Dead Trees Spewing Greenhouse Gases

PEARL RIVER — Katrina’s surge up the Pearl River damaged or killed many thousands of trees, and the tons of debris left to rot is slowly spewing greenhouse gases into the atmosphere, according to new research released Thursday by Tulane University.

The scientists, led by Tulane ecologist Jeffrey Chambers, hand-counted trees along the river and compared satellite images from before and after the storm to identify damage hot spots. From their data, published in the journal Science, Chambers said trees felled in just a few hours in a small area would decay, eventually releasing enough carbon gases to erase what every healthy forest in the United States could suck up in a year.

It was surprising, he said, "that one storm could essentially upset the gains for an entire year."

Should climate-change predictions of stronger and more frequent hurricanes be true, and more trees suffer Katrina-level beatings, the role of forests as a moderator of greenhouse gases could reverse, he said. Healthy forests are a "carbon sink," pulling in more greenhouse gases than they release during natural decay. Katrina’s damage has had the opposite effect, Chambers said.

Large swaths of decaying trees could one day add to global warming.

As pines and other hardwoods that take years to mature start regaining their footing, invasive plants such as the Chinese tallow, or popcorn tree, are now growing underfoot, he said. Levees kept saltwater that surged in from flowing out, creating timber graveyards. There are fewer trees to suck up carbon gases, and more decaying debris to release them.

But "things start growing back fast here," said Jim Shepard, department head of the College of Forestry Resources at Mississippi State University. And what grows back will start taking in greenhouse gases again. Shepard and his colleagues have conducted their own counts of damaged trees relative to lost harvest by timber and allied industries. Hancock County, which is separated from Louisiana by the Pearl River, lost about 40 percent of its timber, he said. Some areas lost every tree.

At first blush, he said, 320 million trees snapped or killed by hurricanes Katrina and Rita in Louisiana and Mississippi may be an overestimate, and the 100 metric tons of carbon gas they’re supposed to release may not take new growth into account. But actual numbers aside, the leap to climate change may be warranted.

"The gist is certainly true if we’re going to have worse storms," he said.

And though trees were lost en masse during the Southern California wildfires, study co-author George Hurtt, a natural resources expert at the University of New Hampshire, said their carbon output pales in comparison with Katrina. His early calculations show Katrina’s dead trees releasing 10 times the amount of carbon gas as the wildfires because in California, there were fewer, smaller trees and the acreage is naturally controlled by fire.

The damage to forests during Katrina was so severe, said Shepard, that for the first time the federal government responded to a major storm by giving millions of dollars for forest recovery. In July 2006 the U.S. Department of Agriculture earmarked $504 million for emergency forest conservation for private landowners with significant timber loss in areas damaged by the five major hurricanes in 2005, including Katrina.

But more than a year later, most of the money is still sitting around, Shepard said, and the department will soon launch a study into why.

"There was a lot of money available. The program could have been run better," he said.