Forest growth responds more to air pollution than soil acidification

Abstract

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. SO2 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 SO2 concentration by 90% between the late 1980s and 2010s. In this study we oserved that annual tree ring width (TRW) declined in the 1970s and subsequently recovered in the 1990s, tracking SO2 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 rime (originating from more pronounced decline of alkaline dust than SO2 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 where we tracked soil chemistry. Instead, statistically significant recovery in TRW is linked to the trajectory of annual SO2 concentrations or sulfur deposition at all three stands.

Fig 1. Study site locations in Ore Mts., Czech Republic.

The maps are based on data from OpenStreetMap and OpenStreetMap Foundation publicly available without special privileges under a CC BY-SA 2.0 licence from the OpenStreetMap contributors (https://www.openstreetmap.org/copyright/en) available from www.openstreetmap.org.

Fig 2. Coal mining and dust emissions in the Czech Republic between 1850–2017.

Fig 3. SO₂ emissions (Czech Republic) between 1850–2017 and ambient annual SO₂ concentrations measured at Cínovec (Zinwald) at the Czech/German border between 1979–2017.

Fig 4. Modelled and measured annual precipitation pH and SO₂/dust ratio (1850–2017) for the Czech Republic.

Modelled values from 1850 to 2013 for pH, 1850 to 2013 for SO₂/dust ratio.

Fig 5. Modelled (1850–1993) and measured (1994–2017) annual sulfur deposition at study sites in the Ore Mts.

Fig 6. Long-term changes of soil chemistry at the Načetín control research site (1994–2018).

Fig 7. Soil pH and base saturation in 2018.

Fig 8

Modelled and measured (a) soil base saturation, (b) soil solution pH and (c) soil solution Bc/Al ratio (1850–2050) for all investigated sites.

Fig 9

(A) Raw (dotted line) and standardised (full line) TRW chronologies of all individual sites as well as Ore (Krušné) Mts. Mean correlation coefficient (CC) and mean GL (Gleichläufgkeit; Eckstein, Bauch 1969) indicate similarity among the site chronologies. (B) Indexed mean TRW chronology for Ore Mts. truncated for sample size (dotted line) of at least 20 TRW series. Black dots indicated negative extremes.