Old is Better than Young: The Carbon Sequestration Potential of Letting Forests Mature
ARTICLE: Moomaw, W.R., S. A. Masino, and E. K. Faison. 2019. Intact forests in the United States: proforestation mitigates climate change and serves the greatest good. 2019. Frontiers in Climate and Global Change, 11 June 2019. https://doi.org/10.3389/ffgc.2019.00027.
“Globally, terrestrial ecosystems currently remove an amount of atmospheric carbon equal to one-third of what humans emit from burning fossil fuels…. Forests are responsible for the largest share of the removal.” (Moomaw et al. 2019). What if we could increase the amount of CO2 forests extract from the air? We can!
REDUCING ATMOSPHERIC CO2 WITH FORESTS
According to NOAA (2019), atmospheric CO2 concentrations have increased dramatically over the past 60 years, from under 320 ppm (parts per million) in 1959 to nearly 410 ppm in 2019. Increased CO2 and other greenhouse gases (such as methane) in the atmosphere are trapping heat, raising global temperature and altering other aspects of the Earth system. A recent report by the Intergovernmental Panel on Climate Change warned about the serious consequences of an increase of more than 1.5oC in global average temperature (IPCC 2018).
Figure 1 shows that CO2 circulates around the Earth in a massive cycle that involves the atmosphere, oceans, and land (including both the surface and fossil fuels underground).
The increase in atmospheric CO2 concentrations follows basic math: more CO2 is being emitted into the atmosphere from human-caused fossil fuel burning and land use than is being taken out of the air by terrestrial and aquatic ecosystems.
It could be worse. Some of the extra CO2 pumped into the air is actually being taken up by oceans and the land. Terrestrial ecosystems alone are extracting about 1/3 of the CO2 emitted as a result of human activities. Most of that enhanced uptake is by forests. This has mitigated the amount of warming that would have happened otherwise.
This carbon sequestration fact suggests a possible pathway for moderating climate change even more. Whereas most strategies have focused on reducing carbon emissions, why not also promote enhanced removal of extra CO2 from the atmosphere by forests? Scientists have focused largely on two strategies for increasing the power of the Earth’s forests to take up CO2. First, lands that are not currently supporting forests could be planted, creating new forest lands—called afforestation. Second, lands that were recently harvested could be reforested. There’s potential through both of these mechanisms to increase the amount of photosynthesizing leaf area, thereby enhancing the CO2 sucking power of forests across the planet.
Underappreciated is the even larger CO2 extraction power of proforestation—allowing current forests to grow without harvest. Calculations have shown that allowing intact forests to mature would result in much more carbon sequestration than either afforestation or reforestation, especially in the crucial next several decades. To achieve substantial CO2 drawdown, afforestation would have to occur over very large areas, with serious negative tradeoffs with food production and other uses. Many years of tree growth would be required for reforested ecosystems to take up much CO2. In contrast, current intact forests are already extracting considerable amounts of CO2, and proforestation would provide other ecological and societal benefits that would not accrue to the same extent with the other two approaches. In a recent article in the Frontiers in Forests and Global Change, William Moomaw, of Tufts University, and his colleagues (2019) provide a detailed review of the striking environmental potential of proforestation.
THE POTENTIAL OF PROFORESTATION TO SEQUESTER CARBON
To illustrate the power of proforestation, Moomaw et al. point out that, “If deforestation were halted, and secondary forests were allowed to continue growing, they would” extract and sequester 1/7 of the extra CO2 emitted by human-associated activities. This is remarkable, but it also illustrates that there is no silver bullet to solve the climate change problem; solutions will have to be manifold, including both reducing emissions and removing CO2 from the atmosphere.
The amount of forestland that is largely free from logging and other resource extraction is small: about 20% worldwide, 6-7% in the U.S., and only about 3% in New England. Figure 2 shows that large parts of New England are covered by forest, but most is subject to harvesting.
This timber cutting accounts for 86% of the total CO2 loss from these forests back into the atmosphere. Reducing harvesting could, therefore, greatly increase carbon retention in these forests. According to William Keeton (2011), secondary forests (harvested in the past but allowed to grow back to forest) in the region have the potential to increase forest uptake of CO2 by 2.3 to 4.2 times current rates if management were changed. At a global level, changing forest management has the potential to increase uptake of carbon two-fold. That’s a lot given the enormous amount of CO2 already being extracted by forests.
Allowing forests to age produces large trees, which suck large amounts of CO2 out of the atmosphere and store tremendous amounts of carbon. Moomaw et al. (2019) cite studies showing that the largest 1% of trees account for 30-50% of the biomass in forests, depending on the location, and trees over one meter in diameter (about 39”) take up the carbon equivalent of an entire 10-20 cm (4-8”) diameter tree in one year.
The old dogma about old forests being unproductive and “decadent” has also been overturned by recent research. Extensive measurements in old-growth forests have revealed that these ecosystems continue to take up more CO2 than they emit, with no apparent age plateau, an issue reviewed by Keeton (2018) in a recent edited book on old-growth forests. Old forests are anything but wastelands; science has shown us that they are diverse, thriving ecosystems (see lead photo above, showing Big Reed Reserve old growth in Maine).
