Mangroves

Florida's Amazing Mangroves

Florida Department of Environmental Protection estimates that there are about 600,000 acres of mangroves in Florida. They are so important for protecting Florida's coastal ecosystems and provide habitat for a wide variety of marine organisms including many of our most important fish that there are regulations for their protection, but local laws vary. Be sure to check with officials in your area prior to taking any action, including trimming, to determine if it's allowed or if a permit is required.

This mangrove poster by the Wildlife Conservation Society shows just some of the many services that mangroves provide. Read on for more details...

Mangroves

According to NOAA, “Mangroves are a group of trees and shrubs that live in the coastal intertidal zone.” They are not necessarily related to each other botanically, but are defined by their ability to survive in saltwater and brackish water environments.

The term "mangrove" can describe either a single plant, a single species, or the whole plant community. Mangroves may not require salty environments for good growth, but they tend to grow in salty environments because of reduced competition from other plants. Salty environments are harsh for plants, but the mangroves use various energy-consuming processes, including extensive filtration and the exudation of salt through special pores to filter out the salt and take in enough usable freshwater for growth. Most mangroves have developed various methods to conserve their freshwater (like desert plants), such as extensive root systems, succulence, thick cuticles, and relatively low rates of water evaporation through pores on the leaves or bark. They may also absorb freshwater from rain through their bark, especially through the surfaces of their drop roots, which are roots that arise from the branches and are usually above mean high tide.

Some mangrove species have roots that exclude most salt using a combination of membranes and reverse osmosis.

The surface of a white mangrove leaf showing the salt deposits, which is why its common name is "white."

Mangrove Reproduction

Most seed-producing plants (both angiosperms and gymnosperms) develop seeds, which are then separated from the parent plant one way or another in a dormant state. When properly stimulated, usually with moisture and contact with soil, the seeds germinate and begin to grow into a new plant. If the seeds do not end up in the right environment, they do not germinate. Some seeds can remain viable for some time, but most do not.

Mangroves are viviparous, which means their seeds germinate while still attached to the parent plant and seedlings are housed in structures called propagules, which can float. When dropped, the seedlings drift away with the tides. Many can float for weeks, but then can quickly establish themselves—because energy is already available from the photosynthesizing seedling in the propagule. (They have already germinated.) This strategy is an important adaptation for their salty, liquid environment and greatly increases the viability of the seeds.

Unlike most woody plants, mangroves don't have obvious annual rings because they grow where there's  a constant water supply and relatively consistent temperatures. Also, they do not spread vegetatively, so they do not send up suckers from the roots that could become another whole tree. In addition, if a branch is bent over and touches the mud, it won't take root there. This means that people wishing to plant mangroves must acquire separate seedlings for each tree: they cannot take cuttings from branches and root them, which is standard practice used to start new plants for most trees and shrubs.

Red mangrove propagules are actual seedlings since they have sprouted while attached to the parent plant.

Rooted mangrove seedlings have spaced themselves  out on the mud flats.

Florida's 3 Mangrove Species

While there are many mangrove species around the world, only three mangrove species are native to Florida: red, black, and white, and because it often grows in the same habitat, green buttonwood, is often thought of as a mangrove associate. When all of these mangrove species, along with the buttonwood, grow along the same shoreline, each tends to grow in a different area. This zonation is determined by tidal flows, elevation of the land, plus the salinity of the soil and water. Mangroves are tropical and subtropical, and so have been mostly in South Florida and Central Florida, but in the last decade or so, the red and black mangroves have been moving northward and often start on the waterside edges of established salt marshes. 

Red mangrove: (Rhizophora mangle) is in the Rhizophoraceae family. When the three mangrove species grow together, this species usually grows farthest out in the water. Tangles of prop roots, which emerge from the trunk, and drop roots, which emerge from branches, act as stilts and increase stability. Since the prop roots disturb and slow down the water flow, they facilitate the capturing of sediments from the water. The red mangrove propagules are six to ten inches long and look like an elongated seed pod, but it is a plant and is capable of rooting quickly when it lands in a suitable location. They are native to most of Florida’s coastal areas.

Black mangrove: (Avicennia germinans) is in the Acanthaceae family. It tends to grow in waters shallower than the red mangroves and deeper than the white mangroves. It produces pneumatophores, straw-like extensions from its roots, to supply needed oxygen and gas exchange to the roots. The propagules contain a fruit with a germinated seed inside and are about the size of a lima bean, but when they land, four layers split open and the fuzzy root emerges first. Black mangroves are found in most of Florida’s coastal areas. Its range somewhat more extensive and contiguous than the red mangrove.

White mangrove: (Laguncularia racemosa) is in the Combretaceae family. This species is less cold tolerant and has not migrated as far north as the black and red mangroves. When growing with other mangrove species, the white mangrove usually occurs on the landward side of the other two and sometimes has root adaptations similar to the black mangrove, but these are less extensive. Like the others, it handles the salt environment, but one of its adaptations is that it exudes salt through its leaf pores, so the leaves often look white. The propagules are small vase-shaped fruits with a sprouted seed inside, which is ready to grow roots quickly when it lands.

Buttonwood (Conocarpus erectus) is sometimes grouped with the mangroves, because it naturally occurs on the landward side of mangrove ecosystems in South Florida, and because they are in the same family as the white mangrove. It’s not a true mangrove because it's not viviparous and it grows further upland.

Red mangrove (Rhizophora mangle) stilt roots hold the plant above the sea bottom. 

This illustration from "Living Shorelines for Florida" shows how the 3 mangrove species usually arrange themselves on the shorelines with the red mangroves occurring farthest into the water and the white mangroves closest to the shore.

