WASHINGTON — New satellite imaging has shown that Hurricanes Katrina and Rita produced the largest single forestry disaster on record in America — an essentially unreported ecological catastrophe that killed or severely damaged some 320 million trees in Mississippi and Louisiana.

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

In addition, the downing of so many trees has opened vast and sometimes fragile tracts of land 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 Gulf Coast land owners to replant and fight the invasive species. Congress appropriated the money in 2005 and added to it in 2007, but officials involved with the emergency conservation program say that only about $70 million has been processed or dispensed so far. Local advocates say onerous bureaucratic hurdles and low compensation rates are major reasons why.

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

Bengt “Skip” Hyberg, a U.S. Department of Agriculture Farm Service Agency economist and policy analyst, said changes were made in the program this year to make it more attractive to landowners.

The U.S. Forest Service and Farm Service Agency had made estimates of the forest damage from 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 trees killed or severely damaged comes from a study to be released Friday in the journal Science, written primarily by researchers at Tulane University in New Orleans who studied images from two NASA satellites.

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

“I was amazed at the quantitative impact of the storm,” Mr. Chambers said. Of the 320 million trees harmed, he said, about two-thirds soon died. “I certainly didn’t expect that big an impact.” Mr. Chambers was even more surprised when his team calculated the amount of carbon that will be released into the atmosphere as the trees and other storm-damaged vegetation decomposes. The total came out to be about 1.1 billion tons, which is equal to the amount of carbon that all the trees in America capture and take out of the atmosphere in a year.

Much of the forest damage occurred in Mississippi on land often owned in small parcels by individuals. Larry Payne, director for cooperative forestry for the Forest Service, said the congressional effort to begin restoring the forest was largely aimed at helping out those small landowners, who often used their timber land as a bank account. The program was created as an emergency add-on to the popular federal Conservation Reserve Program, which pays landowners “rent” for returning marginal or environmentally sensitive land to more natural conditions.