A new study mapped the ages of forests around the world. Forests in the Congo Basin, as seen in this image, were found to be younger at the end of the study than they were at the beginning. Credit: United Nations Organization Stabilization Mission in the Democratic Republic of the Congo/ Abel Kavanagh, CC BY-SA 2.0 The towering trees of old forests store massive amounts of carbon in their trunks, branches, and leaves. When these ancient giants are replaced by a younger cohort after logging, wildfire, or other disturbances, much of this carbon stock is lost.

“We’ve known for a long time that forest age is a key component of the carbon cycle,” said Simon Besnard, a remote sensing expert at the GFZ Helmholtz Centre for Geosciences in Potsdam, Germany. “We wanted to actually quantify what it means if an old forest becomes young.”

The resulting study, published in Nature Ecology and Evolution, measured the regional net aging of forests around the world across all age classes between 2010 and 2020, as well as the impact of these changes on aboveground carbon.

To do this, the team developed a new high-resolution global forest age dataset based on more than 40,000 forest inventory plots, biomass and height measurements, remote sensing observations, and climate data. They combined this information withbiomass data from the European Space Agency and atmospheric carbon dioxide observations.

The results point to large regional differences. While forests in Europe, North America, and China have aged during this time, those in the Amazon, Southeast Asia, and the Congo Basin were younger in 2020 than 10 years prior.

A number of recent studies have shown that forests are getting younger, but the new analysis quantifies the impact of this shift on a global level, said Robin Chazdon, a tropical forest ecologist at the University of the Sunshine Coast in Queensland, Australia, who was not involved in the study. “That’s noteworthy and a very important concept to grasp because this has global implications, and it points out where in the world these trends are strongest.”

Carbon Impact

The study identifies the tropics, home to some of the world’s oldest forests, as a key region where younger forests are replacing older ones.

In this image from 2020, old-growth forests are most evident in tropical areas in South America, Africa, and Southeast Asia. Credit: Besnard et al., 2021, https://doi.org/10.5194/essd-13-4881-2021, CC BY 4.0 On average, forests that are at least 200 years old store 77.8 tons of carbon per hectare, compared to 23.8 tons per hectare in the case of forests younger than 20 years old.

The implications for carbon sequestration are more nuanced, however. Fast-growing young forests, for instance, can absorb carbon much more quickly than old ones, especially in the tropics, where the difference is 20-fold. But even this rate of sequestration is not enough to replace the old forests’ carbon stock.

Ultimately, said Besnard, “when it comes to a forest as a carbon sink, the stock is more important than the sink factor.”

In the study, only 1% of the total forest area transitioned from old to young, primarily in tropical regions. This tiny percentage, however, accounted for more than a third of the lost aboveground carbon documented in the research— approximately 140 million out of the total 380 million tons.

“It’s usually more cost-, carbon-, and biodiversity-effective to keep the forest standing than it is to try to regrow it after the fact. I think this paper shows that well,” said Susan Cook-Patton, a reforestation scientist at the Nature Conservancy in Arlington, Va., who was not involved in the study. “But we do need to draw additional carbon from the atmosphere, and putting trees back in the landscape represents one of the most cost-effective carbon removal solutions we have.”

The increased resolution and details provided by the study can help experts better understand how to manage forests effectively as climate solutions, she said. “But forest-based solutions are not a substitute for fossil fuel emissions reductions.”

Open Questions

When carbon stored in trees is released into the atmosphere depends on what happens after the trees are removed from the forest. The carbon can be stored in wooden products for a long time or released gradually through decomposition. Burning, whether in a forest fire, through slash-and-burn farming, or as fuel, releases the carbon almost instantly.

“I think there is a research gap here: What is the fate of the biomass being removed?” asked Besnard, pointing out that these effects have not yet been quantified on a global scale.

Differentiating between natural, managed, and planted forests, which this study lumps together, would also offer more clarity, said Chazdon: “That all forests are being put in this basket makes it a little bit more challenging to understand the consequences not only for carbon but for biodiversity.”

She would also like to see future research on forest age transitions focus on issues beyond carbon: “Biodiversity issues are really paramount, and it’s not as easy to numerically display the consequences of that as it is for carbon.”

“We are only looking at one metric, which is carbon, but a forest is more than that. It’s biodiversity, it’s water, it’s community, it’s many things,” agreed Besnard.

This article originally appeared in EOS Magazine.