The John Pennekamp State Park on Key Largo includes almost 3,000 acres of mangrove forest. https://pennekamppark.com/

The massive root system of this black mangrove has been revealed by shoreline erosion.

Mangroves and Carbon Sequestration

The presence of the mangroves' multiple trunks and complex root systems in the shallows slows down water from the sea and from the shore so it drops sand, dirt, & whatever else it was carrying under and around these trees. The trees also slow down the coastal winds so that the air also drops whatever it was carrying. Wind drops sand when it is slowed to less than 13 mph. This process of removing sediment from the water improves the local water quality for other marine ecosystems where silt carried in the water is a problem for organisms. This is especially important for coral reefs and seagrass meadows.

Illustration from "Living Shorelines for Florida"
showing how the mangroves slow down water
& wind, which causes deposits of sand or
whatever the wind or water were carrying.
The mangroves also provide critical habitat.

In addition, the mangroves create habitat for many types of wildlife including reptiles, amphibians, birds, fish, and a wide assortment of invertebrates, including crustaceans. All of these organisms also leave their waste as they live and die, which also adds to the soil buildup under mangroves.

All of this buildup of sand and waste from plants and animals forms a type of peat soil because it's under water which is an oxygen-poor environment. In normal, oxygen-rich upland soils, there are microbes, worms, insect larvae and other organisms that make their living by decomposing the soil. When there is no oxygen, the soil organisms can't survive, so the soil continues to build up and the carbon from the sugars in the plant and animal waste is sequestered for the long term.

Quick review: Green plants take in carbon dioxide & water and produce sugar & oxygen through photosynthesis. Organisms that consume plants also contain carbon.

As shown in the graphic below, scientists analyzed a 30-foot core of soil under a mangrove forest to the rock layer and found that the soil at the bottom was more than 7,000 years old. This means that the carbon dioxide was removed from the air by plants and stored away in the soil for the long term. Since this happens under water, mangrove ecosystems are included as part of the Coastal Blue Carbon system. (See diagram below.)

Because of their complex root systems, mangroves provide protected habitat for many types of both marine and terrestrial wildlife. Also, their complex structure reduces water current, so sand & other materials settle
on the bottom.

Analysis of a 30-foot core of the peat soil under a mangrove forest in the Yucatan Peninsula showed that the soil at the bottom
was more than 7,000 years old.

Blue carbon is a term used to describe the removal of carbon dioxide from the atmosphere by the oceanic and coastal ecosystems. The plants involved in this carbon sequestration include various types of algae, phytoplankton, seagrasses, and mangroves, and other plants growing in salt marshes and coastal wetlands. This carbon is sequestered below sea level, even for the coastal ecosystems. Globally, these plants remove up to 83% of the carbon dioxide from the air and produce up to 80% of the world's oxygen. Coastal habitats cover less than two percent of the total ocean area, but they account for approximately half of the total carbon sequestered in sediments. Seagrass meadows, salt marshes, and mangrove forests are sometimes referred to as "blue forests" in contrast to upland-based "green forests."

Carbon sequestration along the coastal areas is mostly under water and so is referred to as "Blue Carbon." Diagram by NOAA Pacific Marine Environmental Laboratory.

Mangroves & Living Shorelines

Living shorelines are nature-based methods of coastal management and shoreline stabilization that prioritize natural coastal solutions including native coastal vegetation and possibly oyster reefs, and while reducing wave energy with partial barriers, the water is part of the solution bringing sand, silt, and water-based flora & fauna into the area. This is opposed to seawalls, riprap, and other hard coastal armoring structures.

In tropical & subtropical areas, mangroves are widely used in living shorelines since they grow well in salt & brackish waters and are so effective in damping wave action, storm surges, tidal flows, and various other currents. 

My photos of the Snook Islands living shoreline project illustrates how red mangroves were used in this Intracoastal Waterway project in Palm Beach County and also shows a couple of configurations of living shorelines. We included these and more in our peer-reviewed book, "Living Shorelines for Florida," to help illustrate the many different arrangements that living shorelines can take. 

Red mangroves planted on an artificial sand dune as part of a living shoreline that's protected by a stone wall.

A C-shaped wall was built to protect this living shoreline installation.

"Living Shorelines for Florida:
A Practical Guide for Building
Coastal Resilience"
Click here for more info &
to order it from our publisher.

Our peer-reviewed book, "Living Shorelines for Florida: A Practical Guide for Building Coastal Resilience," is a comprehensive resource on ecologically informed shoreline management and its application in Florida published by University Press of Florida and written by:
- Savanna Barry, a regional specialized extension agent With Florida Sea Grant and the university Of Florida Institute Of Food and Agricultural Sciences at the Nature Coast Biological Station.
- Vincent Encomio, a county extension agent With Florida Sea Grant and the University Of Florida Institute Of Food and Agricultural Sciences.
- Marjorie Shropshire, a visual artist whose work is deeply concerned with the conservation of Florida's natural areas.
- Ginny Stibolt, a botanist, naturalist, and award-winning author or coauthor of six books.

As part of their Sea Grant activities, both Savanna and Vincent run workshops to teach contractors how to build living shorelines. Since they used much of the curriculum from that course in writing the book, they are now working with Sea Grant to adopt our book as the text for that course on a state-wide basis. 

The whole idea for this book came from Marjorie and her illustrations throughout the book make the various concepts easier to visualize and understand. As a botanist & writer, I brought in practical details on shoreline-friendly gardening practices.

Working together as four authors was a challenge and its own learning experience. We all learned from each other and from the other resources we found on various topics. But I think the end product is certainly worth all the effort in making this book both readable and scientifically accurate. 

Green Gardening Matters
Ginny Stibolt