Why new forests are better at sequestering carbon than old ones

From The Pacific Standard:

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 United Kingdom 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 data set produced by the University of Maryland. Their results, published recently in Proceedings of the National Academy of Sciences, 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. …

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. …

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.

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