NASA's Earth Observatory

September 10, 2008

OLD GROWTH FORESTS ARE VALUABLE CARBON SINKS

Contrary to 40 years of conventional wisdom, a new analysis to be

published Friday in the journal Nature suggests that old growth

forests are usually "carbon sinks" - they continue to absorb carbon

dioxide from the atmosphere and mitigate climate change for centuries.

However, these old growth forests around the world are not protected

by international treaties and have been considered of no significance

in the national "carbon budgets" as outlined in the Kyoto Protocol.

That perspective was largely based on findings of a single study from

the late 1960s which had become accepted theory, and scientists now

say it needs to be changed.

"Carbon accounting rules for forests should give credit for leaving

old growth forest intact," researchers from Oregon State University

and several other institutions concluded in their report. "Much of

this carbon, even soil carbon, will move back to the atmosphere if

these forests are disturbed."

The analysis of 519 different plot studies found that about 15 percent

of the forest land in the Northern Hemisphere is unmanaged primary

forests with large amounts of old growth, and that rather than being

irrelevant to the Earth's carbon budget, they may account for as much

as 10 percent of the global net uptake of carbon dioxide.

In forests anywhere between 15 and 800 years of age, the study said,

the net carbon balance of the forest and soils is usually positive –

meaning they absorb more carbon dioxide than they release.

"If you are concerned about offsetting greenhouse gas emissions and

look at old forests from nothing more than a carbon perspective, the

best thing to do is leave them alone," said Beverly Law, professor of

forest science at OSU and director of the AmeriFlux network, a group

of 90 research sites in North and Central America that helps to

monitor the current global "budget" of carbon dioxide.

Forests use carbon dioxide as building blocks for organic molecules

and store it in woody tissues, but that process is not indefinite. In

the 1960s, a study using 10 years worth of data from a single

plantation suggested that forests 150 or more years old give off as

much carbon as they take up from the atmosphere, and are thus "carbon

neutral."

"That's the story that we all learned for decades in ecology classes,"

Law said. "But it was just based on observations in a single study of

one type of forest, and it simply doesn't apply in all cases. The

current data now makes it clear that carbon accumulation can continue

in forests that are centuries old."

When an old growth forest is harvested, Law said, studies show that

there's a new input of carbon to the atmosphere for about 5-20 years,

before the growing young trees begin to absorb and sequester more

carbon than they give off. The creation of new forests, whether

naturally or by humans, is often associated with disturbance to soil

and the previous vegetation, resulting in decomposition that exceeds

for some period the net primary productivity of re-growth.

Old growth forests, the study said, continue to sequester carbon for

many centuries. And when individual trees die due to lightning,

insects, fungal attack or other causes, there is generally a second

canopy layer waiting in the shade to take over and maintain productivity.

One implication of the study, Law said, is that nations with

significant amounts of old forests may find it somewhat easier to

offset greenhouse gas emissions if those forests are left intact. It

will also be necessary, she said, for land surface models that attempt

to define carbon balance to better characterize function of old forests.

Many of the conclusions from the study were based on data acquired

from the AmeriFlux and CarboEurope programs, researchers said.

Multiple funding sources included the U.S. Department of Energy,

CarboEurope, the European Union, and others. Authors were from

institutions in the U.S., Belgium, Germany, Switzerland, France and

the United Kingdom.

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Contact:

Beverly Law

Oregon State University

541-737-6111

bev.law@oregonstate.edu

This text derived from:

http://oregonstate.edu/dept/ncs/newsarch/2008/Sep08/oldgrowthcarbon.html