Mangroves - Walking Trees

Mangroves

‘Walking Trees’

Jonathan Lowrie

A mangrove is a type of tree that grows in tropical regions at river mouths, bays, coastal lagoons and islands.  They occur in many regions through out the world; some of the best-studied mangrove forests are in the Florida ecosystem. In the Florida Keys, they create a fringing network around most islands and grow at hundreds of shallow locations offshore.  They are one of Florida’s true native species.  They thrive in their salty environment because they are able to obtain freshwater from saltwater.  Some of the species do this by excreting salt through their leaves; others block absorption of salt at their roots.

Mangrove roots act as a physical traps to filter the water systems. They trap debris and silt, stabilizing the near shore environment, and clarifying adjacent waters, and facilitate photosynthesis in other marine plants.  They also provide an attachment substrate for various marine organisms.  Many of these attached organisms filter water through their bodies and, in turn trap and cycle nutrients. Sponges, barnacles, oysters, mussels, shrimps and oysters are all efficient filter feeders that attach to mangrove root systems.  The Florida ecosystem has an estimated 470,000 acres of mangrove forests, and they all contribute to the purification of the state’s water quality.  This ecosystem traps and cycles various organic nutrients, chemical elements, and acts as a nutrient sink for important nutrients. Mangroves shed and drop about 7.5 tons of leaf litter per acre per year.  The constantly shed leaves are quickly broken down by bacteria and fungi and released into the water, providing food for sealife.

I cannot overemphasize the relationship between mangroves and their associated wildlife.  Mangroves provide a secure and safe haven for young fishes, crustaceans, and molluscs.  They also provide food for many marine species like snapper, damselfish, tarpon, and shrimp.  Without a healthy mangrove system, the vitality and health of the sport and commercial fisheries would decline.  74% of the game fish and 90% of the commercially valuable sealife in Florida depends on the mangrove.

Most animals find shelter in the roots, or the complex branch structures of mangroves.  The upper branches serve, as rookeries for coastal birds, like the brown pelican.  The roots also offer habitat for mammals, amphibians, reptiles, countless unique plants, and other invertebrate life. This root structure that penetrates into the water provides substrate for a variety of bivalves, and macro-algaes to attach.  This dense coverage provides shelter for juvenile invertebrates and fish.

Worldwide, more than 50 species of mangroves exist.  Of these, only three are found in Florida waters.  The best known is the Florida Mangrove, Rhizophora mangle.  It’s characterized by aerial roots and concealed prop roots, which provide support for soft muds and stabilize sediments. It typically grows along the water’s edge, where tidal flushing is sporadic and the water is nutrient poor.  The red mangrove is easily identified by the tangled mass of reddish roots called prop roots.  These projections from the trunk have earned this mangrove the name ‘walking mangrove’.  This tree can easily reach 30 feet in nature.

The Black Mangrove, Avicennia germanans, usually occupies higher elevations than the Red Mangrove.  They are characterized by the presence of small pencil-like vertical root shots called pneumatephores.  These root shoots stand in dense arrays near the high tide line, enabling the tree to get oxygen from the atmosphere.  The underside of the leaf surface has a whitish residue, which is excreted salt.  This will remain unless rinsed by a passing thundershower.

The White Mangrove, Laguncularia racemosa usually occupies the highest elevations farther upland than the red or black mangroves.  It grows on elevated grounds above the high tide mark.  Unlike its counterparts, it has no visible aerial root system, as the root system resembles that of most terrestrial trees.  The leaves are thick and succulent, rounded at both ends, and appears uniform in color on both sides. 

Many threats abound to mangrove habitats.  Hurricanes can damage 100,00 acres in a few hours, as did Hurricane Donna in 1960. Even the recent Hurricane Georges that swept across Puerto Rico and the keys was responsible for mangrove habitat damage. However, all of the storm damage cannot equal the impact humans have had on these forests.  Shoreline development has replaced Mangroves with marinas, dredged channels, airports, seawalls and commercial and residential construction.  Over 55% of shallow water mangroves were lost in the upper keys in past 15 years.  Forty percent of the loss was from filling of the habitat to make way for new construction.  This staggering loss occurs not just in the keys, but all over the Florida coastline.  Other threats include illegal dumping, oil spills, agricultural run-off that contains herbicides and pesticides.  Freshwater and street water runoff has also altered the salinity in some habitats causing mangrove die-backs.

Many organizations are studying habitat loss of the mangroves.  Looking at aerial photos of the same habitats from the 1940’s and 1950’s, scientists can see how pronounced the habitat destruction is.  In Florida, state and local laws were enacted to protect the mangroves.  Local laws vary, but most in municipalities it’s illegal to take any rooted plant, or disturb the trees or associated wildlife in any way.  The penalty includes heavy fines and possible jail time.

Mangroves are even now being kept in home aquariums.  They certainly can make a lovely habitat tank, allowing a touch of realisms for the shoreline tidal flat, or mangrove forest.  With the uptake of nutrients, they will certainly contribute to the overall vitality of the captive ecosystem, but because of slow growth, an uptake, they are not ‘miracle natural filters’.  The most common species for home aquariums is the Red Mangrove. It’s available as a propagule, and ships well, in moist bags.

This species does well in most seawater tanks, given a deep substrate to plant it in, and plenty of light.  I have had success with normal fluorescent bulbs, but have found compact fluorescent bulbs or metal halides to work best.  Salinity is less critical, as long as rapid fluctuations do not occur.  One way to jump-start these ‘seeds’ is to soak them in a separate container of water containing a small quantity of miracle Grow or similar fertilizer.  Its important to note that you should never put the fertilizer in your main aquarium display.  These propagules should be firmly embedded in the sediment, so that the bottom 2 to 3 inches is covered.  If you have a healthy propagule, then you should see the beginning of growth in just a few weeks.  I have also had some that take as long as six months before noticeable growth shows- so patience is vital.  Its best not to disturb the root system once it has begun to grow.

Before any mangroves are added to the tank, a few considerations must be made.  They are coastal vegetation, and are not found on the reef, but as an associated habitat.  One could never find mangroves, and small polyped corals grouped together.  Planting one in a typical reef, would only cause problems for the tree or the corals.  Mangroves are relatively slow growers, yet seem to expand by leaps and bounds when confined in a small aquarium.  Considering the overall height of 45 feet, they do grow slowly, but they can get over three feet tall in an aquarium in less than 6 months with adequate lighting.  The size of aquarium, and how much room you have to offer the tree plays a critical role in keeping mangroves.

I have some mangroves in a lagoonal system, complete with sea grasses, and some coastal corals, like Sidastrea, and Condylactus anemones.  I also have a system utilizing mangroves from Indo-Pacific and have the roots structured to create a land mass, where mudskippers, archer fish, can call home.  The possibilities for little captive microcosms are nearly endless.  Its best to get the mangroves as propagules with just a few roots starting.  If the plant is more established, it commonly fails from system shock.

Mangroves are a vital habitat we should all strive to preserve.  They most certainly add beauty and habitat to our home aquariums, and open up many new avenues of aquascaping.  If you ever have a chance to visit the coastal regions of Florida, I highly recommend a visit to a mangrove forest, as the abundance of wildlife makes this habitat one of the most diverse anywhere on earth.