New satellite imaging has revealed that hurricanes Katrina and Rita produced the largest single forestry disaster on record in the nation — an essentially unreported ecological catastrophe that killed or severely damaged about 320 million trees in Mississippi and Louisiana.

The die-off, caused initially by wind and later by weeks-long pooling of stagnant water, was so massive that researchers say it will add significantly to the global greenhouse gas buildup — ultimately putting as much carbon from dying vegetation into the air as the rest of the nation’s forest takes out in a year of photosynthesis.

In addition, the downing of so many trees has opened vast and sometimes fragile tracts to several aggressive and fast-growing exotic species that are already squeezing out far more environmentally productive native species.

Efforts to limit the damage have been handicapped by the ineffectiveness of a $504 million federal program to help Gulf Coast landowners replant and fight the invasive species. Congress appropriated the money in 2005 and added to it in 2007, but officials acknowledge that the program got off to a slow start and that only about $70 million has been promised or dispensed so far. Local advocates said onerous bureaucratic hurdles and low compensation rates are major reasons.

“This is the worst environmental disaster in the United States since the Exxon Valdez accident . . . and the greatest forest destruction in modern times,” said James Cummins, executive director of the conservation group Wildlife Mississippi and a board member of the Mississippi Forestry Commission. “It needs a really broad and aggressive response, and so far that just hasn’t happened.”

The U.S. Forest Service and Farm Service Agency have made estimates of the forest damage from the two 2005 hurricanes, but they have generally focused on economic losses — $2 billion, or 5.5 billion board feet, worth of timber.

The new assessment of tree damage comes from a study being published today in the journal Science, written primarily by researchers at Tulane University who studied images from two NASA satellites.

Lead author Jeffrey Q. Chambers said that to assess the damage, which occurred to a greater or lesser extent over an area the size of Maine, the team used a before-and-after method perfected by researchers who study logging in the Amazon River basin. The satellite images identified green vegetation before the storm, and wood, dead vegetation and surface litter after it. The team then visited the areas of greatest damage to make their overall assessment.

“I was amazed at the quantitative impact of the storm,” Chambers said. Of the 320 million trees harmed, he said, about two-thirds soon died. “I certainly didn’t expect that big an impact.”

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