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The impact of afforestation is complex, not only are there obvious and direct effects on the ground ploughed and planted, but unplanted ground and freshwater habitats both within and beyond the forest are affected in varying degrees.

In the situation of continually expanding afforestation, open ground steadily contracts in area and may become increasingly surrounded and "squeezed" between blocks of forest, so that its relative value may change.

EFFECTS ON BIRDS

Species loss: The direct effect of afforestation is to replace the peatland bird assemblage with a woodland bird assemblage.

Different species are affected at varying rates. Some such as dunlin, disappear quickly, while others may linger for several years, though usually in reduced numbers.

The relevant outcome is that, by the time the young forest closes to the thicket stage at 10-15 years, the habitat has been transformed and is useless to most open moorland birds. They disappear and, at the minimum their loss is directly proportional to the area and quality of the ground planted.

There is no corresponding increase in population density in the adjoining unplanted peatland, because of the carrying capacity of peatland. Bird populations in peatlands have an upper limit imposed by the productivity of the habitat. Most of the peatland bird species already have a non-breeding surplus which is excluded from nesting by the territorial behaviour and spacing of the breeding population.

Edge Effects:

Afforestation also leads to thinning out of breeding bird populations on adjoining unplanted moorland beyond the forest boundary or farming enclaves within it. There is a cessation of moorland management, for example grazing and heather burning and after 10-15 years the luxurious vegetation growth is unsuitable for breeding birds who need short vegetation e.g. golden plover and dunlin.

Some bird species tend to avoid the forest edge resulting in a zone of lower density around the perimeter. This can also be due to increased predation associated with the forest.

Afforestation creates a new nesting habitat for carrion and hooded crows and increased cover for foxes, but they rely largely on the ground outside the forest for food and will perforce concentrate their search over the adjoining bogland. Waterfowl are seriously threatened as they use bog pools for nesting. The young remain near pools and rivers after hatching and are readily located by predators.

Only areas greater than 270ha sustain a representative moorland bird assemblage including less common species such as merlin, ring ouzel and short - eared owl.
But predatory birds will need much larger areas of open moorland hunting area than that which will satisfy the feeding and other requirements of their prey species.

In areas of low carrying capacity such as bogs birds of prey need very large areas to hunt over. Loss in their feeding areas of 50% can result in loss of species. e.g. Ravens.

For merlins and Kestrels the crucial issue is the extent of open hunting ground remaining beyond the planting.

In Caithness and Sutherland since forestry began there has been a 19% golden plover, 17% dunlins, 17% greenshanks decline.

After forest rotation ends, it is unlikely that the bird populations of bogs will be restored. The drying out of the bog means that wetter species will never return.

The woodland habitat created is not richer in woodland birds nor is it an adequate substitute for the original peatland habitat. Passerine song birds increase but these are common everywhere and cannot justify creating more forests at the expense of the specialised peatland bird assemblage.

EFFECTS ON VEGETATION

Afforestation causes the replacement of peatland vegetation by dense stands of conifers with few other plants. Drainage and deep ploughing used as standard ground preparation rapidly dry out the surface and cause the loss of strongly moisture-loving species. When the forest closes to thicket, all the ground vegetation disappears, due to intense shading and heavy litter fall.

Leaving patches of unplanted bog in forestry plantations eventually kills their scientific interest because they are cut off from the other peatland mass upon which they depend.

Even limited drainage can have very series effects on the ecology of bogs. Surface drainage affects the water table level and the species composition of the surface vegetation, and peripheral deep drainage can alter the ground-water mound causing shrinkage of the peat mass, and decomposition.

Complete hydrological systems are necessary to protect peatlands.

As the trees are established, higher evaporation rates lower the water - table further.

These crucial alterations in the blanket bog conditions mean that even when felled, blanket bog moisture loving species will never recolonise. More likely a mixed grass-heath will develop.

Surface oxidation destroys the upper layers of the peat mass and therefore destroys the pollen record.

EFFECTS ON ABIOTIC FEATURES

Ploughing increases peak run off and flash flooding. There is erosion of peat and cracking of the peat mass. This causes the build up of peat in streams.

Lowering of the water table dries the peat and allows oxidation to occur. This increases peat acidity. Acidification of stream waters occurs.

The major chemical effect, is nutrient enrichment of peat, soils and waters by fertilisation associated with tree planting. This causes local eutrophication, leading to algae blooms in lakes and reservoirs.

Acidification of rivers affects fish spawning.

OVERALL ASSESSMENT

Afforestation so fundamentally transforms the peatland ecosystem that it is effectively destroyed.

The fauna and flora are lost, and the physical attributes so altered that even if forestry ceased, any reconstitution of the previous ecosystem would be impossible.

The conservation importance of peatlands in their natural state far outweighs any temporary species diversity associated with the early development of the artificial forest.

References

Rathcliffe D. A. and Oswald, P. H. (1987) Editors.
Birds, Bogs and Forestry. The Peatlands of Caithness and Sutherland. Nature Conservancy Council, Peterborough.

Research Publications and Unpublished Reports on the Effects of Forestry in Ireland

Allott, N. (1993) Evaluation of the Effects of Forestry on Surface-Water Chemistry and Fishery potential in Ireland. Final report executive summary. Eolas Contract ER/90/76. 5pp.

Allott, N., Brennan, M. (1993) Impact of Afforestation on Inland Waters. In: Mollan, C. (Ed) Water of Life The Proceedings of a Conference on the Inland Waters of Ireland, held in the Royal Dublin Society from October 7-9, 1992. Royal Dublin Society, Dublin. p 89-98.

Allott, N., Brennan, M., Mills, P. & Eacrett, A. (1993) Stream Chemistry and Forest Cover in Ten Small Western Irish Catchments. In: Watkins, C. (Ed) Ecological Effects of Afforestation. CAB International, UK. P. 165-177.

Evaluation of the Effects of Forestry on Surface Water Chemistry and Forestry Potential in Ireland. The Aquafor Report. Publication date: June 1997. Four volumes:
1. Chemistry of precipitation and throughfall in soil water and stream chemistry.
2. Munster region.
3. Wicklow region.
4. Galway/Mayo region.

Giller, P.F. et al (1993) An Integrated Study of Forested Catchments in Ireland. Irish Forestry 50 (1): 7-83

Mollan, C.& Moloney M. (1991) Editors. The Right Trees in the Right Places. Conference on Forestry and the Environment, R.D.S., Dublin.

Allott, N., (1993) Evaluation of the effects of forestry on surface water chemistry and fishery potential in Ireland. Eolas Cntract ER/90/76. Trinity College Environmental Sciences Unit.
Volume 1: Chemistry of precipitation, throughfall and soil water, Wicklow & Galway Regions.
Volume 2: Steam Chemistry and Biota, Galway-Mayo Region.
Volume 3: Steam Chemistry and Biota, Wicklow Region.