Do forests acidify waters?

The effects of forests and forestry practices on the acidification of soils and waters

This has been the subject of much research over the last 25 years. A principal concern in the UK has been whether upland conifer forests have contributed to the serious problems of acidified waters and loss of salmon in acid sensitive areas. The primary cause of acidification is the deposition of acidifying sulphur and nitrogen pollutants from the combustion of fossil fuels. However, trees can increase the amount of pollutants that are deposited on a given area due to the greater air turbulence created by their ‘rough’ canopies.

The areas of the UK that are worst affected by acidification

These are parts of central and southwest Scotland, Cumbria, the Pennines, Wales and the Mourne Mountains of Northern Ireland.

This is because of their base-poor, slow weathering soils and rocks, which are unable to neutralise the large quantities of acid pollutants that they receive in rainfall. Over the past 100-150 years, stream and loch waters in these areas have gradually acidified until some can no longer support salmon or other sensitive aquatic life.

International action was taken in the 1980’s to tackle this problem by reducing the emissions of acid pollutants by power stations and industry. However, while this has resulted in many waters showing signs of chemical recovery, the response of fish and aquatic invertebrates has generally been small.

Researching the issue of acidification and its causes

Since forests can enhance the deposition of acid pollutants from the atmosphere, they have the potential to further acidify sensitive waters. There is therefore a need to be able to quantify the amount of pollutants that are captured by forests and identify where this could have a significant impact on the water environment. The way that forests are planned, designed and managed will also influence the risk of them contributing to acidification.

Many organisations have been involved in and continue to research the issue of acidification and its causes, both in the UK and across Europe and the wider world. Much research is undertaken in collaborative programmes and is designed to guide emission control policy and practice. We carry out research, monitoring and assessment work in the following areas:

  • The effects of new planting
  • The effects of harvesting
  • Identifying sites at risk
  • Critical loads approach
  • Liming as a potential tool to ameliorate acidification
  • Catchment-based critical loads assessments.

Modelling the long-term response of stream water chemistry to atmospheric pollution and forestry practices in Galloway, SW Scotland

We have recently been working with the James Hutton Institute and other partners to assess the role of forestry in relation to atmospheric pollution and the acidification and recovery of acidified waters. This was to inform discussions between the forestry sector, the Scottish Environment Protection Agency, fisheries groups and other stakeholders on the case for further reductions in forest cover to aid the recovery process in affected areas.

The long term (1860-2100) response of soil and surface water chemistry in selected streams and rivers in Galloway to changes in acidic deposition and forest management practices were assessed using the dynamic model MAGIC (Model of Acidification of Groundwater in Catchments). The results of this work are now available in a joint report (see right), which was jointly funded by the Forestry Commission, the Scottish Government’s Rural and Environment Science and Analytical Services Division, the Department for Environment, Food and Rural Affairs and Trent University, Ontario.