Alaska: Northern forests first grew faster with more carbon emissions, now they grow slower

In addition to his research in the forest, Goetz was using satellite data to study how the spruce-rich forests of northern Canada and Alaska recover after large fires. The burned forest was re-growing as he expected, but the unburned forest was behaving strangely. Since the 1990s, scientists have known that increasing global temperatures have lengthened the growing season in the Arctic. With carbon dioxide, one of the key ingredients in photosynthesis, also on the rise, the forest should have been thriving. But it wasn’t. The forest was getting browner, not greener.

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http://earthobservatory.nasa.gov/Features/BorealThreshold/

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Scott Goetz made his way to the array of light detectors spread
beneath the canopy of the Alaskan forest. The sun sat low on the
horizon, casting long shadows on the forest floor. The long night of
winter was quickly approaching, and the detectors would no longer be
needed for the year.

On the other side of the United States, Alon Angert, a scientist at the University of California, Berkeley, noticed a strange trend in the forest, too. Angert was tracking carbon dioxide in the atmosphere over the Arctic from 1985 to 1994 when he saw that trees weren’t soaking up as much of the gas at the end of the period as at the beginning. It was as if the whole forest had slowed its breathing during that single decade.

“Something big is happening in the high latitudes,” says Rama Nemani, a research scientist at NASA Ames Research Center, in response to the papers that Goetz and Angert published within weeks of each other. Nemani wasn’t surprised that Northern forests now seem to have slowed their growth.

After all, the same theories that predicted that global warming would increase forest growth in the Arctic, theories that Nemani helped prove, also predicted that the forests would eventually reach the limits of the water supply and go into decline. “We knew something like this would happen,” Nemani says. “We didn’t expect that it was going to happen so quickly.”

What is happening to the forests of northern Alaska, Canada, Europe, and Siberia? Why have they slowed their growth when everyone thought they should be expanding for several more decades? Is the trend that Goetz and Angert independently observed a fluke, a temporary downturn in the health of the forests, or is it something more?

Is it a sign that global warming is changing Northern forests more quickly than anyone thought possible? The switch from carbon sink to source is a possibility that scientists anticipated for the next century, but are Goetz and Angert’s observations a sign that the forests are already approaching this threshold?

Get full text; Support da’ word producer:
http://earthobservatory.nasa.gov/Features/BorealThreshold/

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