Forests between the Tides - Life in a Tangled Mangal

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The mangal, as it is called, is the tangle of roots that makes up the mangrove forest biome. There, trees with twisted limbs live in two worlds—one foot on land, the other in the sea. Mangals thrive in saline coastal sediment habitats in the tropics and subtropics.  Red, black and white mangrove trees, along with buttonwood, may all grow along the same shoreline.  When these species are found together, each stakes out a spot in the tidal zone.

Red mangroves are closest to the edge; their prop roots extend into the water from branches above.  The roots capture sediment, stabilising the shore.  Farther inland are black mangroves, their pneumatophores, or aerial roots, pointing upward from coastal soils.  Pneumatophores supply oxygen in anaerobic, or oxygen-free, sediments.  White mangroves, with no special root adaptations for water or sandy mud, are found in the interior mangrove forest, followed by buttonwood in the upland transition area.

Neither solely of land nor of sea, these forests of the tide collectively cover a worldwide area of 53,190 square kilometres in 118 nations—less than one percent of all tropical forests and less than 0.4 percent of the total global forest “estate”. Too often, scientists say mangals are the forgotten forests.  Now researchers are discovering that mangroves are important for many species from birds to fish, oysters to humans.  They are also finding new ways of tracking the health of the planet’s dwindling mangals.

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The challenge for mangroves, maintains conservation biologist Stuart Pimm of Duke University and the organisation, Saving Species, is that when we are not apathetic toward them, we are often downright hostile.  “Some of my neighbours in the Florida Keys build their homes at the sea’s edge, then cut down mangroves to a short hedge,” Pimm says.  “These people have no sense of how angry the ocean can become in a storm, or how well a mangrove forest could protect them.”

How do we begin to turn the tide for mangroves?  Perhaps by knowing how much mangrove habitat there is left - not an easy number to derive. Using a new technique, Chandra Giri of the U.S. Geological Survey (USGS) and Duke University may have the most accurate answer.  Together with his colleagues, Giri developed the first high-resolution, satellite-based global map of mangroves.  The results were published in a 2010 paper in the Journal of Global Ecology and Biogeography

The map suggests that mangroves cover some 12 percent fewer coastlines than reported in previous studies.  That is concerning, says Giri, when 35 percent of Earth’s mangrove ecosystems disappeared between 1980 and 2000. 

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The decline is mainly a result of agriculture, urban development, shrimp farming, climate change, and oil spills like the one in December 2014, in Bangladesh’s Sundarbans region.  Untold numbers of mangroves, home to rare species such as the Irrawaddy dolphin and Bengal tiger, were smothered by oil that spread over 350  square kilometres,. 

The Sundarbans mangrove forest, which lies along the delta of the Ganges, Brahmaputra and Meghna rivers at the Bay of Bengal, is one of the largest such forests in the world--some 140,000 hectares.

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Giri’s map shows that 75 percent of Earth’s mangrove forests are concentrated in just 15 countries.  Asia and Africa have the most mangroves, with 42 percent and 21 percent, respectively; 15 percent are in North and Central America; 12 percent in Pacific islands; and 11 percent in South America.

Before Giri’s effort, mangroves had not been mapped by global land cover projects.  The resolution was not fine enough across small geographic areas, and Landsat satellite mapping was expensive and time-consuming.  Then the USGS began to offer Landsat data free-of-charge.  “And computers have reached the point where we can now process large volumes of data,” says Giri.  “That opens the way to looking at mangroves from another perspective - from space.”

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Satellites offer images of Earth with several degrees of resolution, says Giri, similar to pixel sizes on a computer screen.  Previously, scientists had visualised global land cover in pixels equal to about one square kilometre, not enough detail to identify mangroves.  Giri’s technique used a finer-resolution: 30 square metres.  “That made it much easier to find mangroves in small patches,” he says.

Once scientists know where mangroves are, they can estimate mangroves’ value as bulwarks against sea-level rise, severe storms such as hurricanes, and tsunamis. As a next step in the research, Giri and his colleagues are looking at mangroves through the eyes of remote-sensing techniques such as lidar.  Lidar illuminates a target with a laser and analyses the reflected light to make measurements.  It may offer even more precise estimates of the extent of mangrove forests.

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Essay by Cheryl Lyn Dybas & Photographs by Michael AW ( OG 36)

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