9. CO2 cycle (Masters of 'Solar Chemistry')
The forest: carbon sink or a carbon source?
Forests that are in an ecological balance bind as much CO2 as they release. However, most forest areas are not in an ecological balance. The amount of CO2 stored in the wood of the forests increases in some regions, while it decreases in others.
Where foresters transform intensely used spruce forests into near-natural forests, trees become more diverse, older and larger. As long as the wood supply increases, net CO2 is bound. Such forests act as a carbon sink, i.e., over the course of a year they extract more CO2 from the atmosphere than they release into the air.
From 2009 to 2017, German forests withdrew a net 60 million tonnes of CO2 from the atmosphere per year in already existing forests. In other words, imber growth improved Germany's annual greenhouse gas balance by about 7% (Riedel et al., 2019).
Fast climate change, increasing forest damage
But that's not always the case. In the five-year period from 2003 to 2008, the average annual net carbon capture of German forests fell from 4.7 tonnes CO2 / hectare to only 1.4 tonnes CO2 / hectare compared to the previous period (1990-2002) (Riedel et al 2019). The heat and drought in 2003 and especially the storm Kyrill in 2007 caused a great deal of damage in German forests.
Dry periods as occurred in 2018 and 2019 or storms could turn even near-natural mixed forests into a net CO2 source if more wood dies and decomposes than grows back. Droughts initially lead to less wood growth. But persistent droughts eventually weaken many trees to the point of becoming victims to bark-beetles. In addition, forest fires can destroy large forest stands within a very short time and abruptly release the stored carbon.
Large-scale losses of forests as a result of storms, drought and heat threaten to further increase climate change - unless we succeed in rebuilding the forests so that they can cope with the new weather extremes.
Literature and Links:
- Fischlin A. et al. (2006): CO2-Senken und -Quellen in der Waldwirtschaft – Anrechnung im Rahmen des Kyoto-Protokolls. Umwelt-Wissen Nr. 0602. Bundesamt für Umwelt, Bern. 45 S: https://www.waldwissen.net/wald/klima/wandel_co2/wsl_co2senken/index_DE
- Mrasek, V. (2019): Waldwunschdenken - Kann Aufforstung das Klima retten? Deutschlandfunk, Online: https://www.deutschlandfunk.de/waldwunschdenken-kann-aufforstung-das-klima-retten.740.de.html?dram:article_id=458726
- Lewis S. et al. (2019): Restoring natural forests is the best way to remove atmospheric carbon. Nature. Online: https://www.nature.com/articles/d41586-019-01026-8
- Riedel Th. et al. (2019): Wälder in Deutschland sind eine wichtige Kohlenstoffsenke. AFZ - Der Wald 14/2019. Online: https://www.thuenen.de/media/institute/wo/Waldmonitoring/THG/Projekt/CI2017/AFZ_14_19_Kohlenstoff_Artikel_2_Riedel.pdf
- Palmer et al. (2019): Net carbon emissions from African biosphere dominate pan-tropical atmospheric CO2 signal. Nature Communicationsvolume 10, Article number: 3344 (2019). Online: https://www.nature.com/articles/s41467-019-11097-w
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