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Temperate forests not a fix for global warming

Temperate forests not a fix for global warming

Temperate forests not a fix for global warming
Carbon offsets based on northern plantations may be bunk
August 9, 2007

Carbon sequestration projects in temperate regions — already facing doubts by scientists — were dealt another blow by Duke University-led research that found pine tree stands grown under elevated carbon dioxide conditions only store significant amounts of carbon when they receive sufficient amounts of water and nutrients.

The results — from the ten year Free Air Carbon Enrichment (FACE) experiment in a Duke University forest — “suggest that proposals to bank extra CO2 from human activities in such trees may depend on the vagaries of the weather and large scale forest fertilization efforts” according to Ram Oren, director of the project.

“If water availability decreases to plants at the same time that carbon dioxide increases, then we might not have a net gain in carbon sequestration,” said Oren, a professor of ecology at Duke’s Nicholas School of the Environment and Earth Sciences. “In order to actually have an effect on the atmospheric concentration of CO2, the results suggest a future need to fertilize vast areas… The impact on water quality of fertilizing large areas will be intolerable to society. Water is already a scarce resource. ”

At FACE, computer-controlled valves on rings of towers are administering carbon dioxide to stands of loblolly pines. Photo by Chris Hildreth

“In some areas, the growth is maybe 5 or 10 percent more, and in other areas it’s 40 percent more,” Oren added. “So in sites that are poor in nutrients and water we see very little response. In sites that are rich in both we see a large response.”

The researchers found that enrichment did not change the proportion of carbon stored in wood, leaves, roots and underlying soil.

“Carbon that’s in foliage is going to last a lot shorter time than carbon in the wood, because leaves quickly decay,” explained Heather McCarthy, a former graduate student of Oren. “So elevated CO2 could significantly increase the production of foliage but this would lead to only a very small increase in ecosystem carbon storage.”

The study, presented at a national meeting of the Ecological Society of America, follows studies by FACE scientists that found soil nutrition is key to helping forests absorb more carbon under elevated CO2 conditions. Still earlier FACE research warned that growing tree plantations to remove carbon dioxide from the atmosphere to mitigate global warming could have other environmental detriments, including water and nutrient depletion and increased soil salinity and acidity.

Other research, the Carnegie Institution’s Department of Global Ecology and Lawrence Livermore National Laboratory, has suggested that planting trees at northern latitudes could actually worsen warming with vegetation absorbing more sunlight — and heat — without producing more moisture. The situation is different in the tropics where higher temperatures result in higher rates of evapotranspiration, the process by which forests release water into the atmosphere. Tropical forests may have a net cooling impact relative to northern forests, according to the research.