Trees and soil at the forest’s edge store more carbon than we thought, studies reveal

May 12, 2022
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Trees and soils keep a lot of carbon trapped in the forest, pulling it from the atmosphere, where it can do more harm than good. In fact, roughly 30% of the carbon generated from fossil fuel burning is captured by forests.

“That’s CO2 that’s not in the atmosphere,” said Boston University biogeochemist and ecologist Lucy Hutyra. “We’re not feeling the full effects of climate change because of the terrestrial climate sink. These forests are doing an incredible service to our planet.”

In most parts of the world, forests have been sliced and diced into fragments resulting in a landscape that looks more like a quilted mosaic than a great green expanse. These forest fragments create new environments for the plants and soils that live along their edges, and this can change the amount of carbon they store.

‘We think about forests as big landscapes, but really they are chopped up into all these little segments because of the human world,” Hutyra said. In two separate studies, her research group investigated the differences in carbon storage of trees and soils along forest edges versus the interiors of temperate forests in the northeastern United States.

Hutyra and her team found that trees on the edge of forest fragments grow faster than those deeper in the interior, and that soil on the edge of urban forests has a greater capacity to store carbon than previously known.

“There is great interest in understanding how trees and soils ‘live on the edge,’” Peter Groffman, a professor at the City University of New York’s Advanced Science Research Center, who was not involved in the research, told Mongabay in an email. “These are excellent studies that increase our understanding.”

Trees on the edge

According to the first study, published in Nature Communications, trees growing within 30 meters (100 feet) of the forest edge grow almost twice as fast as interior trees, meaning the edge trees are pulling carbon from the atmosphere and storing it in their tissues at a faster rate.

Luca Morreale, a Ph.D. candidate in Hutyra’s lab and lead author of the study, said this remarkable difference in growth is likely due to light. “Trees on the edge don’t have competition with interior forest, so they get more light,” he said.

Researchers used the U.S. Department of Agriculture’s Forest Inventory and Analysis program, which includes data on tree size, growth, and land use across the United States. They looked at data for more than 48,000 tree plots across 20 northeastern U.S. states.

It may be good news that edge trees are capturing more carbon than previously thought, Morreale said. Nearly 25% of the forest area in the study region is “edge forest,” so the increase in plant growth and the resulting carbon capture is significant.

However, Morreale made it clear that this is not an argument in favor of creating more fragments as a way to sequester more carbon. “The forest that is lost will always outweigh gains made from growth increases in the new edges.”

Although the trees may be growing faster, their exposure also makes them more susceptible to drought, fire, and extreme temperatures that could weaken or kill the trees, releasing the stored carbon.

The effect of edges on tree growth also doesn’t translate to tropical forests, such as the Amazon Rainforest. In fact, studies of fragmentation effects on tropical forests have found the exact opposite: tropical trees on the forest edge suffer large die-offs due to fire, drought, wind damage, and liana invasions. Trees that survive this onslaught grow slower than those in the interior of the forest.

 Soil on the edge

The second study from Hutrya’s lab focused on soil. Worldwide, more carbon is stored in the soil than in all aboveground plants, animals, and the atmosphere combined, so understanding the soil is an important part of the carbon equation.

“Soils contain wild amounts of bacteria, fungi, roots, and microorganisms, and just the way we breathe out CO2 when working and being active, they respire CO2, as well,” Sarah Garvey, a Ph.D. candidate in Hutyra’s lab and lead author of the soils paper published in Global Change Biology, said in a press release“With soil, there is more there than meets the eye.”

Garvey measured the respiration (release of CO2) and temperature of soils on the edge and in the interior of forests in both rural and urban areas. In urban areas within and close to cities, respiration rates were 25% lower at the forest edge relative to the interior, meaning more carbon is stored in places like city parks and urban forests than previously thought.

Warmer temperatures at the edges of the forest speed up the decomposition of leaves and organic matter. This in turn amps up the activity of soil microbes that then release CO2. In urban areas, however, the hot and dry conditions actually caused the microbes to slow their activity and release less carbon.

“Forests store almost half of their carbon below ground,” Garvey said. “Which is why understanding the relationships between the soil and the plant life is so vital to understanding the bigger picture of how forests store carbon for the long term.”

As more countries and communities commit to restoring forests and planting trees, deciding where those trees go is important. Hutyra and colleagues say they hope this research adds another piece to the puzzle.

“We need to think about that as we [decide] what areas to conserve, what to develop, and how to tackle climate change solutions,” Hutyra said. “Is a place like [a city park] where there’s tons of foot traffic just as valuable to save as a remote forest in Maine where three people visit? There’s no easy answer.”

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