Musings on Mangroves

Barnacles and oysters growing on the prop roots of a red mangrove (Rhizophora mangle) in the Indian River Lagoon.

A really cool thing happened as we were heading south in the Atlantic Intracoastal Waterway, down the coast of Florida: we started seeing mangrove trees scattered throughout the salt marsh.  Maybe that’s not as cool as spotting Tom Brady or Lady Gaga, but they’re busy people who don’t have time to just hang out on the banks of the ICW. (Well, maybe Tom has some free time now that he has re-retired.)  But mangroves are super cool and they, too, are busy…busy marching northward, that is.  

Let’s start with a little background.  What are mangroves and why are they so cool?  Well, first, they are trees that live in seawater.  As if that wasn’t cool enough, they’re also ecosystem engineers, meaning they create habitat for other species: mangrove forests.  It’s a forest community in the intertidal zone of the ocean.  Mangrove forests form in protected bays and estuaries in the tropics and subtropics.  In many ways, mangrove forests are the ecological equivalents to salt marshes found in temperate regions.  Lots of marine species, including many that humans like to catch and eat, depend on mangrove forests.  The iconic prop roots of red mangroves provide a solid surface for the attachment of sessile invertebrates such as oysters, bay scallops, and barnacles.  The tangled, 3-dimensional maze of prop roots also shelters countless fish, including prized sport fish such as mangrove snappers, snook, sheepshead, and juvenile goliath grouper. The canopy of the mangrove forest teems with insects, seabirds, shorebirds, and songbirds.  You can even find land mammals, such as raccoons and bobcats, scampering among the branches. 

In addition to providing habitat for a myriad of critters, mangroves also protect human habitat. Their roots trap sediments and stabilize shorelines, helping to protect coastal towns by preventing erosion and buffering the shore from storm-driven waves and floodwaters.  

Mangrove forests, along with salt marshes, seagrass beds, and freshwater wetlands, are also really good at capturing and storing climate-warming carbon dioxide.  Like all plants, mangrove trees “suck” carbon dioxide out of the atmosphere.  They use that CO2, along with sunlight and water, to produce their own food (sugar) via the process of photosynthesis.  A lot of that food that they create is used to build their tissues, such as roots, branches, bark, and leaves, just like how your body will take that ham & cheese sandwich you ate for lunch and turn it into muscle and bone.  Mangroves are very long-lived and their tissues (i.e., wood) are very stable. So once that carbon-containing tissue is built, it takes that carbon out of circulation for decades or centuries, meaning that it won’t be available to heat up the atmosphere for a long time.  But that’s not all, not by a long shot.  A lot of that root tissue that they build is underground in the intertidal mud.  Decomposition in the intertidal sediments occurs very slowly. So when a mangrove tree eventually dies, its roots can remain intact underground for thousands of years.  All that carbon that the tree sucked out of the atmosphere and used to grow its roots, remains locked away for millenia.  Mangrove forests, seagrass beds, salt marshes, and freshwater wetlands are all really good at capturing and storing carbon.  In fact they can store up to five times more carbon than can tropical rainforests.  Because the oceans and coastal wetlands are so good at capturing carbon, that carbon has been given a special name: “blue carbon.”  

All of these ecosystem services provided by mangroves are valuable to us humans. Worldwide, the annual economic value of all these services is estimated to be worth approximately $1.6 trillion.  That's some serious dough, even to the folks on Wall Street. And we get that $1.6 trillion in services for free! All we have to do is keep them from being destroyed and we'll keep reaping those free benefits.  So it’s really in all of our interests to protect these trees of the sea.

Now, the intertidal zone is a challenging place for plants to grow; they need special adaptations to survive there.  If you don’t believe us, try this little experiment: water your house plants with seawater and see how long they last.  (Legal disclaimer: The authors of this blog cannot be held responsible for the injury or death of any plants.)  To survive in the ocean’s intertidal zone, plants need to be able to withstand inundation in salt water and their roots need to deal with soil that is devoid of oxygen (the bacteria that decompose organic material in intertidal mud use up all of the available oxygen). Because few tropical plant species besides mangroves can tolerate these conditions, the mangroves thrive there with little competition from other species. The thing that limits their distribution and prevents them from taking over the coastlines of the entire world is temperature.  They can’t stand the cold.  Similar to mangroves, salt marsh plants, such as cordgrass, can also grow in the intertidal zone and be submerged frequently in saltwater.  In addition, the marsh plants can also withstand freezing.  So salt marsh ecosystems are what you find in colder climates.  Cordgrass and other salt marsh plants can grow in the tropics but they get out-competed for sunlight and space by mangroves that grow taller and develop a dense canopy that intercepts the sunlight before it reaches the grasses.  In these tropical regions, the marsh grasses suffer in the shadows of the mangroves and eventually die out. 

