Old-growth forest carbon sinks overestimated. So what?

Below a short letter drafted together with William R. Moomaw and Brendan Mackey questioning the message of a recently published paper quantifying the carbon uptake rates by old-growth forests.

Even if old-growth forests have long been thought to be carbon neutral, there is evidence that they might act as carbon sinks late into succession (Luyssaert et al. 2008). A recent paper published in Nature (Gundersen et al. 2021) recently suggested old-growth forest sequesters a third less carbon than previously estimated, and called for reexamining their role in the global carbon budget. Their call, however, misses the mitigation value of these forests in limiting atmospheric CO2 concentrations.

Forests’ mitigation value resides in their accumulated carbon stocks rather than their annual net carbon uptake (Mackey et al. 2013). Securing this stock long-term, and reducing emissions from all sources including harvesting is crucial to limit climate change to 1.5°C. Our remaining global emission budget is around 140-190 Gt C, i.e., 8-12 years of emissions (https://www.ipcc.ch/sr15/chapter/spm/). Globally, forests keep 861 Gt C out of the atmosphere (Pan et al. 2011). Since primary forests constitute ~30% of world’s forests, and they store substantially more carbon than managed forests, clearing them would push the climate well beyond the 1.5°C threshold. Even if regrowing forests have higher carbon uptake rates, it would take decades-to-centuries to restore the original carbon stock of the pre-harvested forest (Hudiburg et al 2019). Our priority is to curb emissions now.

We agree the data calls for revisiting the role of old-growth forests in the global C budget but not for changes implied by Gundersen. Rather, we need to better recognize that these older forests contribute disproportionately to climate stabilization for the carbon they store, not their rate of uptake. Forest carbon is quickly emitted but only slowly accumulated.

Francesco Maria Sabatini – German Centre for Integrative Biodiversity Research (iDiv), Halle-Jena-Leipzig, Germany

Brendan Mackey – Griffith Climate Change Response Program, Griffith University, Queensland 4222, Australia

William R. Moomaw – Woodwell Climate Research Center, Falmouth MA, USA

Cited References

Hudiburg, T. W., Law, B. E., Moomaw, W. R., Harmon, M. E. & Stenzel, J. E. Meeting GHG reduction targets requires accounting for all forest sector emissions. Environmental Research Letters 14, 095005 (2019).

Gundersen, P. et al. Old-growth forest carbon sinks overestimated. Nature 591, E21-E23 (2021).

Luyssaert, S. et al. Old-growth forests as global carbon sinks. Nature 455, 213-215 (2008).

Mackey, B. et al. Untangling the confusion around land carbon science and climate change mitigation policy. Nature Climate Change 3, 552-557 (2013).

Pan, Y. D. et al. A Large and Persistent Carbon Sink in the World’s Forests. Science 333, 988-993 (2011).