Florida's Marvelous Mangroves

Too many native mangrove stands have been removed from the edges of Florida's waterways over decades of development, and as a result shorelines are more vulnerable to tropical storms and our native bird and fish populations are in steep decline. Mangroves growing in thickets along tropical and sub-tropical shorelines absorb the wave action from open waters, build new land as they slow down and hold onto passing sediment, and create fabulous habitat for many types of wildlife. Many types of birds inhabit mangrove thickets and some of them are endangered or have declining populations. Some examples are roseate spoonbills, limpkins, white ibis, herons, bitterns, anhingas, osprey, peregrine falcons, and bald eagles.

Mangroves are so important for the health of the shorelines that Florida has passed regulations to govern their treatment.  We mentioned mangroves last week as one of Florida's important water resources and habitats in We ALL Live in a Watershed!

Mangroves as far as you can see at Pennekamp State Park on Key Largo.

The term mangrove describes either a single species or the whole plant community that grows along saline shorelines. These plants are not related taxonomically, but are grouped by their ability to grow at the edge of the sea. There are 50 species worldwide, but only three mangrove species are native to Florida: red, black, and white, and because it so often grows in the same habitat, another native, green buttonwood, is often thought of as Florida's unofficial mangrove. The mangrove ecosystem ranges from shallow waters into the shore. Typically the red mangrove with its built-in stilt-like roots grows farthest out in the water, followed by black and then white as you move toward shore. The green buttonwood grows mostly on dry land, but can withstand periodic flooding.


Red mangrove propagules.



While mangrove species are salt tolerant, they can also grow in brackish or fresh water. Red and black mangroves filter out much of the salt with a waxy coating on the roots and they can absorb water through their bark. The white mangrove exudes salt through pores on its leaves and the coating of salt makes the leaves look white—hence the name.

One of the features that all mangroves share is that their seeds germinate while still attached to the parent tree—this is called viviparous germination or viviparity. Once germinated, the seed forms a propagule, a seedling that has roots, a stem, and leaves. When dropped, it can float away vertically, root-end downward, until it lands on a favorable site where it can quickly send out roots to anchor itself. The leaves and green stem are already capable of photosynthesis at the point of landing and can provide the energy necessary to fuel the quick root penetration. A propagule may float for many months with no ill effects.

There are three species of mangrove plus a frequent companion plant native to Florida.  All of them are desirable, salt-tolerant, wind-resistant, slow-growing evergreen trees:

 - Red mangrove: (Rhizophora mangle) This species grows farthest out in the water.  Tangles of prop roots act as stilts and increase stability as well as capturing sediments from the surrounding water.  The red mangroves are sometimes called the walking mangrove.  Hardy through USDA's planting zone 9.

 - Black mangrove: (Avicennia germinans) Occurs in waters shallower than the red mangroves and deeper than the white mangroves.  It grows pneumatophores, straw-like extensions from its roots, to supply needed oxygen to the roots.  Hardy through USDA's planting zone 9.

 - White mangrove: (Laguncularia racemosa) This species usually occurs on the landward side of the other two and has no particular root adaptations, but like the others, it handles the salt environment.  Hardy through USDA's planting zone 9b.

Buttonwoods are usually grouped with the mangroves, because they grow on the landward side of mangrove ecosystems, but it is not a true mangrove.

 - Buttonwood: (Conocarpus erectus) A small mangrove-like tree or shrub, it's limited to coastal areas.  It grows best in full sun or partial shade at the edges of fresh, brackish, or saltwater marshes.  It can withstand the rigors of urban settings and makes a durable street or parking lot tree.  Hardy through USDA's planting zone 9b.


Red mangroves have stilts for roots and occur
farthest out from the shoreline

 Establishing a Stand of Mangroves

If your property or property in your community abuts a body of water, consider mangroves for your waterfront instead of, or in front of, a bulkhead or other shoreline hardscape. They provide beauty and habitat, but maybe most important for waterfront property owners, mangroves protect the shoreline from wave-induced erosion and stormwater erosion from the land. Mangroves also absorb nutrients that may run off the land, which improves the overall water quality.



Mangroves are protected species in Florida and you are not to trim them back if they are over ten feet tall or collect them from the wild—not even a propagule that may float by. There are a number of sources online that service the aquarium trade, but be sure to purchase only the four Florida natives mentioned here and for the greatest chance of success, purchase propagules instead of seeds. Mangroves are usually available from nurseries that specialize in native plants.

