Trees bulking up and growing faster on excess carbon dioxide in atmosphere, study suggests

Increased levels of carbon dioxide in the atmosphere have contributed to a rise in wood volume or biomass of forests in the US, according to a new study.

The research, published recently in the Journal Nature Communications, found that elevated carbon levels have led to a consistent increase in wood volume in 10 different temperate forest groups across the US.

By bulking up this way, trees are helping shield Earth’s ecosystem from the impacts of global warming, say scientists, including those from the Ohio State University.

“Forests are taking carbon out of the atmosphere at a rate of about 13 per cent of our gross emissions. While we’re putting billions of tons of carbon dioxide into the atmosphere, we’re actually taking much of it out just by letting our forests grow,” Brent Sohngen, co-author of the study, said in a statement.

Previous studies have shown that over the last five decades in the US, the per-hectare volume of wood in trees has increased.

However, it has been unclear whether this increase was primarily driven by forest management and their recovery from past land uses, such as agriculture, or other environmental factors such as elevated carbon dioxide, nitrogen deposition, or climate change.

Research has also shown that through a phenomenon called carbon fertilisation, plants use an influx of carbon dioxide from the atmosphere to increase their rate of photosynthesis – a process by which plants combine energy from the sun, water, and other nutrients to spur their growth.

“It’s well known that when you put a ton of carbon dioxide in the atmosphere, it doesn’t stay up there forever. A massive amount of it falls into the oceans, while the rest of it is taken up by trees and wetlands and those kinds of areas,” Dr Sohngen explained.

In the study, researchers noted that forests in the US have sequestered about 700-800 million tons of carbon dioxide each year, accounting for roughly about a tenth of the country’s total carbon emissions.

Scientists say compared to the trees that lived about 30 years ago, modern trees in the US forests have about 20-30 per cent more biomass.

Even older large trees, they say, are continuing to add biomass as they age because of elevated carbon dioxide levels.

“We show that elevated carbon dioxide has had a strong and consistently positive effect on wood volume,” scientists wrote in the study.

For the research, scientists used historical data from the US Forest Service Forest Inventory and Analysis Program (USFS-FIA) to compare how the volume of wood in some forest groups has changed over the last few decades.

Researchers estimated from the data that there has been a significant increase in trees’ wood volume between 1970 and 2015, correlating with a distinct rise in carbon emissions.

They also found that trees that were planted by humans respond to carbon dioxide levels in the same way natural ones do.

These findings, according to researchers, highlight the value of trees in mitigating climate change.

“Carbon fertilisation certainly makes it cheaper to plant trees, avoid deforestation, or do other activities related to trying to enhance the carbon sink in forests. We should be planting more trees and preserving older ones, because at the end of the day they’re probably our best bet for mitigating climate change,” Dr Sohngen said.

Scientists say the findings can help policymakers and other stakeholders better account for the role of forests in helping curtail global warming within the 1.5 degree Celsius target that was agreed upon at the Paris Climate Accords in 2015.