Katrina rated largest U.S. ecodisaster

No surprise: They love trees in Poplarville, Miss.

So when hurricane Katrina ripped out tulip poplars, bent black gum to the ground, and scattered loblolly pines like pick-up sticks, local tree enthusiasts such as Julia Anderson not only had a rude aesthetic shock, but many also sensed that the destruction had shaken the very roots of the region's ecological balance.

Now, scientists using NASA satellite imagery have at least partly confirmed those suspicions. From vast slash pine plantations to river-bottom hardwood stands, hurricane Katrina killed or damaged about 320 million trees across Florida, Mississippi, Louisiana, Alabama, and Texas – the largest ecological disaster in US history, new estimates reveal. Confronting a potential 100 million metric tons of greenhouse gases seeping from rotting logs and leaves, the proliferation of nonnative plants, and a spike in wildfire risks, scientists and residents alike are raising new questions about the storm's environmental legacy.

Perhaps the most critical one: Can Katrina-like storms contribute to an ecological "feedback loop," in which carbon being released from fallen, decaying forests raises the occurrences of storms and, in turn, intensifies the effects of global warming? The good news is that resilient and fast-growing Southern forests, with the help of humans, may be able to temper the phenomenon.

"The problem with feedback is that it'll make climate change worse than the current scenarios are envisioning," says George Hurtt, a natural-resources professor at the University of New Hampshire in Durham and coauthor of a new damage assessment published Friday in the journal Science. "Katrina left a huge carbon footprint, and there are going to be constant reminders of that."

The storm killed or damaged nearly one big tree for every American, and the total load of carbon dioxide produced by their decay surpasses the amount of CO2 that all healthy forests in the US could photosynthesize back into oxygen in a year's time. Moreover, a recent Louisiana State University study showed that, across history, wildfires have consumed hurricane-wrecked areas on the Gulf Coast. Such fires, too, can play into the carbon feedback loop, says Professor Hurtt.

At the same time, escaped Asian ornamentals – including Chinese tallow, the pesky privet bush, and congongrass (a forest-floor bully that swamps all competitors) – will probably complicate or delay regrowth in some parts of the forest.

"It actually is hard to find a silver lining," says Jim Shephard, a forestry professor at Mississippi State University (MSU) in Starkville.

Some critics, however, say that in their reach to connect Katrina's damage to global warming, researchers from UNH and Tulane University in New Orleans may have done better by employing forestry techniques such as on-site surveys.

The Tulane and UNH estimate of the storm's damage is "way, way, way too much," says Wayne Tucker of the Mississippi Institute for Forest Inventory (MIFI) in Jackson.

Instead of looking at a small number of test plots, MIFI conducted more than 150 on-the-ground surveys on randomly selected plots in each affected county. Revising early estimates of about 3 billion lost board feet, state forestry experts finally figured the storm ruined about 1 billion board feet – one-fifth of the estimate published in Science, according to Mr. Tucker.

"Their analysis on imagery is probably pretty good, but where they fell down was their sampling method," he says.

Mississippi researchers agree, however, that the damage transformed the ecological and economic equation for landowners and residents. Tellingly, paper and lumber companies are switching from fast-growing but relatively fragile loblolly pines – which fell in droves during Katrina – to the slower-growing but deeper-rooted longleaf pine. Private and state nurseries are sold out of longleaf seedlings.

What's more, Congress for the first time approved money – $504 million – to help states replant lost forests, but only a fraction of that money has so far been allocated. One problem: Because the US has never before compensated tree owners, the program employs a formula used to assess cotton crop damages – a poor fit that has put the project behind schedule.

Congress also provided tens of millions of dollars for states to buy firefighting equipment, which has already been used to cut hundreds of miles of new firebreaks. In fact, with about half the fallen lumber salvaged, the worst fire fears are over, says Mr. Shephard at MSU.

But trees define life in Mississippi more than in simply dollar terms, says Ms. Anderson, the Poplarville resident and coordinator of a replanting effort in Pearl River County. Today, the resuscitation effort is driven largely by residents keen to see the return of the canopy, which defined many people's sense of place.

Replant South Mississippi, a private effort, hopes in the next three years to replace 300,000 lost trees, including elder, pecan, poplar, black gum, Chickasaw plum, bald cypress, red buckeye, pawpaw, 10 varieties of oak, and bigleaf magnolia.

"My best, most fun activity was to walk in the woods, and now almost every tree is gone," says Anderson. "I can't even go walk where I used to because of all the dead trees. I've gained 20 pounds since the storm."

Blessed with long growing seasons and favorable weather, the region is expected to see much of the canopy regrow within 15 years, experts say.