shrubs, often
with rich bryophyte and lichen communities, wet and dry heaths are especially
widespread in Scotland. Heather moorland sustains a rich invertebrate fauna, and
is prime habitat for a suite of bird and other wildlife species. Heather is
itself regarded as one of Britain's 'keystone species'. Countering a tendency to
revert to woodland, most heaths below the natural treeline are maintained by
grazing and burning. Under low-intensity land management, many species
associated with the original woodland ground layer can persist.
5.2 With changes in soil moisture, blanket mires, wet heaths and dry heaths
grade into each other. The oceanic, cool, wet climate of the west favours the
development of wet heaths. Dry heaths become more predominant in the east.
Maritime heaths are also to be found on coastal cliff top and dunes, where
vegetation is also influenced by exposure and salt spray.
6. Trends
6.1 Over 41 years, between c.1947 and c.1988, the area of heather moorland in
Scotland is estimated to have declined by 23%: a mean annual rate of 80 kmē
.
6.2 As drained heather-dominated mire was re-classified as heather moorland
(i.e. no-longer considered to be actively peat forming), that under-estimates
the reduction of true' heather moorland. Eliminating the compensating effect of
drained mire, heather moorland declined by 28%: a mean annual rate of 100 km2
per annum.
6.3 The 'corrected trend' illustrates a near continuous rate of decline from the
1940s to the 1980s.
6.4 Half of the true heather decline (53%) was brought about by afforestation
and one-third (37%) by grassland expansion. Although heather also colonised
grassland in places, the overall trend was of grassland expansion at the expense
of heather.
6.5 There was evidence of bracken expansion (6%), but this was confounded by
improved detection of bracken as the quality of photography improved.
6.6 A range of smaller changes accounted for the rest.
6.7 The pattern of afforestation and grassland conversion was replicated
throughout Scotland. Consequently, heather moorland contracted throughout most
of its range. Exceptions were in the Shetland and Western Isles where apparent
expansion was associated with mire drainage.
6.8 Overall heather moorland contracted from around 14,600 km2 in the 1940s to
11,300 kmē in the 1980s.
7. Rough Grassland
7.1 Rough ('unimproved' or 'semi-natural') grassland is associated mainly with
the unenclosed uplands. There, acidic or calcifugous grassland is most extensive
throughout Scotland. Neutral or mesotrophic grasslands would occur in lowland
areas on moderate to nutrient-rich soils, but they have been greatly diminished
by agricultural improvement. Where they still occur, they tend to be confined to
inaccessible areas. Lime-loving or calcicolous grasslands establish in areas
where soils are derived from a chalk or limestone geology. Machair is a
calcicolous grassland derived from wind-blown calcareous shell sand.
7.2 Except at high altitude, on exposed coasts or in extreme northern areas
where climate limits the growth of trees, grasslands in Scotland would generally
undergo natural succession to woodland if unmanaged or ungrazed. The character
of grassland communities, in terms of the range and abundance of plant and
animal species, is governed mainly by soil type and drainage, as
well as by the degree of disturbance or alteration through land management. In
the uplands, grazing and browsing by sheep and deer, as well as the 'muirburn'
of purple moor grass in the West Highlands, have exerted a strong influence on
the composition of grassland plant communities.
8. Trends
8.1 Over 41 years, between c.1947 and c.1988, the area of rough grassland in
Scotland is estimated to have declined by 10%: a mean annual rate of 29 kmē.
8.2 Drained grass-dominated mire was reclassified as rough grassland (i.e.
no-longer considered to be actively peat forming). That under-estimates the
reduction of 'true' rough grassland. Eliminating the compensating effect of
drained mire, rough grassland declined by 19%: a mean annual rate of about 56
kmē per annum.
8.3 The 'corrected trend' illustrates a near continuous rate of decline from the
1940s to the 1980s.
8.4 The relatively small overall change in extent conceals substantial areas of
rough grassland that were afforested or, to a lesser extent, altered by
grassland improvement. This is because reductions in the extent of rough
grassland were offset by heather moorland becoming grassier and by blanket mire
drainage (since grass-dominated blanket mire was re-classified as rough
grassland).
8.5 Changes in the extent of rough grassland exhibit a distinct north-south
divide:
In the north, an apparent increase in the area of rough grassland occurred due
to the effect of re-classifying drained mire and heather moorland becoming
grassier. These outweighed reductions due to afforestation and grassland
improvement.
In the south, rough grassland was reduced by afforestation and grassland
improvement.
8.6 The extent of rough grassland contracted from around 12,300 km2 (16% of
Scotland) in the 1940s to 11,100 kmē (14% of Scotland) in the 1980s.
9. Source
9.1 Statistical estimates are from the 'National Countryside Monitoring Scheme',
a sample survey which interpreted air-photography representing 7.5% of
Scotland's land area (Mackey et al., 1998). Although sampling is not optimal for
this purpose, change in extent is mapped here according to the 21 Natural
Heritage Zones.
9.2 The geographical distribution of habitats was mapped using the nearest
equivalent features from the Land Cover of Scotland 1988 dataset (MLURI, 1993).
10. References
Macaulay Land Use Research Institute (1993).
The Land Cover of Scotland 1998:
Final Report. Aberdeen: MLURI.
Mackey, E.C., Shewry, M.C. and Tudor, G.J. (1998).
Land Cover Change: Scotland
from the 1940s to the 1980s. Edinburgh: The Stationery Office.
11. Further Information
Further information on other topics covered by Natural Heritage Trends: Land
Cover 1947-1988 can be found in Information & Advisory Note No. 123.
12. Contact
To obtain further information about any of the issues raised in this l&A Note,
please contact
Mr Ed Mackey and Mr Mike Shewry
Chief Scientist's Unit
Environmental Audit Group
Scottish Natural Heritage
2 Anderson Place
Edinburgh
EH6 5NP
Tel: 0131-446 2415
Fax:0131-446 2405
E-mail: ed.mackey@snh.gov.uk
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