Rapid recovery of stem increment in Norway spruce at reduced SO2 levels in the Harz Mountains, Germany

► SO2 pollution caused the decline of exposed mountain spruce forests in Europe. ► Soil acidification versus foliar damage were discussed as alternative mechanisms. ► Exposure to high SO2 levels reduced tree-ring width in Norway spruce. ► Recently, clean air acts caused a rapid decrease of SO2 pollution. ► Rapid recovery of stem growth suggests a key role of foliar damage in spruce decline.