Walls of mud and roots

At low tide the shore near Dam Doi is a grey flat of mud, pierced by thousands of roots that stand up like pencils. Mr Hai, a shrimp farmer in his fifties, walks out along a line of bamboo fencing to check the young trees planted behind it. Eight years ago the sea took his front pond in a single winter storm. "The dyke was concrete and the waves simply walked over it," he says. "Then the dyke went, and the pond went with it."

Mangroves are salt-tolerant trees whose tangled roots slow incoming waves and trap silt, so engineers increasingly describe them as natural sea defences. Studies elsewhere in Asia suggest that a belt a few hundred metres wide can take a great deal of force out of a wave. Vietnam once had far more of them. Chemical defoliants sprayed during the war destroyed large areas, among them the forests of Can Gio near Ho Chi Minh City. Those were replanted by hand from the late 1970s and later became a UNESCO biosphere reserve. Afterwards, shrimp ponds claimed much of what was left.

The cost of that trade is now plain to see. In places the Mekong Delta is sinking because groundwater is pumped out faster than it is replaced, and dams upstream hold back the sediment that once fed the coast. Together these have left a large share of the shoreline eroding, in a few spots by tens of metres a year. Farmers like Mr Hai are caught between the sea in front of them and the salt water creeping into the canals behind. Rebuilding the old green belt is no longer a question of conservation alone. It is a question of whether people can stay.

Provincial programmes now pay for soft engineering: bamboo fences to hold the mud, then seedlings behind them. Success has been mixed. Young trees are fragile, and planting in the wrong zone or in water that is too deep can fail within a season. The approach that appeals most to farmers pairs the forest with the pond. Under these schemes, farmers keep mangrove on much of their land and raise shrimp among the trees. They give up some yield in return for natural feed, fewer diseases and a better price from buyers who want certified shrimp.

Mr Hai switched to this model five years ago. His harvest per hectare is lower than it used to be, he admits, and the first two years were lean. Yet he no longer lies awake during the stormy months, and his fence has collected nearly a metre of new mud. Whether such schemes can spread fast enough is another matter. Most farmers cannot afford to wait for trees to mature, and the land will keep sinking unless pumping is brought under control. "You cannot plant your way out of everything," says an engineer who works on the coast. "But you can stop making it worse."