Old Growth Forest Must No Longer Be Ignored In Carbon Balances

In the carbon cycle, forests help to slow down the build-up of atmospheric carbon dioxide by absorbing this gas, thereby mitigating climate change. Specifically, forests use CO₂ to synthesise the organic molecules that are stored in trees, and thereafter in organic soil matter and dead leaves, which decompose slowly.

The ability of forests to fix carbon dioxide depends on the balance between the amounts removed from the atmosphere through photosynthesis and those released as a result of plant respiration.

In the late 1960's, an American scientist named Eugene Odum put forward the hypothesis that the carbon dioxide fixing and release rates in old growth forests, (i.e. woodland more than 150 years old) reached an equilibrium, rendering them neutral from a carbon balance perspective. Although little empirical evidence was produced in support of this hypothesis, it was nonetheless accepted by the vast majority of ecologists and "non-ecologists" alike. As a result, old growth forests were disregarded in the Kyoto protocol.

An international research team that includeds scientists from CEA's Laboratory of Climate and Environmental Science (LSCE - Laboratoire des Sciences du Climat et de l'Environnement) has compiled a new database using measurements made by the "CarboEurope" and "AmeriFlux" observatory networks, with the aim of accurately assessing Odum's hypothesis. "Old growth forests may actually still be accumulating carbon, in defiance of Odum's equilibrium hypothesis, explains Philippe Ciais, Deputy Director of LSCE, and co-author of the study. More than 30% of the planet's total forested area is unmanaged primary forest, half of which is located in temperate regions in the northern hemisphere. The database established for this study reveals that these ancient forests fix between 0.8 and 1.8 billion tonnes of carbon each year, and that 15% of the total forest area that has until now been totally ignored in carbon balances is in fact responsible for at least 10% of all carbon sinking activity."

The study finds that ancient forests accumulate large quantities of carbon over the centuries, which might be released in case of accidental disturbance (e.g. fire, insects, disease, storms, extreme droughts, etc.). In conclusion, carbon balance assessments should take these old growth forests into account.