Some 2,000 square kilometres (800 square miles) of US coastal mangrove habitat are concentrated in three states most threatened by the estimated 5,000 barrels-a-day of crude oozing into the Gulf: Louisiana, Texas and the southern tip of Florida.

"The oil will basically kill the trees," said Jerry Lorenz, a marine biologist and head of research at Audubon of Florida.

Found in both tropical and subtropical zones, mangrove swamps are a thick tangle of salt-tolerant trees and shrubs that thrive in brackish tidal waters.

Their signature maze of exposed roots serve as a nursery for many commercial and recreational fisheries species, including shrimp, spiny lobster, red fish and snook.

They are also the main nesting and foraging area for dozens of species of wading and other fish-eating birds.

Under pressure from upstream damming and development, mangrove forests along the Gulf Coast have diminished by nearly 30 percent in the past three decades, according to a world survey of mangroves published in 2007 by the Food and Agriculture Organisation.

The massive oil spill, experts say, could destroy much of what remains and severely disrupt the flora and fauna that depend on them.

"Fisheries will be wiped out," Lornenz said. "Birds will not be able to find food, and they will become coated with oil and unable to fly."

One of the biggest ecosystem services mangroves provide human communities is protection from storm surges and wind storms, an especially valuable asset along the hurricane-battered Gulf Coast.

An oil spill deadly to mangroves would not only destroy that buffer, it would also leave a huge stockpile of potentially lethal, hardwood projectiles in its wake.

The first line of defence against the oil is to prevent it from reaching these wetland areas.

Clean-up efforts so far have focused on containing the slick by laying more than 11 kilometres (seven miles) of boom, dropping chemical dispersant, and burning off parts of the slick.

"But for the mangroves, we have to be careful the methods used are not worse than the oil itself," said Gilles Bocquene, an ecotoxicologist at the French Research Institute for the Exploitation of the Sea.

Once the viscous crude does reach the seaside swamps -- which most experts say is a matter of "when" not "if" -- there are few options for removing the oil from the gnarled, slow-growing plants.

"The forests are so tightly bunched, exposed above the waterline, that people will not be able to get in there to wipe them down," Bocquene said. "Besides, there are literally hundreds of millions of trees."

One tactic may be to flush the oil using water under pressure and vacuuming the oil, according to a study on managing oil spills in mangrove ecosystems conducted in the mid-1990s by the Louisiana Environment Research Center at McNeese State University in Lake Charles.

It may also be possible to use natural materials such as matted, dried vegetation to absorb and hold oil and prevent it from settling into the oxygen-poor soil, where microbial degradation takes much longer, according to the study.

Christophe Rousseau of France's Centre for Documentation, Research and Experimentation on Accidental Water Pollution says that the only way to save some forests may be to carefully scrape away a thin surface layer of soil so that the plants can grow.

"But you have to be very, very careful not to push the oil under the surface or to take out the rhizomes, or rootstalks, just under the surface," he said.

And if the oil is abundant, rising and falling tides will simply add new coats of crude with every cycle.

The BP-leased Deepwater Horizon off-shore drilling rig sank on April 22, two days after an explosion that killed 11 workers.

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