Old-Growth Forests Help Combat Climate Change

Don’t forget: there is more to the relationship between mature &
old-growth forests and climate stability than carbon sequestration.

ASW

Scientific American
News –  September 11, 2008

Old-Growth Forests Help Combat Climate Change
Mature forests in colder climes may continue to store more carbon
than they emit, helping stave off global warming

By David Biello

Rare is the forest untouched by man. Whether logging or clearing land
for agriculture, the bulk of the world’s forests have fallen to
crops, cattle or younger trees. According to some estimates, less
than 10 percent of forests worldwide can be considered old growth, or
undisturbed for more than a century. And that is not just a tragedy
for the plants and animals that require mature forests-it is also a
tragedy for the world’s climate, according to a study published today
in Nature.

Laborious research in the 1960s by the late pioneering U.S. ecologist
Eugene Odum seemed to indicate that forests achieve a balance between
the amount of carbon dioxide (CO2) absorbed by growing trees and
plants and the amount of CO2 released back into the atmosphere by the
decomposition of dead plant matter.

But it seems that old forests may be more efficient than previously
believed. Biologist Sebastiaan Luyssaert of the University of Antwerp
in Belgium and his colleagues surveyed all the existing measurements
of how much carbon is absorbed and released from old-growth forests
(exclusively in temperate and boreal forests due to a lack of
extensive data on tropical forests). Their findings, Luyssaert says:
“old-growth forests continued to accumulate carbon.”

In fact, not only do old trees continue to store carbon in their
wood, forest soils also appear to be actively capturing carbon over
time, although direct observations of this process are lacking. All
told, by Luyssaert’s calculations the relatively small remaining
stands of old-growth forests in the U.S. Pacific Northwest as well as
Canada and Russia consume “8 to 20 percent of the global terrestrial
carbon sink,” or roughly 440.9 million tons (0.4 gigatonnes) of
carbon per year.

That is not even close to enough to balance the 1.8 billion tons (1.6
gigatonnes) released into the atmosphere by deforestation or
crop-clearing. But it remains important-if unrecognized-in the
present battle to combat climate change. Luyssaert suggests that
credit-and money-should be given to protect such old-growth forests
under carbon trading schemes and other economic mechanisms to combat
climate change.

“Any kind of existing program that gives credit to reforestation
could give credits to forest preservation,” such as the carbon
offsets based on tree planting, he says. “Instead of investing the
money in a new forest, it could as well be used to protect an old
forest.”

But the case for old forests as carbon sinks is not airtight. The
measurements used by Luyssaert rely on the flux of CO2 levels over
the forest, but this kind of metric can be skewed by young stands of
trees within an old-growth forest or an increase in growth as a
result of higher atmospheric carbon dioxide levels, according to
forest ecologist Mark Harmon of Oregon State University in Corvallis,
who was not involved in the study.

“To really test this, one would need a far better data set that had
different ages in the same system: that is very young, mature,
old-growth and super old-growth in each system,” he says. But “older
forests should not be written off as places to store more carbon.
Even if they aren’t taking up more carbon, their harvest releases a
great deal.”

It remains unclear whether tropical forests, such as those of the
Amazon or Congo, produce the same effect, due to much faster
decomposition of dead plant matter in these climes. But preliminary
results suggest they do. “The data that are available show that, like
the boreal and temperate forests, tropical old-growth forests also
continue to take up and sequester carbon,” says forest scientist
Eugenie Euskirchen of the University of Alaska Fairbanks, who was not
involved in this research.

Protecting old-growth temperate and subpolar forests might prove a
boon to the fight against global warming, also because of the soils
they currently shade. “Many old boreal forests tend to be underlain
by permafrost soils, which can contain many times more carbon than
that stored in the vegetation,” Euskirchen notes. Melting those soils
is an ongoing climate calamity.

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