Passage IV

NATURAL SCIENCE: This passage is adapted from the article “ Forests of the Tide” by Kennedy Warne (©2007 by National Geographic Society).

Mangroves live life on the edge. With one foot on land and one in the sea, these botanical amphibians occupy a zone of desiccating heat, choking mud and salt levels that would kill an ordinary plant within hours. Yet the forests mangroves form are among the most productive and biologically complex ecosystems on Earth. Birds roost in the canopy, shellfish attach themselves to the roots, and snakes and crocodiles come to hunt. Mangroves provide nursery grounds for fish; a food source for monkeys, deer, tree-climbing crabs, even kangaroos; and a nectar source for bats and honeybees.

As a group, mangroves can't be defined too closely. There are some 70 species from two dozen families—among them palm, hibiscus, holly, plumbago, and legumes. They range from prostrate shrubs to 200-foot-high timber trees. Though most prolific in Southeast Asia, where they are thought to have originated mangroves circle the globe. Most live within 30 degrees of the Equator, but a few hardy types have adapted to temperate climates and one lives as far from the tropical sun as New Zealand. Wherever they live, they share one thing in common: They're brilliant adapters. Each mangrove bas an ultrafiltration system to keep much of the salt out and a complex root system that allows it to survive in the intertidal zone. Some have snorkel-like roots called pneumatophores that stick out of the mud to help them take in air; others use prop roots Or buttresses to keep their trunks upright in the soft sediments at the tide's edge.

These plants are also landbuilders par excellence. The plants' interlocking roots stop riverborne sediments from coursing out to sea, and their trunks and branches serve as a palisade that diminishes the erosive power of waves.

Bangladesh has not lost sight of that logic, putting a great premium on the ability of mangroves to stabilize shores and trap sediments. A low-lying country with a long, vulnerable coastline, Bangladesh is also land starved, with a population density of 2,500 persons per square mile. By planting mangroves on delta sediments washed down from the Himalaya, it has gained over 300,000 acres of new land on the Bay of Bengal. The plantings are relatively new, but there have been mangroves here for as long as the Ganges, Brahmaputra, and Meghna Rivers have been draining into the bay. The vast tidal woodland they form is known as the Sundarbans—literally "beautiful forest." Today, it is the largest single tract of mangroves in the world.

For more than 25 years Jin Eong Ong, a retired professor of marine and coastal studies in Penang, Malaysia, has been exploring a less obvious mangrove contribution: What role might these forests play in climate change? Ong and his colleagues have been studying the carbon budget of mangroves the balance sheet that compares all the carbon inputs and outputs of the mangrove ecosystem and they've found that these forests arc highly effective carbon sinks. They absorb carbon dioxide, taking carbon out of circulation and reducing the amount of excess greenhouse gas in the atmosphere.

By measuring photosynthesis, sap flow, and other processes in the leaves of the forest canopy, Ong and his team and can tell how much carbon is assimilated into mangrove leaves, how much is stored in living trees and how much eventually makes its way into nearby waterways. The measurements suggest that mangroves may have the highest net processing of carbon of any natural ecosystem (about a hundred pounds per acre per day) and that as much as a third of this may be exported in the form of organic compounds to mudflats. Mangroves, it seems, are carbon factories, and their demolition robs the marine environment of a vital element.

Ong's team has also shown that a significant portion of the carbon ends up in forest sediments, remaining sequestered there for thousands of years. Conversion of a mangrove forest to a shrimp pond changes a carbon sink into a carbon source, liberating the accumulated carbon back into the atmosphere—but 50 times faster than it was sequestered.

If mangroves were to become recognized as carbon-storage assets, that could radically alter the way these forests are valued, says Ong.

"Take Indonesia, which has the largest total area of mangroves of any country in the world.It can't afford to save them for nothing" Ong says. "But if the Indonesians could trade the carbon-storage potential of their mangroves as a commodity, that would create great a incentive to stop bulldozing them for shrimp ponds or chipping them for the production of rayon."