Forest edges store more carbon than expected

Even though they don’t have lungs, soils and trees are constantly breathing in and out. Carbon dioxide (CO2) is absorbed by trees, which is then converted to oxygen through photosynthesis and stored in their trunks. When the leaves fall to the ground, soil bacteria break down the leaves and other organic debris, releasing carbon dioxide into the atmosphere.

Forests actually retain more carbon dioxide than they release, which is good news for us. Forests absorb roughly 30 percent of carbon emissions from fossil fuel combustion, a phenomenon known as the terrestrial carbon sink.

“That’s CO2 that’s not in the atmosphere. 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,” said Professor Lucy Hutyra, a biogeochemist and ecologist at Boston University.

As a result of deforestation, large continuous forest areas are being divided into smaller forest regions. This type of forest fragmentation has been occurring for more than a decade. Professor Hutyra’s research is focused on determining what happens to the planet’s “lungs” due to forest fragmentation. 

“We think about forests as big landscapes, but really they are chopped up into all these little segments because of the human world.” Professor Hutyra further explained how forests are being broken up into smaller plots to make room for roads, buildings, agriculture, and solar farms, which is one of the leading causes of forest loss in Massachusetts. Forest edges – literally, the trees at the forest’s farthest edge – are created as a result of these land use changes.

The assumption has been that forest fringes release and retain carbon at the same rate as forest interiors, but the BU team has revealed that this is not the case. Soils and trees along the borders of temperate forests in the Northeast United States behave differently than those further away from humans.

The researchers discovered that border trees grow quicker than their rural cousins deep in the forest, and that soil in urban areas can store more carbon dioxide than previously assumed. The findings may cast doubt on popular notions of conservation, and shed light on the usefulness of urban forests as more than just recreational areas.

Professor Hutyra and her study team, which included researchers from the Harvard Forest, investigated the development rates of edge trees in comparison to the rest of the forest in one of the most extensive studies of temperate forest edges to date.

The experts examined more than 48,000 forest plots in the Northeast United States using data from the US Department of Agriculture’s Forest Inventory and Analysis program, which tracks tree size, growth, and land use across the country. They discovered that trees on the margins grow nearly twice as quickly as trees in the interior, which are around 100 feet away from the edge.

“This is likely because the trees on the edge don’t have competition with interior forest, so they get more light,” said study lead author and PhD candidate Luca Morreale. She added that as a tree grows, the amount of carbon it can take in increases.

This is good news, because edges encompass almost a quarter of the area in the Northeast United States. However, more forest fragmentation isn’t the answer to sucking more carbon out of the atmosphere. Carbon storage along the edges of fragmented areas does not come even close to compensating for the negative consequences of losing forests, such as releasing carbon long stored underground back into the atmosphere.

The research, published in the journal Nature Communications, emphasizes the need to better understand and protect existing forest borders, which are often considered more disposable.

“We are underestimating how much carbon is being taken up by temperate forest edges,” said Hutyra. “We also need to think about how susceptible they might be in the future to climate change. Because previous studies have shown that even though these trees are growing faster from more sunlight, hotter temperatures cause growth rates of edge trees to plummet.” 

In a second study, Professor Hutyra collaborated with BU scientist Pamela Templer to discover that soils along the forest boundary, like trees, felt the consequences of forest fragmentation.

“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. With soil, there is more there than meets the eye,” said 

Sarah Garvey, lead author of the study published in Global Change Biology.

Garvey discovered that forest edge soil not only releases more carbon than inside forest soil, but that it also behaves differently in rural and urban forests. She measured the concentrations of carbon emitted from the soils at eight field sites in developed and undeveloped areas of Massachusetts every two weeks for a year and a half (except the winter, when the ground is covered in ice). 

The researchers measured the temperature and moisture levels of the soil at the forest edge, then went 300 feet farther into the forest to take more measurements.

Warmer temperatures near the forest edge caused leaves and organic waste to decay rapidly in rural regions with fewer people and buildings, causing soil microorganisms to work harder and release more carbon dioxide than their cooler, more shadowed counterparts in the forest. 

However, soils in urban forests, where the Earth was substantially hotter and drier, did not release as much carbon. “It’s so hot and dry that the microbes are not happy and they’re not doing their thing,” said Hutyra. 

“The long-term effect of unhappy soil is uncertain, but the findings also mean that urban soils, like those in Franklin Park, the largest public park in Boston, could have a greater capacity to store carbon than previously expected,” said Garvey, who plans to delve more deeply into the mechanisms that underpin the various carbon release and storage rates.

Although finding that urban trees and soils store more carbon may appear to be “a double whammy of a positive thing,” said Hutyra, it is uncertain whether this increase in carbon uptake will remain as the world warms.

Climate change may aggravate carbon losses from soil, and trees on the outskirts of forests in rural and urban areas may become more susceptible to excessive heat and drought.

“Forests store almost half of their carbon below ground. 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.” explained Garvey.

With cities and countries pledging to plant more trees in an effort to mitigate the effects of climate change, Hutyra’s lab agrees that considering the larger context of trees and soils, as well as where new trees are planted, is critical. When looking at long-term climate change estimates, it’s also important to consider the increased amounts of carbon deposited by forest edges.

“We need to think about that as we (decide) what areas to conserve, what to develop, and how to tackle climate change solutions. Is a place like Franklin 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,” said Hutyra.

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By Ashikha Raoof, Earth.com Staff Writer