Years of drought have turned German pine forest into CO₂ source — study

Repeated heatwaves and drought have caused long-term, potentially irreversible damage to a pine forest in southwestern Germany, transforming it from a net carbon sink into a carbon source, according to researchers from the University of Freiburg. Over 60 percent of the Scots Pine trees in a forest near Hartheim am Rhein have died since 2018, and are being replaced by deciduous trees, which is fundamentally shifting the forest’s behaviour. “Although the pines are slowly being replaced by deciduous trees, this cannot yet compensate for the negative CO₂ balance,” said Simon Haberstroh, first author of the study from the University of Freiburg.

The study is part of a long-term effort to understand the carbon dynamics of European forests, as part of the Integrated Carbon Observation System of which the University of Freiburg is part. The researchers have been measuring the carbon behaviour of the Hartheim forest since 2003. During pre-drought years (2003–2006), the forest absorbed around 391 grams of carbon per square meter annually. However, after 2018, this balance became neutral or negative, peaking at 329 grams of carbon emitted in the hot year of 2022.

”If this effect observed in Hartheim occurs on a large scale, we would lose the function of forests to partially bind man-made CO₂ emissions and mitigate the climate impact of our emissions. Instead, this would further accelerate climate change," said co-author Andreas Christen.

Forests usually act as natural carbon sinks, playing an important role in achieving climate targets by balancing out emissions that are hard to avoid. The German climate law already prescribes annual net-negative emissions targets for the land use sector (LULUCF, which also includes other carbon sinks and sources, for example peatland) for 2030, 2040 and 2045, and the government is currently working on a long-term strategy for negative emissions. One goal of the strategy is to strengthen natural carbon sinks, including forests.

But German forests are in a poor state, as four out of five trees of the most common species – spruce, pine, beech and oak – are damaged, according to the agriculture ministry's forest condition assessment last year. In recent years, German forests have been damaged heavily by droughts, bark beetle infestations, storms and forest fires. Another report published by Germany’s Ministry of Agriculture last year found that the country’s forests had become net carbon emitters.