But minnows and trout barely survive in its clear, cool lakes and streams. Sensitive bog mosses and lichens - which form a fragile crust, clothing large tracts of the moor - are in decline. And grassland, which is poor in wildlife, may be extending its hold at the expense of species-rich heathers.
This SSSI is far from being alone. The UK is committed to reducing emissions of sulphur dioxide (SO2 ), the major component of acid rain. But even when we have done so, more than 1,000 SSSIs in the UK will still be at risk of acidification resulting from the deposition of sulphur. Many of these SSSIs - our most important locations for wildlife - are moorlands, uplands and mountains. Others are broad-leaved woodlands, lowland heaths, flower and insect-rich meadows and pastures, wetlands and coastal habitats.
This startling finding is published today by the Joint Nature Conservation Committee, following an assessment of SSSIs at risk carried out by the Government's statutory conservation agencies, the Countryside Council for Wales, English Nature and Scottish Natural Heritage. It is the first time that anyone has quantified how much of the UK's wildlife is at risk from acid rain.
The assessment has used something called the critical loads model. A critical load is the largest amount of a pollutant that will not cause harm-
ful effects on the most sensitive ecosystems.
To construct a critical-loads map for soils across the country, the model uses information on the dominant soil for each square kilometre of land. At one extreme, soils rich in carbonates - such as those derived from chalk or limestone - weather rapidly and have a high capacity to neutralise any acid rain that falls on them. At the other extreme, soils rich in quartz and other acidic minerals produce naturally acid soils. Consequently, they have little or no ability to neutralise any more acid that comes their way from man-made sources.
Knowing a soil's critical load is only part of the picture. To complete it, one has to estimate how much sulphur is falling to the ground and where.
There are many sources of SO2 . The largest are coal and oil-burning power stations. Industry contributes its share, too. The pollution falls back to the ground, often a long way from its source, dissolved in rain and mist or as dry dust.
According to the most recent estimate, 3.6m tonnes of SO2 are deposited each year on Britain's green and pleasant land. The quantity is declining. That is due, in good part, to implementation of the European Community's Large Combustion Plant Directive to which the UK government is a signatory. This stipulates a 60 per cent reduction in SO2 emissions by 2003 (using 1980 as a baseline).
The computer modelling done for the assessment of SSSIs at risk has assumed that this 60 per cent reduction will be achieved. There are no fewer than 1,242 SSSIs in Britain in areas where - even after the reduction in emissions - the critical loads for SO2 will be exceeded. Removing from this list those sites that are unlikely to be affected by such pollution - caves or important bat roosts, for instance - leaves 1,023 SSSIs at risk. This represents an area of nearly half a million hectares of land.
Most of the sites are in the upland areas of north and mid-Wales, north- west England and south-west Scotland. North Wales (Gwynedd and Clwyd) is the most severely affected region of the UK. No less than 77 of its SSSIs - an area of around 52,000 hectares - will be at risk. That represents more than 60 per cent of the SSSI area in the region.
Extensive areas of upland - mountain plateaux in Snowdonia and large expanses of wet moorland, blanket bog, upland heath and grassland - dominate the picture. They are hearth and home to a wide range of flowering plants, including rarities such as Snowdon Lily, and to numerous species of bog mosses and lichens that are particularly susceptible to acid rain damage. Many of the oak wood SSSIs in north Wales - some significant because of their rich populations of mosses and liverworts - will also remain at risk in 2003.
If SO2 emissions were cut by more than 60 per cent, the soil's critical load would be exceeded in fewer areas of the UK. The conservation agencies therefore also assessed the number of SSSIs at risk if the quantity of the remaining SO2 emissions in 2003 were to be halved. That is an 80 per cent reduction of 1980 levels, rather than 60 per cent.
Instead of 1023 SSSIs, the number at risk UK-wide falls to 405, an area of 225,000 hectares. It still represents 12 per cent of Britain's existing SSSI area, mainly because many large uplands would remain acidified. In north Wales, the number of at-risk SSSIs would fall from 77 to 44, and the area affected from 52,000 hectares to 26,000 hectares.
An 80 per cent reduction, rather than 60 per cent, would be a considerable gain for the UK's most important wildlife habitats. But even more severe reductions are necessary in the amount of SO2 if these sites are to gain the protection they deserve.
Some plant communities are sensitive to acid pollution but do not depend on soil for sustenance. Examples are lichens that grow on tree bark.
The critical-loads model, together with this first-ever count of the UK's most important wildlife sites affected by acidification, is the best, and only, such assessment that exists. It is being used by the UK government in the UN Economic Commission on Europe negotiations taking place in Geneva. These will decide, by the end of this year, the next European reduction target for sulphur dioxide under the Convention on Long Range Transboundary Air Pollution.
But as sulphur dioxide pollution falls, environmental experts are turning their attention to those other villains of the pollution piece, nitrogen oxides. Around 2.8m tons - from car exhausts, power stations and industrial chimneys - are deposited on the UK each year. And as our roads clog with vehicles, the quantity is rising. Nitrogen oxides dissolve in rain water to produce nitric acid. They also have a fertiliser effect, stimulating growth of some plants at the expense of others, upsetting the fragile ecological balance in habitats such as heaths and grasslands.
The author is chief ecologist of the Countryside Council for Wales.
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