Washington, September 11 : A new analysis has suggested that old growth forests are usually valuable "carbon sinks", and they continue to absorb carbon dioxide (CO2) from the atmosphere and mitigate climate change for centuries.
The analysis of 519 different plot studies, by researchers from Oregon State University (OSU) and several other institutions, 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.
According to the study, in forests anywhere between 15 and 800 years of age, 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.
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. 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," Law said.
"The current data now makes it clear that carbon accumulation can continue in forests that are centuries old," he added.
According to Law, when an old growth forest is harvested, 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, according to the study, continue to sequester carbon for many centuries.
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.
The study will be necessary for land surface models that attempt to define carbon balance to better characterize function of old forests.