PLANT OF THE WEEK

Dr. T. Ombrello - UCC Biology Department

 

RED MANGROVE

 

Common name:             Red Mangrove  

 

Scientific name:            Rhizophora mangle

 

Explanation of scientific name:       

Rhizophora   -  from the Greek “rhizo” meaning root and “phora” meaning bear or carry, in reference to the numerous roots that arise from the stems of this plant.

mangle           -  from the Spanish “mangle,” which is from the word for this plant used by the Taino people of the Caribbean Islands.  

A first impression of the Red Mangrove is that it certainly is not an impressive tree.  Its tangle of branches with multitudes of stilted prop roots makes it appear more like a grotesque shrub than anything else.  Growing in odorous muck, surrounded by stinging and biting insects, it is no wonder that until recently the Red Mangrove did not receive much attention or respect.  Today we know that it plays a vital role in the lives of most creatures that inhabit the tropical coasts lining the Atlantic and Pacific Oceans.

Mangroves growing along the shoreline of a bay on St. John, U.S. Virgin Islands.

The Red Mangrove has an extensive range, and is found in the estuaries and lagoons along the west coast of Africa, on the other side of the Atlantic from northern Brazil through Central America to the southern coasts of Florida, and on the Pacific coast from Ecuador to Baja California.  It thrives in a habitat where few trees can even survive, and is often found growing in pure stands with little or no vegetative competition.  In contrast to most trees, the Red Mangrove grows in sea or brackish water.  It does not need land, instead it roots into the soft muck in shallow water.  In order to survive, the tree has adaptations that include: 

bulletthick waxy green leaves that lose little precious fresh water to evaporation,
bulleta multitude of aerial and submerged prop roots to anchor the plant in place and collect water-borne silt and debris to build soil beneath it,
bulletspecial provisions to transport oxygen from the aerial roots to the roots  submerged and embedded in the anaerobic muck below, and
bulleta truly amazing way of dispersing its offspring such that they end up in a suitable environment to grow.

The Red Mangrove bears viviparous seedlings.  This means that the seeds germinate and begin to grow before the fruit is dropped from the tree.  Upon maturity, the single seeded fruits sprout, grow 6 – 12 inches and appear as cylindrical pods.  But they really are seedlings that consist mostly of a long tap root topped by a small bud.  These unique seedlings are called propagules.

If a propagule falls from the tree into shallow water, it will spear itself into the muck and grow there.  If it is high tide or the water is naturally deeper the propagule will float horizontally, carried by the currents for upwards of a year, potentially over hundreds of miles.  With time the root tip absorbs water, and pulls the propagule vertical.  When it reaches shallow water it will root in.  Brackish water will be absorbed faster than seawater, so the propagule is more likely to tip vertical near shore where the seedling must lodge if it is to survive.  The stilted roots coupled with their unusual mechanism of dispersal give the appearance that the Red Mangroves are walking on water and advancing seaward over time.

Native peoples have made use of the Red Mangrove throughout its native habitat.  Its dark red, fine grained and hard wood is used for charcoal.  It is seldom used for lumber because the species does not produce tall single trunks.  In fact, the largest specimen in the United States, found on Sanibel Island, Florida, is only 40 feet high with a 29-foot spread and a trunk circumference of 3 feet 5 inches.  Tannins extracted from the bark and leaves have been used in tanning hides.  The tannins are so abundant that they naturally stain the water a rust brown near large groves of the species.  A red dye from the young shoots was important in the past, but today is of little commercial value.

The Red Mangrove’s greatest significance involves its role in nature.  Coastlines fringed with Red Mangroves are quite stable, and protected against the ravages of erosion and storm damage.  Long-time Florida coast inhabitants claim that the safest place to be during a hurricane is in a stand of Red Mangroves.  The state of Florida, concerned about the future of its coastline threatened by extensive development, has enacted legislation making it illegal to cut or destroy Red Mangroves without appropriate approval.

While it has long been recognized that the coastal areas play an essential role in the food webs of organisms that not only spend their entire lives along the coast but also well beyond, it has just recently been recognized how central the Red Mangrove is to these food webs.  Red Mangroves shed over 3 tons of leaves per acre each year.  As the leaves decompose, they provide food for crabs, worms, shrimp and others that make up the lower links of a food web.  These little creatures are in turn fed on by larger and larger fish and eventually birds and mammals including us. Almost everything is connected back to the Red Mangrove.

It seems that the tree that nobody likes is probably more important than most people suspect.