Black mangroves and their  pneumatophores



Before you start planting, remove Australian pines or other invasive species that may be in the wet areas. Plant several individuals, even for the smallest shoreline, because one plant will not create the grove ecosystem that you are looking to create. Plant the red mangrove in the deepest water and the black mangrove closer to shore where the soil may be exposed some of the time. Neither of these plants will survive in a dry environment. Plant mangroves that will thrive in your climate.  For instance, in the Vero Beach area, the white mangrove and the buttonwood might survive, but they would be at the edge of their hardiness range. It's more sustainable to plant specimens that you know will thrive in your area.

Plan for the adult size of these trees, which may eventually reach 30 to 40 feet or more; so don't plant under low overhead wires, and leave at least three feet between them. Bury one third of the propagule in the mud. You sometimes see mangroves planted in a regular checkerboard pattern, and while this spacing will eventually be obscured as the mangroves mature, wouldn't it be better to plant them in a more random pattern so it looks like Mother Nature has arranged them?

If the planting site receives continuous wave action, you'll need to fashion some type of anchor and probably some type of protection to keep the new mangrove seedlings from being washed away or damaged by the flotsam and jetsam until they become established. Some folks use a cylinder of galvanized hardware cloth attached to a deeply driven stake to surround the seeding and keep it in place--others use a PVC pipe. This cylinder also protects young seedlings from raccoons, deer, and other munching critters. Plan to remove the cylinder later as the tree grows, but other than that, no particular care or fertilization is required.

At Blowing Rocks: White mangrove is in
foreground;  black mangrove is darker behind it.

Mangrove Restoration Projects

Blowing Rocks, a Nature Conservancy preserve, in Jupiter has ongoing mangrove restoration projects. Andrea Graves summarized them:

"We consider the mangrove and tidal creek restoration along the Lagoon Boardwalk to be very successful. This habitat contains all three species of mangroves found in Florida: red, black and white. After the initial restoration in the mid-80’s, many of the trees you see today moved in on their own and colonized the area, a process known as natural recruitment. Another sign of success is the large number of fiddler crabs that can be observed at low tide foraging in the detritus. The falling leaves from the mangroves decay and form the base of the food chain for the Indian River Lagoon. We often see wading birds such as white ibis or little blue herons, feeding on the fiddler crabs in these mangrove wetlands. In addition to their habitat value, mangroves help reduce shoreline erosion and improve water quality by filtering pollutants.

White mangrove leaf.
Note the indentation at the tip.

"We have also had success restoring red mangroves along our Lagoon Restoration Trail, south of the Hawley Education Center. Here, visitors can see restored red mangroves that started out as seedlings placed in white PVC pipe. The casing protects the young tree from waves caused by heavy boat traffic. Once the mangrove becomes established, the PVC material is carefully removed."

Buttonwood branch with fruits.  Buttonwood is not a
mangrove, but it often grows inland from mangrove forests. 

 

Mangroves are Pioneers

Because the propagules can float in the sea for so long, they can be the first plants to become established on a newly emerged sand bar. Over the years the mangroves form thickets of vegetation, then birds come to roost bringing more types of seeds. Meanwhile oyster spat cling to the mangrove roots, fish and crabs find habitat amongst the roots, and a whole new island ecosystem has formed. Without the mangrove as a pioneer, it would still be a sandbar that could erode away at any time.

Be a pioneer in your neighborhood and establish some wondrous mangroves along the shorelines. You might be rewarded with some roseate spoonbills nesting in the mangrove thickets and soaring bald eagles. Now wouldn't that improve your view??

A buttonwood tree with its peeling bark.
Buttonwoods are a good addition to
south Florida landscapes

References:

A Nature Convserancy article on the services mangrove forests provide: www.nature.org/magazine/summer2010
/features/art31631.html

Mangrove trimming regulations: www.dep.state.fl.us/southwest/erp/mangroves.htm

The Florida Museum of Natural History website provides more details on birds that inhabit mangrove thickets and other general information: www.flmnh.ufl.edu/fish/SouthFlorida/
mangrove/lifebirds.html.

Ginny Stibolt

photos by Ginny and Sue Dingwell