Forest growth responds more to air pollution than soil acidification

The forests of central Europe have undergone remarkable transitions in the past 40 years as air quality has improved dramatically. Retrospective analysis of Norway spruce ( Picea abies ) tree rings in the Czech Republic shows that air pollution (e.g. SO 2 concentrations, high acidic deposition to the forest canopy) plays a dominant role in driving forest health. Extensive soil acidification occurred in the highly polluted "Black Triangle" in Central Europe, and upper mineral soils are still acidified. In contrast, acidic atmospheric deposition declined by 80% and atmospheric SO 2 concentration by 90% between the late 1980s and 2010s. Annual tree ring width (TRW) declined in the 1970s and subsequently recovered in the 1990s, tracking SO 2 concentrations closely. Furthermore, recovery of TRW was similar in unlimed and limed stands. Despite large increases in soil base saturation, as well as soil pH, as a result of repeated liming starting in 1981, TRW growth was similar in limed and unlimed plots. TRW recovery was interrupted in 1996 when highly acidic rim (originating from more pronounced decline of alkaline dust than SO 2 from local power plants) injured the spruce canopy, but recovered soon to the pre-episode growth. Across the long-term site history, changes in soil chemistry (pH, base saturation, Bc/Al soil solution ratio) cannot explain observed changes in TRW at the two study sites at which we tracked soil chemistry. Instead, statistically significant recovery in TRW is linked to the trajectory of annual SO 2 concentrations or sulfur deposition at all three stands.