The death of the trees from wind damage and soaking in saltwater will ultimately release about 367 million tons of carbon dioxide as they decompose — about the same amount that is absorbed by all US forests in a year, according to the study published in the journal Science.

Considered on the vast scale of global climate change, Katrina’s impact is small. But as a one-time event, its infusion of carbon is significant, exceeding an entire season’s worth of emissions from US forest fires. Most of the lost trees in the Gulf region stood 70 to 100 feet tall, and others will not grow back for decades, if ever, experts said. Hurricane Katrina damaged 5 million acres of forests in August 2005, 80 per cent of them in Mississippi.

Biologist Jeffrey Q. Chambers of Tulane University and his colleagues said the deforested land, once covered with native species such as longleaf pine, oak and cypress, is being taken over by invasive species that are changing the ecology of the area. One of the most prolific, the Chinese tallow, oozes a milky, toxic sap that creates an inhospitable environment for insects, birds and small animals.

The trees that are growing back represent only a fraction of those destroyed, and they are surrounded by large quantities of decomposing lumber. Combined with the drought now punishing the South, that is the recipe for a devastating forest fire, said James L. Cummins, executive director of the Mississippi Fish and Wildlife Foundation.

Experts said they expected that most of the lost forestland would never be replanted. Most of the people who lost trees in the hurricane were tree farmers, and replacing the felled trees in the months after the storm was not a high priority.

Farming trees “is hardly ever the only thing a private landowner does,” said Amanda Box, a spokeswoman for the Mississippi Forestry Assn. in Jackson, Mississippi. “I may also have cattle, and I may be a schoolteacher.”

The federal government earmarked $504 million for tree replacement, but the programme typically provides only half the cost of replanting, along with a small stipend — as low as a net of $100 per acre in some areas — to keep the trees growing for a decade.

Owners struggling to rebuild their homes and businesses in the wake of Katrina weren’t able to make up the difference and many sold their land to people who used it for residential construction. Only about $70 million in government funds has been disbursed, and no more than 100,000 acres have been replanted, Cummins said.