Are we overestimating how much trees will help fight climate change?

This story
was originally published by Undark and is reproduced here as part of
the Climate Desk
collaboration.

Bob Marra navigated his way to the back of a dusty barn in
Hamden, Connecticut, belonging to the state’s Agricultural
Experiment Station. There, past piles of empty beehives, on a wall
of metal shelves, were stacks of wooden disks — all that remains
of 39 trees taken down in 2014 from Great Mountain Forest in the
northwest corner of the state.

These cross-sections of tree trunks, known as stem disks — or
more informally as cookies — are telling a potentially worrisome
tale about the ability of forests to be critical hedges against
accelerating climate change. As anyone following the fires
burning in the Amazon rainforest
knows by now, trees play an
important role in helping to offset global warming by storing
carbon from atmospheric carbon dioxide — a major contributor to
rising temperatures — in their wood, leaves, and roots. The
worldwide level of CO2 is currently averaging more than
400 parts per million
— the highest amount by far in the last
800,000 years.

But Marra, a forest pathologist at the Experiment Station with a
PhD in plant pathology from Cornell University, has documented from
studying his fallen trees that internal decay has the capacity to
significantly reduce the amount of carbon stored within.

His research,
published in Environmental Research Letters late last year and
funded by the National Science Foundation, focused on a technique
to see inside trees — a kind of scan known as tomography (the
“T” in CAT scan). This particular tomography was developed for
use by arborists to detect decay in urban and suburban trees,
mainly for safety purposes. Marra, however, may be the first to
deploy it for measuring carbon content and loss associated with
internal decay. Where there is decay there is less carbon, he
explains, and where there is a cavity, there is no carbon at
all.

“What we’re suggesting is that internal decay in trees has
just not been properly accounted for,” says Marra.

This tree trunk section,
or cookie, shows a large hollow in the center. Marra argues that
traditional methods can miss such decay, and therefore overestimate
how much forests will contribute to storing carbon. Jan Ellen
Spiegel

While the first round of his research was a proof of concept
that necessitated the destruction of 39 trees to show that
tomography is accurate, his ultimate goal is a nondestructive
technique to enable better assessments of carbon sequestration than
those done annually by the U.S. Forest
Service.
Under the United Nations Framework Convention
on Climate Change
, ratified in 1994, governments are required
to report annual estimates of carbon holdings in all their managed
lands. The most recent Forest Service figures show that U.S.
forests offset about 14 percent of the nation’s carbon emissions
each year.

The Forest Service estimates that carbon makes up 48 to 50
percent of a tree’s biomass, so ones with decay will be less
dense and therefore hold less carbon. But Marra contends that the
visual signs monitored by the Forest Service, such as canopy and
tree size, along with conspicuous problems such as lesions or
cankers, don’t accurately reflect internal decay — a tree that
looks healthy may have decay and one that appears problematic may
be fine inside.

In addition, he says, foresters typically use a mallet to hammer
a tree to register a sound that might indicate it’s hollow.
“You know that there may be a hollow, but you don’t know how
big the hollow is,” Marra says. As a result, he believes the
government’s baseline data used to estimate carbon storage are
not accurate.

“There are a lot of ways to improve our estimates of carbon
being stored above ground in forests, and this decay component
could certainly prove to be important,” says Andrew Reinmann, an
ecologist and biogeochemist with the City University of New
York’s Advanced Science Research Center. But, he added, “We
haven’t really had the technology to explore this before —
it’s still a little bit of an unknown.”

Marra used a two-stage system for his research: sonic
tomography, which sends sound waves through the tree, followed by
electrical resistance tomography, which transmits an electric
current. Both processes are necessary to fine-tune each other’s
readings.

The system, which costs about $25,000 and fits in a backpack, is
cheap and small by scientific equipment standards. Each reading
takes no more than a few minutes and computerized visual renderings
of the results appear instantly.

