Forestry covers about 10% of the land area of Ireland. Of this number, about 80% represent quick-growing commercial coniferous species, such as Sitka spruce. The ‘Strategic Plan for the Development of the Forestry Sector in Ireland’ is set to increase this forestry cover to 17% by 2030.2623254482_91846c98a2_z

Forestry planting and harvesting can have damaging effects on the aquatic environment, especially if efforts are poorly sited or managed. These include:

  • Acidification;
  • Siltation;
  • Eutrophication

Acidification of waters due to conifer plantations, particularly in sensitive areas such as the Wicklow Mountains and West Galway-Mayo, can have serious long-term effects on aquatic life. The canopies of coniferous forests are efficient scavengers of particulate and gaseous contaminants in polluted air. This results in a more acidic deposition under forest canopies. Chemical processes at the roots of coniferous trees then further acidify soil and soil water in forest catchments. When forests are located on poorly buffered soils (soils without the chemical constituents necessary to neutralise the acidity), these processes can lead to significant acidification of the run-off water and consequent damage to nearby streams and lakes.
The toxic action of acidity is most pronounced in its effect on fish and macroinvertebrates. Extreme cases of acidification can lead to the death of these organisms, while lower level cases may produce non-lethal effects such as respiratory difficulties and impaired growth and reproductive ability, which can lead to elimination in the long-term. Artificial acidification of surface waters is not extremely widespread in Ireland, being largely confined to areas in the country where conifer afforestation has taken place on acid-sensitive soils.
Siltation is potentially one of the most serious impacts of clearfelling (the harvesting of all or most of the trees from a forested area) on aquatic systems. All forestry operations result in soil disturbance during the initial planting, road construction and eventual harvesting. Sediments from soils are then released and washed into streams, where they may remain suspended in the water or settle onto the streambed. Such siltation can affect stream ecology by settling on and damaging fish gills, or by altering the streambed habitat necessary for fish spawning and the survival of streambed invertebrates.

Several forestry activities, primarily fertiliser application and clearfelling, can lead to the eutrophication of water bodies in the catchment area. Nitrates and phosphates in fertilisers, as well as nutrients in soil disturbed during harvesting operations, can lead to the enrichment of run-off water, which feeds into surrounding rivers and lakes. Management measures such as reduced fertiliser use, the creation of buffer zones around water bodies, and the phased harvesting of forestry areas to limit the land area subject to clearfelling at any one time can help reduce eutrophication impacts.IMG_1110