City trees and soil are sucking more carbon out of the atmosphere than previously thought

Date:
February 16, 2022
Source:
Boston University
Summary:
Research uncovers new information about the role that forest edges
play in buffering global impacts of climate change and urbanization.



FULL STORY ==========================================================================
They may not have lungs like we do, but the soil and trees are breathing
in and out all of the time. Trees take in carbon dioxide (CO2), release
oxygen by way of photosynthesis, and store carbon in their trunks. And
when the leaves land on the ground, soil microbes work to decompose the
leaves and other organic matter, which releases carbon dioxide.


========================================================================== Forests actually store more carbon dioxide than they release, which is
great news for us: about 30 percent of carbon emissions from burning
fossil fuels are taken in by forests, an effect called the terrestrial
carbon sink.

"That's CO2 that's not in the atmosphere," says 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." For more than
a decade, Hutyra has been investigating what happens to the planet's
"lungs" when large forests are cut down into smaller patches, a process researchers call 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," says Hutyra, a
BU College of Arts & Sciences professor of Earth and environment. Forests
get cut into smaller parcels, as chunks are taken down to make space for
roads, buildings, agriculture, and solar farms -- one of the biggest
drivers of forest loss in Massachusetts. These alterations to forests
create more areas called forest edges -- literally, the trees at the
outermost edge of a forest.

It has long been assumed that these forest edges release and store carbon
at similar rates as forest interiors, but Hutyra and researchers in her
lab at BU have discovered this isn't true. Soils and trees in temperate
forest edges in the Northeast United States are acting differently
than those farther away from people. In two recent research papers,
Hutyra's team found edge trees grow faster than their country cousins
deep in the forest, and that soil in urban areas can hoard more carbon
dioxide than previously thought. Their results can challenge current
ideas about conservation and the value of urban forests as more than
places for recreation.



========================================================================== Breathing In CO2 In one of the most detailed looks at temperate forest
edges to date, Hutyra and her research team, including collaborators at
the Harvard Forest, examined the growth rates of edge trees compared to
the rest of the forest.

Using data from the US Department of Agriculture's Forest Inventory
and Analysis program -- which monitors tree size, growth, and land use
across the country -- Hutyra's team looked at more than 48,000 forest
plots in the Northeast United States. They found trees on the edges grow
nearly twice as fast as interior trees -- those roughly 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," says Luca Morreale, a
PhD candidate in Hutyra's lab and lead author on the paper, published in
Nature Communications, outlining the findings. And the more a tree grows,
the more carbon it is taking in.

This is good news, considering over 25 percent of the landscape in the Northeast United States is covered by an edge. But this doesn't mean that
more forest fragmentation is a solution for sucking more carbon out of
the atmosphere; carbon storage along the edges of fragmented areas does
not come close to offsetting the negative side effects of losing forests
-- like releasing carbon long stored underground back into the atmosphere.



========================================================================== According to Morreale and Hutyra, their study instead points to the
need to better understand and conserve existing forest edges, which
are typically seen as more disposable. "We are underestimating how much
carbon is being taken up by temperate forest edges," Hutyra says. "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.

Breathing Out CO2 In a second related study, Hutyra teamed up with BU
biologist Pamela Templer to find that soils at the forest edge felt the
effects of forest fragmentation, just like the trees.

"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," says Sarah Garvey, a PhD candidate in Hutyra's
lab and lead author of a paper on forest edge soils published in Global
Change Biology."With soil, there is more there than meets the eye."
Garvey found that not only does forest edge soil release more carbon
than interior forest soil, but that the soil is acting dramatically
differently in rural and urban forests.

She visited eight field sites in developed and undeveloped areas of Massachusetts every two weeks for a year and a half (skipping the winter,
when the ground is covered in ice) to measure the levels of carbon
being released from the soils. She and her team would take a snapshot
of the temperature and moisture levels of the soil at the forest edge,
and then walk about 300 feet into the forest to take measurements again.

They saw that in rural areas with fewer people and buildings, warmer temperatures at the edge of the forest caused leaves and organic matter
to decompose faster, forcing the soil microorganisms to work harder and
release more carbon dioxide than their cooler, more shaded peers in the
forest interior. But, in urban forests, where the ground was significantly hotter and drier, those soils stopped releasing as much carbon.

"It's so hot and dry that the microbes are not happy and they're not
doing their thing," Hutyra says. 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, says Garvey. Her
next project will look deeper into the possible mechanisms behind the
different carbon release and storage rates.

Critical for Our Future Although discovering that urban trees and soils
store more carbon might seem like "a double whammy of a good thing,"
Hutyra says, it's unclear if this boost in carbon uptake will last as
the planet continues to warm.

Climate change could exacerbate carbon losses from soil, and the trees
at the edge of forests in rural or urban areas could be more vulnerable
to extreme heat and drought.

"Forests store almost half of their carbon below ground," Garvey
says. "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." With cities and countries
making commitments to plant more trees in an effort to curb the impacts of climate change, the researchers in Hutyra's lab all agree that thinking
about the greater context of the trees and soils, and where new trees
are planted, is extremely important. Factoring in the elevated amounts of carbon stored by forest edges should be taken into account when looking
at long-term projections of climate change, as well.

"We need to think about that as we [decide] what areas to
conserve, what to develop, and how to tackle climate change
solutions," Hutyra says. "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." ========================================================================== Story Source: Materials provided by Boston_University. Original written
by Jessica Colarossi.

Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Sarah M. Garvey, Pamela H. Templer, Erin A. Pierce, Andrew
B. Reinmann,
Lucy R. Hutyra. Diverging patterns at the forest edge: Soil
respiration dynamics of fragmented forests in urban and rural
areas. Global Change Biology, 2022; DOI: 10.1111/gcb.16099 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/02/220216153902.htm

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