New satellite imaging has revealed 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.