Mangroves protect coastlines, store carbon – and are expanding with climate change
Posted by admin on 9th February 2018
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With the help of technology, humans can traverse virtually every part of our planet’s surface. But animals and plants are less mobile. Most species can only live in zones where temperature and rain fall within specific ranges.

As regions become warmer due to climate change, plants and animals in those areas will either move to more appropriate climates or be replaced by newcomers who are well-suited to the new conditions. These changes are already occurring. For example, many plants, animals and birds in the Northern Hemisphere have shifted their ranges northward.

My research team studies mangroves – salt-tolerant trees with branches that intertwine like dense jungle gyms. Mangroves line the world’s coastlines and prefer warm temperatures, so they have traditionally been restricted to subtropical and tropical environments. But they have many features that have enabled them to survive major climate shifts in the past. Now, in a harbinger of climate change, mangroves are expanding from tropical zones into temperate areas. Scientists are finding them at higher and higher latitudes in North America, South America, Asia, Africa, Australia and Latin America.

Working with other ecologists in the shadow of the huge launch complex at Florida’s Kennedy Space Center, we have found that mangroves have increased in abundance by 70 percent in just seven years over an area of 220 square miles (567 square kilometers). This is a dramatic change in the plant community along this stretch of the Atlantic coast. Unlike many other impacts of climate change, we expect these shifting ranges to produce some benefits, including increased carbon storage and storm surge protection.

The world’s mangrove forests in 2000.
Giri et al., Journal of Biogeography (2008)., CC BY-SA

Traveling by water

Plants have less ability to move than animals, but some – particularly mangroves – can disperse via water over thousands of miles. Mangroves release reproductive structures called propagules, similar to seeds, which can produce new plants. They float and are distributed by ocean currents and, sometimes, big storms. As mangrove propagules drift north along the Atlantic coast, they are reaching areas where winter freeze events that could kill them are becoming less common due to climate change. Similar movements are occurring in other locations around the world.

In the Gulf of Mexico and Florida, mangroves are increasingly found in areas recently dominated by salt marshes, which typically occur in cooler zones. Using satellite images and land-based field studies in our ongoing study of mangroves, we can see this spread of mangroves occurring faster than we could have expected based on climate data alone.

Though these shifts also likely happened in the past due to hurricanes and freeze events, the recent changes in mangrove range are still dramatic. One study from Florida shows that mangroves from northern populations may be reproducing earlier than normal and producing bigger propagules, which could help them take over salt marshes.

Mangrove propagules drifting near Australia’s Great Barrier Reef.
Brian Gratwicke, CC BY

Stabilizing coastlines

In a recent modeling effort, we examined how mangroves protect NASA facilities at the Kennedy Space Center. We found that a 2-meter-wide strip of mangroves along the shore can reduce wave height by 90 percent. In contrast, it takes 20 meters of salt marsh habitat to reduce waves by the same amount. Other studies have found that mangrove forests helped reduce shoreline damage during the catastrophic 2004 Indian Ocean tsunami and Tropical Storm Wilma in Belize in 2005.

Mangroves around the world have been severely reduced by human activities, particularly clearance for aquaculture. Scientists estimate that at least 35 percent of global mangrove habitat was lost between 1980 and 2000. One recent estimate suggests that mangrove deforestation rates in recent decades have been three to five times faster than other forests around the globe.

All types of coastal wetlands help to prevent millions of dollars in damage from flooding and save hundreds of hours of labor to repair storm damage. Mangrove restoration efforts are ongoing in many parts of the world, including the Tampa Bay estuary and southern China, but some projects have been major failures. To succeed, these initiatives need to consider mangrove habitat needs, particularly hydrology.

Though mangroves may protect coastlines even more effectively than salt marshes, it is important to note that marsh plants provide important habitats for numerous species of birds and fish. We don’t yet know how these animals will fare as mangroves replace marshes, nor do we yet understand other downsides of plant range shifts due to climate change.

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