By Dr. Ruscena Wiederholt
Mangroves are fairly mundane at first glance. They can range in size from small trees or shrubs to forests of trees exceeding 50 feet in height. Found worldwide, these trees are specially adapted to live in salty waters along tropical and subtropical coastlines. As a superhero, their “kryptonite” is cold weather. In fact, the global extent of coastal mangrove forests is limited by the frequency and severity of frost events. That is why you don’t see them around Boston Harbor or lining the fjords of Norway.
South Florida is fortunate. The Everglades supports over 550 square miles of mangroves, the largest mangrove forest in the continental U.S. We have 3 species here in Florida: the red mangrove (Rhizophora mangle), the black mangrove (Avicennia germinans), and the white mangrove (Laguncularia racemosa).
Red mangroves, nicknamed “walking trees” for their arching aerial prop roots, are likely a familiar sight. As a first line of defense, red mangroves typically grow along the shoreline with their characteristic roots providing stabilization. Black mangroves inhabit more inland and basin areas, and are known for their strange pencil-like root projections (called pneumatophores) protruding from the ground, which help their roots breathe like snorkels. White mangroves also produce pneumatophores, but can be found across a range of coastal habitats.
Plants of Steel
Mangroves are very tough, dodging every blow coming their way, or at least the equivalent, from a plant’s perspective. Mangroves survive in all sorts of precarious situations that would kill other plants. Living so close to the coast, mangroves are subjected to stresses like salt, flooding, anoxia, high temperatures, and even toxins like hydrogen sulfide. They have a number of “special powers,” or adaptations, that help them survive in these difficult conditions. Their prop roots and pneumatophores help them aerate the soil, creating a force-field that protects their roots in harsh environments. To deal with salt, red mangroves perform reverse osmosis at the root level to prevent salt from entering the plant. Others, like the black and white mangroves, excrete salt from the leaves, just as we mere mortals sweat out excess salt. Since they have limited access to freshwater, like desert plants, mangroves can minimize water loss through their leaves during the hottest times of the day.
For plants, mangroves have an unusual birth. Along the coast, you may have seen large green cigar-shaped pods floating in the water or collecting along the shore. These are the “propagules” of red mangroves. Much more developed than a seed, these propagules are living and growing while still attached to their parents. After falling into the water, they continue to develop as they are dispersed by the tides and currents. Mangrove propagules can survive a whole year before taking root in a suitable location.
Despite their presence along our coastline, mangroves can even grow in freshwater. You can collect propagules from the coast and grow them hydroponically in freshwater, or even in regular potting soil. Just keep them wet.
Guardians of the Coastline
Mangroves are our first line of defense in the face of tropical storms. Lining our coastlines, they slow the flow of water and lower surface waves, reducing the impact from storm surges. For instance, during Hurricane Irma, mangroves saved an estimated $1.5 billion in storm surge-related damage to properties in Florida, and spared an estimated 626,000 people from being flooded. On top of that, mangroves provide all sorts of other benefits like preventing shoreline erosion, providing critical habitat for a variety of wildlife, and improving water quality.
Mangroves adopt a new color fighting the insidious forces of climate change, a patriotic shade of blue. Blue carbon refers to carbon removed from the atmosphere by coastal ecosystems. The mangroves in Everglades National Park are highly productive and represent a large carbon sink. Consequently, they store a lot of carbon in their soils and vegetation. Disturbing this forest would be a bad idea, since this could lead to carbon emissions equivalent to burning ~190 to 230 billion pounds of coal. By increasing freshwater flows to the coast, Everglades restoration will improve the health of these forests and increase their carbon-capturing capacity.
During the worst of hurricane season, the Everglades has historically provided an essential “buffer” that slows storms’ intensity, providing protection for populated areas throughout South Florida. Mangroves close to the coast also help stabilize the shoreline, reducing flooding from storm surge and protecting wildlife habitats.
Everglades restoration will reinstate a freshwater flow to keep mangroves healthy so they can continue their many beneficial natural functions, including protecting Florida’s coastline.
Mangroves in Florida are threatened primarily by habitat loss and changes in freshwater flow. Globally, we’ve lost 20 to 35% of our mangroves since the 1980s, and we may be losing an additional 1 to 8% each year. Habitat fragmentation, pollution, and storms, as well as climate change and sea level rise pose a threat. For instance, Everglades mangroves have shifted inland since the 1960s from both sea level rise and reduced freshwater flow. However, as mangroves migrate landward to safety, another foe awaits.
Under the combination of sea level rise, low freshwater flow (because of water management) and saltwater intrusion, peat soil can break down and disappear faster than it can accumulate. If peat collapse occurs before mangroves are established, the affected area will transform to open water, not to the mangrove forest. Unfortunately, in the coastal Everglades, this process is already occurring.
EVERGLADES END GAME
If mangroves had a savior, it would be Everglades restoration. Restoration will help slow the impacts of sea level rise and saltwater intrusion around the coast, protecting habitats that are vulnerable to rapid loss from exposure to saltwater. The increased freshwater flow expected from Everglades restoration should also help improve mangrove habitat.
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