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Mysterious Mangroves

by Richard Garrigues

In the nebulous zone between high and low tide, where freshwater meets saltwater and the ground is neither liquid nor solid, grow the mangroves.

Inhabitants of inter-tidal zones throughout the tropics and subtropics, mangroves are a most curious collection of plants. It is easy to make the erroneous assumption that the different kinds of mangrove trees are closely related species adapted to the unique conditions in which they grow. Even their English common names (red mangrove, black mangrove, white mangrove, buttonwood mangrove, etc.) lend to the idea that these are merely different species of the same plant family, as if they were maples or oaks.

In reality, the mangroves are a wonderful example of convergent evolution—a situation in which totally unrelated organisms have evolved certain similarities simply because those are the characteristics best suited for making use of a particular resource. And in this case the resource is a place where a plant might grow if it can overcome the two major difficulties faced by mangroves: the salinity of the sea water which saturates the ground they grow in and the absence of oxygen in that same saturated mud.

In Costa Rica there are seven species of mangrove trees from four very different plant families: the red mangroves (Rhizophora mangle and R. harrisonii, Rhizophoraceae), the black mangroves (Avicennia germinans and A. bicolor, Verbenaceae), the tea mangrove (Pelliciera rhizophorae, Theaceae), the white mangrove (Laguncularia racemosa, Combretaceae), and the buttonwood mangrove (Conocarpus erectus, Combretaceae). In Costa Rica, any and all of these plants are called mangle (man-gley) and the association in which they grow a manglar.

Of the mangroves found in Costa Rica, the red mangrove is the most easily recognized with its striking aerial prop roots, which often branch one or more times before reaching the ground. The primary function of these roots is not to support the tree but to aid in the aeration of the plant's sap system.

The black mangrove, and to some extent the white mangrove, cope with the lack of air in the mucky substrate by developing vertical extensions from their roots which stick above the soil level and (at low tide) accomplish oxygen exchange. The tea mangrove has pronounced buttresses which act as aerators. The buttonwood mangrove effectively avoids this problem by growing on the back edges or higher ground within a mangrove swamp, thus reducing the likelihood of having the soil around its roots supersaturated except at extreme high tides.

Mangroves have developed various ways to deal with the problem of high salt concentrations in the water around them. Some species secrete salt from their roots and/or leaves. Pacific coast black mangroves can be observed in the dry season with salt crystals along the outer edges of their leaves (secretion of salt through the leaves happens throughout the year, but is not usually observed in the rainy season because the rain washes off the salt).

In some mangroves, such as the red mangrove, salt is stored in the older leaves which soon fall off the tree. And in other species, it appears that salt is simply tolerated in much higher levels than is common in most plants.

As a result of the rather severe conditions where mangroves grow, there is not much plant diversity. Correspondingly, the animal life associated with mangroves is not nearly as diverse as it is in other lowland habitats in Costa Rica. Nevertheless, what mangrove swamps may lack in numbers of species they make up for with numbers of individuals.

At certain times of the year the tops of mangrove trees are filled with nesting birds. In the Tempisque River, the Isla de Pájaros, or Bird Island, is an impressive example of this phenomenon. Each year hundreds of Wood Storks, Roseate Spoonbills, Anhingas, and Neotropical Cormorants reproduce on this protected patch of mid-river mangroves.

Part of the reason for the concentration of nesting and roosting birds in the mangroves could be that it serves as a sanctuary from terrestrial and climbing predators. The bases of the trees are under water almost nearly of the time and even when they are exposed by receding tides, the soft mud that surrounds them is a deterrent to many creatures.

Below the water's surface in areas where mangroves grow one can find a high diversity of life forms. Among the mangrove's root systems many marine organisms live or spend a portion of their lives. Such creatures include algae, corals, barnacles, sponges, oysters, crabs, lobsters, shrimps, octopi, and fishes. The importance of mangroves to the health of the marine ecosystem is immeasurable.

Unfortunately, in Costa Rica and the rest of the world, mangrove forests are being destroyed and their sites converted to fish pens, rice paddies, salt-drying ponds, cattle pasture, tourist developments and human settlements. Mangrove wood makes good fuel and excellent charcoal, but over-harvesting has contributed to their demise. Additionally, the red mangrove is an important source of tannin (used in processing leather), but the stripping of the bark to get the tannin kills the individual trees.

All mangroves in Costa Rica are protected by law, but there is not always someone around to enforce the law. Nevertheless, there are still large areas of mangroves lining estuaries and mouths of rivers and streams along the Pacific coast of Costa Rica. Mangrove development is not as common on the Caribbean coast, because there is little variation in the height of the tides, but one area where some very tall red mangroves can be seen is along the extensive canal system between Limón and Tortuguero.

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