- A recent study finds young forests sequester more carbon per year than old-growth forests. In total, it estimates that intact, old-growth forests sequestered 950 million to 1.11 billion metric tons of carbon per year while younger forests – those that have been growing less than 140 years – stored between 1.17 and 1.66 billion metric tons per year.
- The study also estimates that the world’s regenerating forests stand to uptake a further 50 billion metric tons of carbon as they grow.
- These findings upend conventional wisdom that old-growth tropical rainforests are the planet’s biggest carbon sinks.
- The authors say their research could be used to improve forest management and help mitigate climate change.
Forests store vast quantities of carbon and play a huge role in the world’s carbon cycle – as well as in human hopes of mitigating global warming. Tropical rainforests, the revered “lungs of the planet,” were once thought to take the cake when it comes to carbon sequestration. But a new study adds to a growing body of evidence that other types of forest may actually be better at sucking CO2 out of the atmosphere. Specifically it finds that young temperate forests may be more effective carbon sinks than are old rainforests.
Researchers at the Birmingham Institute of Forest Research (BIFoR) in the UK modeled carbon storage in old-growth and regrown forests between 1981 and 2010 using recent data on forest ages as well as the latest global land cover change dataset produced by the University of Maryland in the U.S. Their results, published recently in Proceedings of the National Academy of Sciences (PNAS), reveal that intact, old-growth forests sequestered 950 million to 1.11 billion metric tons of carbon per year while younger forests – those that have been growing less than 140 years – stored between 1.17 and 1.66 billion metric tons per year.
This isn’t the first time younger, regenerated forests have attracted the climate spotlight. Another UK study, published just last month in Quaternary Science Reviews, found that a period of cooling during the 16th and 17th centuries called the Little Ice Age was caused by forest regrowth following the deaths of millions of indigenous people in the Americas due to European colonization.
But why are younger forests better at storing carbon? One reason, the researchers write, may be that newly deforested areas are open and sunny and are easily recolonized by fast-growing species. These plants are able to extract carbon from the air and incorporate it into their biomass more quickly than mature trees that must contend with more neighbors and less sunlight.
“It’s important to get a clear sense of where and why this carbon uptake is happening, because this helps us to make targeted and informed decisions about forest management,” said Tom Pugh, a scientist at BIFoR and lead author of the study.
Younger forests include areas in the eastern U.S. that were cleared for agriculture long ago but then later abandoned as colonization pushed west in the late 1800s and subsequently incorporated into the U.S. National Forest system and allowed to regrow. Large tracts of boreal forest in Canada, Russia and Europe have also grown back after deforestation by fire and logging.
Concerted reforestation efforts may also be contributing to this carbon sink. For instance, a massive government-driven program in China was recently lauded for playing a big part in the world’s recent overall increase in tree cover (however, researchers question whether China’s efforts qualify as “reforestation”). Meanwhile, country-led efforts in Africa are vying to restore 100 million hectares of degraded land on the continent by 2030.
In addition to modeling the past, the BIFoR researchers also looked at the future. They estimate that, under a business-as-usual scenario, forest regrowth stands to capture around 50 billion metric tons of carbon. This potential, they write, will be split fairly evenly between the tropics and temperate areas.
“The remaining uptake potential in forest biomass under current disturbance rates is equivalent to five years of emissions from fossil fuel burning at 2016 levels,” they write.
The researchers caution that there are still many unknowns. For instance, how will forest regrowth be affected by climate change-induced stressors like drought and direct human pressures like road-building and small fires? They also say forests won’t be able turn around climate change all on their own.
“The amount of CO2 that can be taken up by forests is a finite amount: ultimately reforestation programs will only be effective if we simultaneously work to reduce our emissions,” Pugh said.
Citation: Pugh, T. A., Lindeskog, M., Smith, B., Poulter, B., Arneth, A., Haverd, V., & Calle, L. (2019). Role of forest regrowth in global carbon sink dynamics. Proceedings of the National Academy of Sciences, 201810512.
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