A secondary forest in a portion of the Brazilian Amazon takes up carbon at only about twice the rate of primary forest, as compared to carbon accumulation at up to 11 times in other parts of the world; that could be bad news if similar findings are confirmed elsewhere in the Amazon and the tropics, according to scientists.

The Bragantina region of Pará state where the study occurred has been used agriculturally for hundreds of years, until today, almost no primary forest remains. It is unlikely these degraded forests will return to their original levels of carbon storage and biodiversity on “politically meaningful timescales,” the researchers said.

The results indicate that future researchers should be more cautious in estimating the absorption capacity of atmospheric carbon by regenerating tropical forests to mitigate the impacts of climate change, as that capacity is variable depending on multiple factors and may be overestimated.

The findings could also put in doubt Brazil’s plan to meet its Paris Climate Agreement carbon reduction pledges by replanting forest. The nation promises to restore 12 million hectares of forest by 2030. But the actual carbon storage value of these new secondary forests, including tree plantations, could be far lower than expected.

Forests capture carbon from the air and store it as they grow, providing an essential service to a world where humanity is rapidly increasing atmospheric CO 2 levels and escalating the climate crisis. Secondary forests — forests that are growing back after clearing — are known to have a high potential for carbon capture, with some studies suggesting that the carbon uptake of these regenerating forests can be up to 11 times that of primary forests.

However, that isn’t always true — which could be bad news for humanity. In a study recently published in the journal Ecology, researchers found that secondary forests in the Bragantina region of Pará state in the Brazilian Amazon possessed carbon accumulation rates only around twice that recorded in primary forests — much lower than expected.

“Our results offered a much less optimistic picture than other studies, including our own studies for other regions of Pará,” said lead author Fernando Elias, a PhD candidate at the Federal University of Pará. “We cannot always assume that secondary forests will develop quickly or completely.”

The Bragantina region has been used agriculturally for hundreds of years. Today, almost no primary forest remains in the landscape. And, according to the study, it is unlikely these forests will return to their original levels of carbon storage and biodiversity on “politically meaningful timescales.”

The researchers investigated tree species recovery and the influence of climate and landscape on carbon dynamics in Bragantina forests between 6 and 60 years old. They found that secondary forests have only 41.1% of the average carbon and 56% of the tree diversity of primary forests in the region.

“Our results indicate that we should be cautious about the success of solutions to mitigate the impacts of climate change by regenerating secondary forests,” says Elias, “as the absorption capacity of atmospheric carbon by these forests may be overestimated.”

The study’s findings could also have a detrimental bearing on one of Brazil’s major proposals for curbing its greenhouse gas emissions under the 2015 Paris Agreement. The nation has pledged to restore 12 million hectares (46,332 square miles) of forest by 2030. But the carbon storage value of these new secondary forests could turn out to be far lower than hoped for, especially since Brazil is currently pushing restoration via plantings of eucalyptus and other plantation tree crops.

While the United Nations currently counts tree plantations as forest restoration, numerous studies show that such plantations — including eucalyptus used to make toilet paper, oil palm, and pines used for biofuel wood pellets — often have considerably less value for sequestering carbon than native forests, partly because some types of plantation trees are regularly harvested, repeatedly resetting their carbon storage capacity back to zero.

Forest regrowth rates — and the carbon they are capable of storing — can vary greatly from place to place and depend on soil properties, climatic conditions, and previous land use, among other factors. Much of what we know about the regrowth of tropical secondary forests is from studies in places with greater forest cover then the Bragantina region and with a less intense land use history — with better conditions for growth and for biodiversity recovery. This study offers a clearer glimpse of how forest recovery occurs in non-optimal conditions.

This study “shows that even the Amazon forest, which is believed to be highly resilient to disturbances, can take more than a century to recover its original carbon stocks and biodiversity levels,” says Catarina Jacovac of Wageningen University & Research, who studies secondary forests. “Conserving the old-growth forests is, therefore, crucial for guaranteeing the resilience of the Amazon ecosystem as a whole.”

Intense droughts associated with El Niño events over the past decade have also harmed the recovery of secondary forests. The researchers recorded an increase in temperature in the Bragantina region by 0.1° Celsius per decade and a reduction in the growth of plants during drier times.

The link with climate was surprising, according to Elias. The team observed less carbon uptake during drier times, “an alarming finding given much of the Arc of Deforestation in the Amazon is suffering longer dry seasons and becoming hotter.” The Arc of Deforestation is a vast crescent-shaped swathe of the Brazilian Amazon stretching from from Pará state in the East, to Rondônia in the West — the Arc’s primary forests have been penetrated by roads and degraded by cattle ranches and soy plantations.

“If secondary forests are going to play a key role in mitigating climate change, then we need to ensure that we first protect the primary forests that supply the seed sources and regulate regional climates,” study co-author Jos Barlow of Lancaster University told Mongabay. “We also need to ensure climate change itself is limited, by reinforcing reductions in emissions across the world. If climate change continues, we may lose one of the most effective tools we have to absorb carbon dioxide.”

Banner Image caption: A toucan in the Brazilian rainforest. Image by Rhett A. Butler / Mongabay.

Citation:

Elias F, Ferreira J, Lennox GD, et al (2020) Assessing the growth and climate sensitivity of secondary forests in highly deforested Amazonian landscapes. Ecology. doi: 10.1002/ecy.2954.

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