Wales planted fast-growing conifer trees for timber, leading to more acidic rivers and fewer salmon. Decades later, researchers observed these environmental effects in affected areas. Image Credit: Wikimedia CommonsOver the last century, Wales has planted large areas of fast-growing conifer trees to meet demand for timber. As a result, the country developed large commercial forests. However, decades later, researchers found that some rivers flowing through heavily forested catchments had become more acidic, with fewer Atlantic salmon in affected areas.This history has drawn renewed attention as UK governments continue to plant more trees to meet climate and biodiversity goals. Scientists say the experience of Wales highlights an important lesson: planting trees alone is not always enough. The choice of species, location and management practices can shape how forests affect surrounding ecosystems.Forest expansion in WalesAccording to the UK government’s Forestry Statistics 2024 report, the UK has around 3.28 million hectares of woodland, covering about 13.5 percent of the country’s land area. In Wales, woodland covers roughly 312,000 hectares, or around 15 percent of the nation’s land area.Conifer forests account for approximately 139,000 hectares in Wales. Large-scale conifer planting expanded across many upland areas of Wales during the post-war decades, with fast-growing species such as Sitka spruce and lodgepole pine widely planted on acid-sensitive soils.According to Forest Research, these plantations played an important role in creating a reliable domestic timber supply. However, over time, scientists began to examine the wider environmental effects of these large-scale plantations. Improved forest management and reduced pollution have since aided river ecosystem recovery. Future tree planting must consider species, location, and management for ecological balance. Image Credit: Wikimedia CommonsHeavy tree cover in catchments was associated with acidified riversResearchers found that some rivers flowing from conifer plantations showed increased acidity. One of the key studies on this subject was conducted by the UK government’s Acid Waters Monitoring Network, which monitored the impact of acid deposition on lakes and streams.The research found that upland catchments with dense conifer plantations could experience greater acidification than nearby non-forested areas. This was linked to the ability of evergreen trees to intercept airborne pollutants, particularly sulphur and nitrogen compounds, which could later enter soils and streams through rainfall and drainage.As explained in the Forestry Commission research note “Forestry and Surface Water Acidification,” surface waters near dense conifer plantations in acid-sensitive areas can become more acidic before harvest. The process occurs because tree canopies capture pollutants from the atmosphere, allowing them to be deposited into surrounding ecosystems.Trees themselves are not the source of pollution; instead, their dense evergreen canopies capture airborne pollutants more effectively than open land, and those pollutants can later reach streams through rainfall and soil drainage.Atlantic salmon were among the species affectedAcidic rivers create difficult conditions for freshwater animals, including Atlantic salmon. Research has shown that acidification can reduce salmon survival by increasing the movement of harmful forms of aluminium from soils into streams.Salmon juveniles are especially vulnerable in their early life stages, when even small changes in water chemistry can reduce survival. Scientists have emphasised that forestry was only one part of a much larger environmental problem.Acid rain caused by industrial emissions across Europe was the primary driver behind widespread freshwater acidification. The influence of conifer plantations was most significant in particular catchments where forests increased the movement of pollutants into streams.Better forest management is improving river ecosystemsConditions are now much improved compared with previous decades. According to the UK Centre for Ecology & Hydrology, the UK’s sulphur dioxide emissions have declined sharply in recent decades because of tighter emission controls and reduced use of coal and oil.As pollution levels have fallen, several lakes and rivers have shown improvements in their chemical conditions, with lower concentrations of sulphur and nitrogen compounds. Forestry management practices have also changed.Current guidance from Forestry Research promotes measures such as creating buffer zones near rivers, developing more mixed woodland, adopting sustainable harvesting practices and avoiding dense conifer planting in environmentally sensitive locations. These measures are intended to minimise impacts on water quality while preserving the benefits of forestry.Lessons from Wales for future climate strategiesWoodland creation in the UK has continued to increase. According to Forestry Statistics 2024 from Forest Research, 20,660 hectares of new woodland were created across the UK during 2023/24 as part of ongoing efforts to expand forests for climate goals.Scientists say that future tree planting should take both climate change and local ecological conditions into account. UK forests are an important national resource. However, species choice, site selection and management all affect how surrounding ecosystems respond.The history of forestry in Wales provides important lessons for future tree-planting programmes. While forests remain among the UK’s most valuable natural resources, their ecological impact depends on where they are planted, which species are selected and how they are managed.