Forestry and surface water acidification

Forests and forest management practices can affect surface water acidification in a number of ways. The primary mechanism is the ability of tree canopies to capture more sulphur and nitrogen pollutants from the atmosphere than other types of vegetation. Pollutant scavenging is expected to have peaked in the 1970s when emissions were greatest and led to surface waters draining catchments dominated by forestry being more acidic. The introduction of emission control policies in the 1980s has achieved major improvements in air quality and studies show forest sites to be recovering in line with their moorland counterparts. However, forest streams remain more impacted, requiring continued restrictions on new tree planting and restocking. Tree planting can influence acidification by the scavenging of acid deposition, base cation uptake, the scavenging and concentration of sea salts, soil drying and the formation of an acid litter layer at the soil surface. Cultivation, drainage and road building, fertiliser use, felling and harvesting, and restocking also have effects. This Research Note considers each of these factors in turn and assesses the role of tree species, planting scale and design. It covers the identification and protection of vulnerable areas, use of critical load and site impact assessments, research and monitoring, and measures to promote recovery. Continued monitoring will be essential to demonstrate whether current measures remain fit for purpose and guide the development of good practice.