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Hurricane Katrina's toll was also environmental. Trees took a wallop, hundreds of millions of them, according to new research. As those uprooted trees decay, they're releasing tons of carbon dioxide, and that makes them part of the climate change problem as well.

NPR's Elizabeth Shogren has that story from Louisiana.

ELIZABETH SHOGREN: Jeff Chambers is one of the lucky ones. He evacuated his house in New Orleans before Katrina hit. He's a biologist. And as he was fleeing, he already was thinking about the research tools he's been using to study the Amazon Forest.

JEFF CHAMBERS: And I thought, you know, this exact same tools would be ideal for quantifying Hurricane Katrina's impact on the Gulf Coast forest.

SHOGREN: Chambers, a professor at Tulane University, uses satellite images and on-the-ground surveys to study the damage caused by wind storms in the forests of Brazil. That put him in a perfect position to study the trees killed by Katrina. To start out, he hunted down two satellite images - one from two years before the storm and one from a year after, both taken the same month on clear days.

CHAMBERS: We'll do this, so we can kind of...

SHOGREN: Chambers shows me how he analyzed those images with expensive software.

CHAMBERS: Okay. So this is the pre-Katrina image. Now, let me show you the post-Katrina image.

SHOGREN: Much of the area that was green, meaning live trees, turns red in the image taken after Katrina. He ran programs to compare the two images, pixel by pixel. Each pixel covers about the size of a basketball court. He picked 25 of them that represent a range of forest damage. Then came the fun part. He zoomed in on each pixel.

CHAMBERS: Punch it into our GPS receiver and went out into the field and located that exact pixel that we had never been to before on the ground. And we measured the number of dead trees and all these data on forest damage.

SHOGREN: One of the most damaged pixels is about an hour's drive north of New Orleans at the Pearl River hunting reserve. I drive there with Chambers to take a look. Wild hogs are about the only thing in season, so there aren't a lot of hunters.

But the ones here, like Paul Noel, say this used to be a gorgeous, wide-open forest with mature hardwood trees.

PAUL NOEL: I'm amazed. I'm absolutely amazed. It is nothing...

SHOGREN: What's changed?

NOEL: There's no trees. You can't walk any place because of the briers.

SHOGREN: Noel just pulled up in his SUV. He's putting on his boots and hunting vest. It's his first time here since the storm.

NOEL: But it ought to be just for the game, you know, it really should be.

SHOGREN: And it is. Not far away, two hunters carry a couple of hogs out of the woods on their shoulders and put them into their red pickup truck. Nearby, we head into the woods. We crawl over, under and around downed trees. Prickly shrubs grabbed at our skin and clothing.

CHAMBERS: This is essentially ground-zero for where Katrina made its final landfall.

SHOGREN: This is what Chambers brought me here to see. The trunks of once massive oaks, sweet gums, pecans and other hardwoods lie in tangles across the forest floor like a massive game of pickup sticks. New growth, trees and sticker bushes, sprout everywhere. Chambers stops at one downed tree that's covered with mushrooms.

CHAMBERS: So here's the fruiting body of some species of fungus that's consuming this piece of dead wood. And as the fungi consume the piece of dead wood, one of their waste products is carbon dioxide.

SHOGREN: Three hundred and twenty million trees releasing carbon dioxide over the next 10 to 20 years. That's a lot of carbon dioxide. Chambers estimates that's about as much as all the trees across the United States soak up over the course of one year.

He's not the only scientist who's tried to measure Katrina's carbon footprint. Steve McNulty of the Forest Service used field surveys and aerial photography to make a similar calculation. His estimate was less than half as much, but McNulty doesn't quarrel with Chambers' findings.

STEVE MCNULTY: My estimates are probably somewhat conservative relative to the total damage just because you can't go out and survey all of the damage.

SHOGREN: Some in the forestry industry have criticized Chambers' study. They point out that most of Louisiana's forests are privately owned, and Chambers didn't do any site surveys on private property. And they stress that Chambers didn't take into consideration the trees that were taken to mills and turned into lumber after the storm.

(SOUNDBITE OF SAW MILL MACHINE)

SHOGREN: About an hour's drive to the northwest, loggers are busy cutting down and loading 60-foot-tall loblolly pines at the Jenkins family tree farm.

KIRK CASANOVA: The trees that are salvaged during this operation, the carbon will not be released because it'll stay in the form of boards and be used for construction and housing.

SHOGREN: That's Kirk Casanova with the Louisiana Department of Forestry. The state estimates that about 20 percent of the trees killed by Katrina were harvested. More than 200 acres of them were at the Jenkins' farm.

Margie Jenkins is the 86-year-old matriarch of the business.

What does it look like after the storm here?

MARGIE JENKINS: Well, it was absolutely devastating. You had these trees broke off halfway. A lot of them were leaning, a lot of them was laying on the ground. It was awful. So sad.

SHOGREN: Walking through a field of knee-high loblolly pines, Mrs. Jenkins' granddaughter, Amelia Levin, says Chambers missed something else. Some landowners, like themselves, have replanted. And the saplings already are absorbing carbon dioxide.

AMELIA LEVIN: By planting baby trees, that's a hell of a countermeasure, I think.

SHOGREN: Chambers concedes that new trees will soak up carbon dioxide, but it will take several decades for them to absorb as much as the trees that were lost.

CHAMBERS: The carbon released from all the deadwood and these patches of forests that were heavily damaged will result in a carbon source for many years. It would take much longer for the gains from photosynthesis and new wood production to make up for those losses.

SHOGREN: Besides, Chambers says, it's more important to ponder the big questions. As climate change makes intense storms like Katrina more frequent, will forests be able to absorb and store as much of the world's greenhouse gas pollution as they do now? Probably not. And if they can't, that will make the challenge of keeping climate change in check even tougher.

Elizabeth Shogren, NPR News.

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