OLD-GROWTH FOREST CARBON SINKS OVERESTIMATED, DANISH STUDY SHOWS

The claim that old-growth forests play a significant role in climate mitigation, based upon the argument that even the oldest forests keep sucking carbon dioxide (CO2) out of the atmosphere, is being refuted by researchers at the University of Copenhagen (UCPH).

They claim this argument is based upon incorrectly analyzed data and that the climate mitigation effect of old and unmanaged forests has been greatly overestimated.

Nevertheless, they reassert the importance of old-growth forest for biodiversity, Science Daily reported late on March 25, 2021. Old and unmanaged forest has become the subject of much debate in recent years, both in Denmark and internationally. In Denmark, setting aside forests as unmanaged has often been argued to play a significant role for climate mitigation.

The entire climate mitigation argument is based upon a widely cited 2008 research article that reports that old-growth forests continue to suck up and sequester large amounts of CO2 from the atmosphere, regardless of whether their trees are 200 years old or older. UCPH researchers scrutinized the article by reanalyzing the data on which it was based. They conclude that the article arrives at a highly overestimated climate effect for which the authors' data presents no evidence.

"The climate mitigation effect of unmanaged forests with trees more than 200 years old is estimated to be at least one-third too high -- and is based solely upon their own data, which, incidentally, is subject to great uncertainty. Thus, the basis for the article's conclusions is very problematic," explains Professor Per Gundersen, of the University of Copenhagen's Department of Geosciences and Natural Resource Management. The original research article concluded that old-growth forests more than 200 years old bind an average of 2.4 tons of carbon per hectare, per year, and that 1.3 tons of this amount is bound in forest soil. According to the UCPH researchers, this claim is particularly unrealistic. Carbon storage in soil requires the addition of a certain amount of externally sourced nitrogen. "The large amounts of nitrogen needed for their numbers to stand up don't exist in the areas of forest which they studied. The rate is equivalent to the soil's carbon content doubling in 100 years, which is also unlikely, as it has taken 10,000 years to build up the soil's current carbon content. It simply isn't possible to bind such large quantities of carbon in soil," said Gundersen, who also wrote a commentary on the matter in the journal Nature.

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