Forestry Focus

Forestry and Water Quality

Research has identified that there are three main potential impacts of forestry on water resources in Ireland i.e. acidification, siltation and eutrophication. Water shortages due to the presence of forests have generally not been an issue in Ireland as we experience ample rainfall throughout the year.


Many of Ireland’s older forests are located on upland and peatland areas where the soils are naturally acidic. Conifers tree species dominate these forests as they are best adapted to exposed sites and acid soils. Conifer plantations, however, can increase site acidity through capturing air borne pollutants and concentrating these in surface waters. Increasing the acidity of already acid systems can seriously affect the aquatic ecosystem and the life dependent on it.

In pre-Guidelines forests it is difficult to mitigate the effects of acidification. Changing the chemistry of surface waters through liming is possible, but difficult and costly to maintain. Liming unless carefully carried out and regularly monitored can change the ecology of the waters and is difficult to sustain in the long term. Slowing down run-off to allow water to remain in contact with mineral soil particles is perhaps the simplest to way of buffering against acidity. Another measure is to reduce the area of conifers within a catchment by creating more open space, or by planting broadleaved species if they will survive. Not replanting is also an option but one which is not favored by the Forest Service or the EU as it implies deforestation.

Following the introduction of the Forestry and Water Guidelines the Forest Service now consults with the Regional Fisheries Boards and Local Authorities on whether or not to proceed with new plantations in areas where there is a perceived risk of acidification. Decisions are made on the basis of criteria such as the importance of the aquatic zone for salmonids, the amount of existing forests in the catchment, the base geology and water chemistry.  Such procedures will ensure that acidification due to forestry should not be an issue in these areas in the future.

Further information on forests and surface water acidification can be accessed at:

Forestry and surface water acidification


Site disturbing operations such as cultivation, road building and harvesting when combined with heavy rainfall pose a high risk of silt runoff into waterways. Great care is therefore required in carrying out these operations to ensure that the measures in the Forests and Water Guidelines are followed.  Silt has the effect of smothering streambeds and decreasing oxygen levels in the water which can greatly affect the fish and micro invertebrate life and even change the structure of the physical habitat.

Further information of forests and siltation can be accessed at:

An evaluation of the role of forests and forest practices in the eutrophication and sedimentation of receiving waters – literature review


Eutrophication is the process by which a body of water acquires a high concentration of nutrients, especially phosphates and nitrates. The effect of eutrophication is to promote excessive growth of algae which when it dies and decomposes, depletes the water of available oxygen causing the death of other organisms such as micro-invertebrates and fish. Eutrophication is a natural, slow-ageing process for a water body, but human activity greatly speeds up the process.

Fertilization and harvesting are the two main forest operations that can cause nutrient run-off to water bodies and contribute to their eutrophication unless mitigating measures are taken. The Forests and Water Quality Guidelines and Forest Harvesting Guidelines describe best practice that must be adopted when carrying out these operations.

The risks from fertilization are the accidental application of fertilizer to the water body and also through the soluble element being carried into the drains and streams before it can be taken up by the site vegetation. Mitigation measures include ensuring that:

  • there is an adequate  buffer zone around the water body on which no fertilizer is applied,
  • fertilizer is applied when vegetation is actively growing and
  • fertilizer is only applied to established crops based on foliage analysis so that excessive amounts are not used.

Nutrient run-off can also be caused by forest harvesting. When trees are felled the timber is removed but the brash (foliage and branches) is usually left on the ground.  As the brash decays nutrients are released and being highly mobile can be transported into streams and rivers through the site drainage system. Peat sites are particularly susceptible to nutrient run-off as the organic soil does not retain nutrients to the same extent as mineral soils.  Mitigation measures include limiting the size and number of felling coups within a catchment at any particular time so that cumulatively large areas of forest are not felled together.  Slowing down the flow of water from a site is also another option so that nutrients can be bound to soil particles or absorbed by vegetation. The riparian zones that are currently being established will help prevent nutrients from entering waterways when they are well established.

Further information of forests and eutrophication can be accessed at:

An evaluation of the role of forests and forest practices in the eutrophication and sedimentation of receiving waters – literature review

Forestry operations and eutrophication

Forestry and Water Guidelines

Despite the acknowledged beneficial effects of forests on water, there has been considerable controversy about the impact of forestry and forestry practices on water bodies, particularly in the upland and western peatland areas of Ireland. The presence of large coniferous plantations on acid sensitive sites and their effects on water courses are of concern to game fisheries.  The acidifying effects of conifers in already acid sensitive catchments, and outflows of nutrients and sediments, particularly from harvesting operations, can seriously affect the reproductive capacity of fish populations and the invertebrate life on which they depend, unless mitigating measures are taken. Forestry and Water Guidelines, however, have been developed by the Forest Service with fishery interests and other stakeholders to give direction to forest planners and contractors in avoiding potential impacts. Coillte has also developed its own harvesting and water quality guidelines to meet the needs of forest certification which go beyond the requirements of the Forest Service Guidelines.

Strict adherence to these guidelines and procedures is required and their implementation is subject to regular inspection by the Forest Service, Regional Fishery Boards and forest certification auditors.  After Health and Safety the impacts on water bodies are perhaps the most closely monitored aspect of forestry operations.

Despite existing guidelines and procedures our wet (and changing) climate poses a continuous challenge to forest managers and contractors. With no predictable dry season, and the fact that over 40% many of Ireland’s forests are on peat soils, many of which are located in the headwaters of important salmonid and fresh water pearl mussel catchments, water issues will continue to dominate forest operations for the future. A key challenge is to maximize the wide range of multi-sectoral forest benefits without detriment to water resources and ecosystem function.