Forest soils store large amounts of organic carbon. When storms devastate the forest, this carbon is lost and a large part enters the atmosphere as CO2. Mountain forests are particularly vulnerable to such carbon losses, shows a study by the Swiss Federal Institute for Forest, Snow and Landscape Research WSL.
In Swiss forests, soils store more CO2 in the form of organic carbon than the entire above-ground biomass, i.e. trunks, branches and leaves. This means that forest soils play an important role in the fight against climate change. But what happens to the carbon in the soil when a storm knocks down the trees above it? That’s what Mathias Mayer and his fellow researchers at WSL have been investigating.
To do so, they studied the soils of forests devastated by hurricanes Vivian (1990) and Lothar (1999). The researchers repeatedly took soil samples from different elevations between 420 and 1550 meters above sea level. While Lothar caused the greatest damage on the Central Plateau, Vivian mainly destroyed mountain forests in the foothills of the Alps.
The comparison of destroyed with undamaged forests showed: In the years after the storms, enormous amounts of carbon were lost from the soil, especially in mountain forests. That’s because without trees, the forest floor becomes warmer and wetter, and that stimulates microbes that break down humus, releasing carbon. Eighteen years after Vivian, the carbon stocks of high-elevation storm areas were still up to ninety percent smaller than those of intact forests. The reason: because of the cooler conditions in mountain forests, leaves and needles break down slowly. This is why a thick layer of humus accumulates, which stores large amounts of carbon. So more can be lost.
In comparison, less soil carbon was lost in destroyed midland forests. As a result, the storage also recovers more quickly. "After just 10 years, soils store the same amount of carbon as before. In mountain forests, on the other hand, we estimate recovery periods of 60 years," says Mayer.
Extrapolated to the whole of Switzerland, this adds up to a large amount of carbon: The researchers’ calculations show that as a result of storms Lothar and Vivian, around 400,000 tons of soil carbon were lost throughout Switzerland. Or, in other words, as much CO2 as 400 airplanes emit flying from Zurich to New York and back. "This is roughly equivalent to the amount of CO2 that forests on an equivalent area fix in their tree biomass in 40 years," says Frank Hagedorn, co-author of the study.
More storm damage in the futureStudies show that forest damage from storms has increased in recent decades. Ongoing climate change is also likely to bring a further increase in storm damage and thus in CO2 emissions from forest soils.
Can anything be done to prevent carbon loss? "Basically, from a soil conservation perspective, it’s good to leave some of the dead wood in place," Mayer says. That reduces erosion, and the deadwood creates niches where young saplings can grow up protected from natural hazards and wildlife. Thus, deadwood could also accelerate the buildup of humus and thereby carbon storage.
Mayer, M.; Rusch, S.; Didion, M.; Baltensweiler, A.; Walthert, L.; Ranft, F.; Rigling, A.; Zimmermann, S.; Hagedorn, F., 2023: Elevation dependent response of soil organic carbon stocks to forest windthrow. Science of the Total Environment, 857: 159694 (9 pp.). doi: 10.1016/j.scitotenv.2022.159694