Marra uses a kind of scan
known as tomography to measure carbon storage and decay in trees.
Jan Ellen Spiegel

Marra experimented with three northern hardwoods — sugar
maple, yellow birch, and American beech — and included more than
two dozen of each, along with some control trees with no decay. The
researchers analyzed the lower bole — the first two meters or so
— of each tree, which is the oldest part and closest to the soil,
where most decay-causing fungi would come from.

A dozen or so nails were tapped in a circle around the trunk and
connected by cables to the tomograph; a sonic hammer then activated
the system to get sound-wave measurements.

For the electric resistance tomography, a second set of nails
was hammered between the first, and electrodes — plus and minus
— were attached to each.

The various nail areas were painted in different colors to
enable the computer renderings to be aligned later with photographs
of the cookies after the trees were cut down.

The cookies, about 4 inches thick and which Marra called “the
truth,” were only taken from where the measurements were made —
the areas with the paint markings.

He analyzed 105 cookies from the 39 trees taken down. In the 11
cases where tomography found no decay, the cookies revealed only
one small cavity. In the 32 cases where incipient, or early, decay
was detected, the cookies showed one additional cavity. The cookies
confirmed the tomography results in 36 cases where active decay was
found, though eight small cavities were also detected. Tomography
correctly identified cavities in the remaining 26 cookies, meaning
that it missed a total of 10 cavities among the 105 cookies.

“One thing to sort of mitigate against this failure, if you
want to call it that — these were very small cavities,” Marra
says of the ones the tomography missed. “So they would have very
little impact on a carbon budget.”

Marra readies a tree for
scanning with electrodes and a tomograph. Jan Ellen Spiegel

Then came the time-consuming process of measuring the actual
amount of carbon in each tree. After air-drying the cookies for a
year, the wood from 500 drilled holes was sent to a gas
chromatography lab at the University of Massachusetts to determine
the carbon levels.

The tomography and lab results were then combined to calculate
how much carbon was stored in the lower boles and to contrast that
with the levels if the trees had been solid wood. Those
calculations took until 2017 to complete.

“You’re looking at anywhere from a 19 percent to a 34
percent carbon loss” for an actively decaying tree among those
studied, Marra says. “But any place there’s a cavity you’ve
lost all of your carbon.”

The upshot of his five years of research, says Marra, is that
accurate tomographic readings are possible in just a few minutes.
“And what our tomography tells us is the carbon content,” he
says.

At the same time, Marra is aware that tomography is not a
practical substitute for the Forest Service’s carbon estimate
system — which itself is a clunky and labor-intensive slog. But
it could provide a valuable way to augment those estimates.

“Those are very, very impressive results,’’ says Kevin
Griffin, a tree physiologist at Columbia University and its
Lamont-Doherty Earth Observatory. “They obviously have obtained a
lot of precision in the techniques.”

“The results are important,” he adds, “but whether
internal tree decay is the single most burning question? Probably
not. There’s probably bigger fish to fry before we get
there.”

Among them, he says are forest growth rates and overall tree
health and age, as well as the impact of harvesting and other kinds
of losses, including disease.

A tree’s architecture and height could also play large roles
in carbon sequestration, says Reinmann of the City University of
New York’s Advanced Science Research Center, as could the makeup
of the forest landscape. His own research, for instance, found
trees grow faster and have more biomass at the edge of fragmented
forest.

“I think they’re making a good point that we’re probably
over-estimating” carbon storage levels, says Aaron Weiskittel,
director of the University of Maine’s Center for Research on
Sustainable Forests.

Even so, Weiskittel and others — including Marra — say the
research needs to be scaled up to many more tree types and full
forests. For his part, Marra would like to sample forests randomly
with many more trees and controlling for factors including species,
age, and soil characteristics.

The goal, he says, is to develop a methodology for generating
data to provide better carbon estimates for more than three tree
types in one small part of the country.

“We need to use tomography to refine models so we’re more
accurately assessing the role that forests are playing as
sequesterers or climate change mitigators,” Marra says. “We
don’t want to be over-estimating the roles that they play.”

This story was originally published by Grist with the headline
Are we overestimating how much trees will help fight climate
change?
on Sep 8, 2019.

Source: FS – All – Science – News
Are we overestimating how much trees will help fight climate change?