There are about 80 species of mangrove trees (and bushes) found throughout the tropical latitudes of the world.  Four species occur in Florida: black mangrove, red mangrove, white mangrove, and buttonwood.  These four species differ from one another regarding their tolerances for cold weather, inundation in saltwater, and exposure to oxygen-depleted soils (intertidal mud).  Red mangrove is the species with those prop roots, and is the most tolerant to inundation by seawater, growing in the water, right at the transition between the subtidal and intertidal zones. Those prop roots keep the rest of the tree suspended above the water like a circus performer on stilts.  Black mangroves are slightly less tolerant of inundation, so they tend to grow at slightly higher elevation than red mangroves (but still within the intertidal zone).  Black mangroves can be recognized by the pencil-shaped projections called pneumatophores that poke up above soil from their roots. These pneumatophores are sort of like snorkels, bringing oxygen from the atmosphere to the tree’s roots.  Among Florida’s mangrove species, the black mangrove is the most cold-tolerant and they tend to be the most common species in North Florida.  White mangroves and buttonwoods grow at even higher elevation (further inland, at the transition between the upper intertidal and upland) than either red or black mangroves, and are the most sensitive to cold temperatures, being restricted to South Florida. 

Red mangroves with their prop roots.

Neumatophores of black mangroves (Avicennia germinans), growing upwards from their roots.  These "snorkel-like" projections transport atmospheric oxygen to the roots.

 

Foliage of a black mangrove.

Climate change is allowing mangroves to expand their ranges northward.  Historically, St. Augustine (at 30 deg. N latitude) has been recognized as the northern limit of mangroves on Florida’s east coast.  Most of the mangroves in the St. Augustine area are black mangroves and they form small patches widely scattered within the salt marsh.  They aren’t able to completely displace the marsh plants because, every few years, a severe cold snap kills off some of the trees.  But between 1984 and 2011, the intertidal land area occupied by mangroves doubled in size in the St. Augustine area (and the area covered by salt marsh declined by the same amount).  Recently, mangroves have been discovered to be well-established at Fort George Island, in the Timucuan Ecological and Historic Preserve, north of Jacksonville (30 deg 25 min N, not that far from the Georgia border).  From the age of these particular trees, they must have germinated in the spring of 2018.  Surprisingly, these northernmost mangroves are not the relatively cold-tolerant black mangroves, but instead are red mangroves.  This is probably because red mangroves have greater seed dispersal than do black mangroves.  The seed pods of black mangroves actually germinate while still attached to the adult plant.  This improves the likelihood of the seedlings surviving, but keeps those offspring living close to their parents.  Red mangroves, on the other hand, produce large, buoyant seed pods that look like overgrown green beans.  Because they drop into the water and float, they can drift with the currents and put down roots far from their parents.  Storms can carry these red mangrove seed pods very long distances.  In the fall of 2017, storm surge and waves driven by Hurricane Irma spread red mangrove seed pods far and wide, and probably was responsible for bringing those pioneer trees to Fort George Island (After Hurricane Irma, we even found some red mangrove seed pods on Sapelo Island, GA).  Newly-germinated seedlings are most vulnerable to cold weather.  Having survived five years, it is possible that the red mangroves are on Fort George for good, and we can expect black mangroves to follow. 

On our trip south, we first started noticing mangrove trees just a few miles north of St. Augustine. By the time we got to Daytona Beach, mangroves were the dominant coastal vegetation and salt marshes were hard to find.

Black mangrove trees growing in the salt marsh at St. Augustine, FL.  These mangroves are shorter and sparser than what you find in South Florida.

Same scene, closer up.

The northward march of mangroves isn’t just the result of increasing average temperatures.  More specifically, it is driven by a decrease in the number of extremely cold days.  It seems that for mangroves, -4°C is a magic threshold (about 2°C lower than the freezing point of seawater).  If it gets colder than that, mangrove trees, especially the tender young ones, begin to die. 

Climate change is reducing the occurrence of these extreme freezing events, making formerly marginal habitat at the northern limit of the mangroves’ range more mangrove-friendly.  Therefore, more mangrove seeds are taking root in salt marshes, surviving to adulthood, and replacing marsh with forest.

The implications of the mangroves’ northward march are not fully clear.  Surely, large ecological changes are in store for the coast of northeast Florida, and probably Georgia, too, over the coming decades as mangrove forests replace salt marshes.  It is also clear that both salt marsh and mangrove habitats are under threat.  Since the 1980s, a startling 35 percent of the world’s mangrove forests have been destroyed by coastal development, such as by the construction of coastal resorts and shrimp aquaculture facilities. The combined effects of rising sea level and coastal development constraining inland migration of the trees threatens another 10-20% of the world’s mangrove forests by 2100. 

As ecologists, it is fascinating to witness the northward spread of mangroves and their replacement of salt marshes.  But the forces driving this northward expansion of mangroves also threaten the existence of both mangroves and salt marshes due to sea level rise.  At the same time, the ability of these coastal wetland habitats to capture and store atmospheric carbon dioxide is unparalleled, and needs to be part of the solution to climate change.  We neglect these natural systems at our own peril.

One of the many threats to mangroves.  Because of the valuable services they provide, mangroves have been given strong legal protections in Florida.  However, it seems pretty easy to obtain exemptions from these laws and it is not that uncommon to hear stories of land owners who simply pay the fines so that they can trim or remove mangroves.  In this case, the mangroves along one stretch of a sea wall were trimmed...poorly.  Most of these trimmed trees appeared to be dead.  

Another threat to mangroves: seawalls, bulkheads, concrete, and pavement.  Mangroves grow in a very narrow range of water depths.  So if they are going to survive the rising level of the ocean and continue providing ecosystem services, they have to be able to spread inland.  But concrete seawalls, sidewalks, roads, and buildings prevent this landward "migration."  And concrete bulkheads like the one shown above do not provide the kind of complex three-dimensional habitat found in a mangrove forest and, therefore, support a less diverse, less productive community of organisms.