Bottom Line: Old forests store tremendous amount of carbon, keeping it out of the atmosphere, and continue to remove CO2 from the air no matter how old they are.
OTHER BENEFITS OF PROFORESTATION
There are, of course, tradeoffs between proforestation and societal benefits that might accrue from harvesting, such as raw materials for building, timber-industry related jobs, and economic development, especially in rural areas. However, Moomaw et al.argue that management and monitoring associated with proforestation will result in its own set of jobs and economic activity. Moreover, mature and old-growth forests are associated with many other crucial ecological benefits, including biological diversity, water quality, and lower soil erosion. Older forests are also key habitats for scientific studies, providing baselines for understanding the structure and processes of complex natural ecosystems, as well as important places for recreation and mental health sustenance.
It’s important to point out that obtaining carbon and other benefits from intact forests is not an either/or proposition. It’s possible to let some of these forestlands mature without intervention, while managing others with harvesting techniques that provide wood, maintain intact canopies, protect biodiversity, and also sequester more carbon than traditional operations, although not as much as full-scale proforestation. Experiments in Upstate New York and Vermont, for example, have shown that targeted harvesting strategies can accelerate forest development toward old-growth structure, maintain high levels of carbon storage, and produce modest flows of timber (Keeton et al. 2018).
Moomaw et al. (2019) argue that, “To date, the simplicity of the idea of proforestation has perhaps been stymied by inaccurate or non-existent terminology to describe it.” In fact, most analyses and policy recommendations regarding forests and carbon sequestration don’t distinguish between young and old forests, harvested and unharvested, often simply relying on cover of any forest, no matter what condition. This seriously hampers efforts to promote proforestation as a strategy for reducing atmospheric CO2concentrations.
Forest management is largely driven by legal frameworks and benefits accruing to managers and owners. Some of these could effectively require or incentivize proforestation. Proforestation could be mandated on portions of federal and state public lands, with the goal of sequestering carbon and promoting mature forest habitat. The mission of U.S. National Forests, for example, includes multiple uses, and carbon and mature forest management could become more prominent than they are today. The development of efficient carbon markets could provide private forest owners with financial compensation for growing rather than harvesting forests. This is, of course, already occurring in some large ownerships, especially associated with carbon markets managed in California. Easier entry for smaller woodland owners could expand the forest area where proforestation makes both environmental and financial sense.
“Taken together, proforestation is a rapid and essential strategy for achieving climate and biodiversity goals and for serving the greatest good.” (Moomaw et al. 2019)
Barton, A.M. and W.S. Keeton. 2018. Ecology and Recovery of Eastern Old-growth Forests(Washington, DC: Island Press).
Intergovernmental Panel on Climate Change (2018).“Summary for Policymakers,” in Global warming of 1.5°C. An IPCC Special Report on the impacts of global warming of 1.5°C Above Pre-Industrial Levels and Related Global Greenhouse Gas Emission Pathways, in the Context of Strengthening the Global Response to the Threat of Climate Change, Sustainable Development, and Efforts to Eradicate Poverty, eds V. Masson-Delmotte, P. Zhai, H. O. Pörtner, D. Roberts, J. Skea, P. R. Shukla, A. Pirani, W. Moufouma-Okia, C. Péan, R. Pidcock, S. Connors, J. B. R. Matthews, Y. Chen, X. Zhou, M. I. Gomis, E. Lonnoy, T. Maycock, M. Tignor, and T. Waterfield. (Geneva: World Meteorological Society). Available online at: https://www.ipcc.ch/sr15/.
Keeton, W. S., Lorimer, C. G., Palik, B. J., and Doyon, F. (2018). “Silviculture for Eastern Old Growth in the Context of Climate Change,” in Ecology and Recovery of Eastern Old-growth Forests, eds A. M. Barton and W. S. Keeton (Washington, DC: Island Press), 237.
Keeton, W. S. (2018). “Source or sink? Carbon dynamics in old-growth forests and their role in climate change mitigation,” in Ecology and Recovery of Eastern Old-growth Forests, eds A. M. Barton and W. S. Keeton (Washington, DC: Island Press), 267.
Keeton, W. S., Whitman, A. A., McGee, G. C., and Goodale, C. L. (2011). Late-successional biomass development in northern hardwood-conifer forests of the Northeastern United States. Forest Sci. 57, 489–505. doi: 10.1093/forestscience/57.6.489
Moomaw, W.R., S. A. Masino, and E. K. Faison. 2019. Intact forests in the United States: proforestation mitigates climate change and serves the greatest good. 2019. Frontiers in Climate and Global Change, 11 June 2019. https://doi.org/10.3389/ffgc.2019.00027.
NOAA. 2019. Global carbon dioxide growth in 2018 reached 4thhighest on record. March 22, 2019. https://www.noaa.gov/news/global-carbon-dioxide-growth-in-2018-reached-4th-highest-on-record[Accessed September 14, 2019]