Carbon Sink

Forests can absorb more U.S. carbon than previously thought
(Photo: Eppic Photography)

(Photo: Eppic Photography)

Oregon State University forestry scientists have a habit of redefining the conversation about carbon and forests. Professors Beverly Law, Mark Harmon and their colleagues have demonstrated that old-growth stands on the west side of the Cascades store as much carbon or more than that held in tropical rain forests.

In 2009, Law reported that forests from the San Francisco Bay Area to the Columbia River could theoretically double the amount of carbon they currently contain.

In 2007 and 2009, her research group determined that Pacific Northwest fires emit less carbon than previously thought. Most emissions were from combustion of the forest floor and understory vegetation, and only about 1 to 3 percent of live tree mass was burned.

Not surprisingly, tree cutting turns forests from carbon sinks to carbon sources. Law has determined that it may take 15 years or more for young trees to begin absorbing more carbon than is lost through decomposition of branches, roots and other dead material. She conducted her studies in ponderosa pine, and her conclusions were later confirmed in an international study of boreal and temperate forests.

Ameriflux Network

Now, Law has co-authored a national study concluding that forests and other terrestrial ecosystems in the lower 48 states can sequester up to 40 percent of the nation’s fossil fuel carbon emissions, a larger amount than previously estimated, unless a large drought or other major disturbance occurs.

Carbon dioxide, when released by the burning of fossil fuels, forest fires or other activities, is a major “greenhouse gas” and factor in global warming. But vegetation, mostly in the form of growing evergreen and deciduous forests, can play an important role in absorbing some of the excess carbon dioxide.

Widespread droughts, such as those that occurred in 2002 and 2006, can cut the amount of carbon sequestered by about 20 percent, Law and her colleagues concluded in a study that was supported by the National Science Foundation and U.S. Department of Energy.

The research, published by scientists from 35 institutions in the journal Agricultural and Forest Meteorology, was based on satellite measurements and data from the AmeriFlux network, a system of nearly 100 carbon-monitoring sites in the Americas.

Not all of these data had been incorporated into earlier estimates, and the new study provides one of the most accurate assessments to date of the nation’s terrestrial carbon balance.

“With climate change, we may get more extreme or frequent weather events in the future than we had before,” Law adds. “About half of the United States was affected by the major droughts in 2002 and 2006, which were unusual in their spatial extent and severity. And we’re now learning that this can have significant effects on the amount of carbon sequestered in a given year.”

Climate Mapping

Such information is important to understand global climate issues and develop policies, the researchers note. This study examined the carbon budget in the United States from 2001 to 2006. Also playing a key role in the analysis was OSU’s PRISM climate database, a sophisticated system to monitor weather on a very localized and specific basis.

The period from 2001 to 2006, the researchers say, had some catastrophic and unusual events, not the least of which was Hurricane Katrina and the massive destruction it caused. It also factored in the 2002 Biscuit Fire in Northern California and southwest Oregon, which burned nearly 500,000 acres and was among the largest forest fires in modern U.S. history.

The research found that temperate forests in eastern states absorbed carbon mainly because of forest re-growth following the abandonment of agricultural lands, while some areas of the Pacific Northwest assimilated carbon during much of the year because of the region’s mild climate.

Croplands were not considered in determining the annual magnitude of the U.S. terrestrial carbon sink, because the carbon they absorb each year during growth will be soon released when the crops are harvested or their biomass burned.

The study was led by Jingfeng Xiao, a research assistant professor at the Complex Systems Research Center, Institute for the Study of Earth, Oceans, and Space, at the University of New Hampshire.

“Our results show that U.S. ecosystems play an important role in slowing down the buildup of carbon dioxide in the atmosphere,” the researchers wrote in their conclusion.

Ø Online: See more about Beverly Law’s terrestrial ecosystem research at terraweb.forestry.oregonstate.edu/

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