The mangroves

What is a mangrove forest?

A mangrove forest is a habitat mainly comprised of a group of mangrove trees, growing on the coastline between the sea and the land. Mangroves are commonly found in wetlands or estuaries, where seawater meets and mixes with freshwater from rivers, intermittent streams, rain runoff or subterranean sources.

What is a mangrove tree?

A mangrove tree is a tree or shrub that features special physiological and anatomical adaptations which allow it to thrive in the extreme conditions typical to the litoral zone between land and sea (Figure 2). The low-lying lands in this area are known for the frequent water intrusions, high salinity, and poor aeration.

Necessary conditions for mangrove development

Generally speaking, a mangrove forest develops in places that gather the following five conditions

1. Mangrove trees need warm temperatures. These should be between 60-86 F (15-30 C). The temperature is a determining factor for mangrove distribution throughout the world (Figure 1), limiting their development to the tropical and subtropical regions between 30 degrees N and 30 degrees S.

2. Areas with low exposure to waves and surges, with small chance of erosion, are ideal in aiding seeds and young plants settlement.

3. Mangrove development is also greater in zones offering alluvial substrates, rich in organic material (such as silt).

Alluvial substrate - deposit of water-transported sediments

Estuary – habitat in which seawater becomes diluted as it mixes with freshwater draining from the mainland.

Littoral zone (intertidal zone) - coastal area under the direct influence of the rising or subsiding des.

Mangrove - tree or shrub with special adaptations which allow it to thrive in high salinity and poorly oxygenated conditions.

Mangrove forest – habitat chiefly comprised of mangrove trees.

Propagule - the mangrove’s viviparous seed, it is a plant in the early stages of development.

Wetland - areas influenced by aqua c systems and terrestrial systems which are frequently flooded.

4. Mangroves occupy land tracts in which strictly terrestrial plants could not survive, such as areas frequently flooded by seawater

5. Coastal regions with low slopes have a wider litoral zone and therefore provide a more expansive area for mangrove establishment.

Mangrove trees can grow up to a height of approximately 148-164 feet (45-50 m) when situated in optimal conditions. This may be observed near the equator, where there is a good availability of nutrients, wide expanses of intertidal zones, and relatively constant, high temperatures. In places with more strenuous conditions (non-op mal temperatures, very arid soil or extremely high salinity), a mangrove tree only grows as a 3-6 feet (1-2 m) tall shrub.

Importance of the mangrove ecosystem

1. Mangrove thickets are connected to the seagrass meadows and coral reefs. They help retain and stabilize the sediments coming from the mainland, and help control substrate erosion. These trees absorb nutrients and pollutants from the water and are therefore key to clear, clean waters. Mangrove thickets also help reduce freshwater influx into the sea, and are highly productive systems. Many fish and crustacean larvae grow and develop in the mangrove thicket habitats before migrating to other marine ecosystems.

2. Mangroves, like any other plant, use solar energy for photosynthesis, producing oxygen in the process. Photosynthesis allows the mangrove tree to transform solar energy into organic material such as leaves, wood or flowers. Organisms capable of this are known as primary producers.

3. Many other species such as crabs, shrimps and butterflies feed on the leaves, flowers and other organic material produced by the mangroves. These organisms are known as primary consumers. The organisms which feed on the primary consumers, such as seagulls and fish, are known as secondary consumers. In this manner, mangrove forests fulfill a vital role as the base of the food chain, and are highly productive ecosystems.

4. Mangrove thickets are important habitats providing food and shelter for many marine species, including mammals, amphibians, birds, reptiles, plants, fish, and invertebrates.

5. They serve as nursery zones (offer shelter to species in their juvenile stages), refuge and feeding grounds for many commercially important species such as snapper, lobster and conch.

6. Mangrove thickets offer a nesting and resting area for several migratory bird species like the prairie warbler, endemic species like the yellow-shouldered blackbird, or resident species like the American yellow warbler.

7. Mangrove roots stabilize and retain sediments along the shore, protecting the coast from erosion driven by wind and wave action. This is particularly important during strong surf events like hurricanes, tsunamis or storms.

8. Mangrove forests offer recreational opportunity es and benefits to both residents and visitors alike.

Primary consumers – Organisms which feed on primary producers.

Primary producers – Organisms which can transform solar energy into matter through photosynthesis.

Secondary consumers - Organisms which feed on primary consumers.

The mangroves of Puerto Rico

The largest mangrove forest in Puerto Rico can be found in the northeast coast (Figure 5), in the Piñones area in Loíza. The north coast mangrove forests are associated with the freshwater influx from rivers and lagoons (basin and riparian mangroves), and from runoff (bank mangroves). Because the Atlantic Ocean features such heavy surf conditions, mangrove forests on the north coast develop only in locations protected from these conditions. Throughout the southern coast, on the other hand, mangrove forests present a thick border on the coasts (border mangroves) and the cays (cay mangroves). The submerged land shelf in this zone is wider and flatter than the one on the north coast, and the surf and wave conditions presented by the Caribbean Sea are much calmer than that of the Atlantic Ocean.

There are around 54 mangrove species worldwide. Of these, about ten can be found within the Caribbean region, and only four within Puerto Rico:

a. Red mangrove (Rhizophora mangle)

b. Black mangrove (Avicennia germinans)

c. White mangrove (Laguncularia racemosa)

d. Buttonwood (Conocarpus erectus)

The four mangrove species found in Puerto Rico can be easily identified by their main distinctive characteristics: leaf shape, flower and fruit shape, presence of aerial prop roots and presence of pneumatophores; as well as their localization inside the mangrove forest. The trees tend to be

distributed in a sequence perpendicular to the coast, from the sea to the mainland. This distribution is known as zona on (Figure 6). Zona on is characterized by the adaptations each mangrove species has to the terrain’s extreme conditions, such as:

• Tolerance to salinity. The species which are better adapted to high salinity are on the soil furthest from the li oral zone. During the low de, this soil is exposed to the wind and the sun; some of the water evaporates and some filters down through the sediment, leaving behind salt crystals. In the zones closer to the sea, on the other hand, the salts are dissolved in water and are therefore less salty.

• Adaptations to life in poorly-aerated soils. Mangrove forests are highly productive habitats with very high levels of organic decomposition (for instance, leaves, wood and flowers). Some of the bacteria in charge of decomposing this organic material draw their energy from oxygen. However, oxygen content in these soils is low, due in part to the soils being flooded and being composed mainly of fine sediments, which limit aeration. Therefore, a good amount of the organic decomposition taking place in mangrove forests is performed by anaerobic bacteria which can survive and work in the absence of oxygen.

• Capacity to become established in unstable substrates (waterlogged and muddy soils). Soils in direct contact with the sea tend to be more saturated with water, are usually more unstable and unconsolidated, whereas those soils which are further from the sea tend to be firmer.

Aerial prop root - root growing outwards from the main tree trunk in a s lt shape.

Pneumatophore - type of root which grows upwards from the soil. They are a feature of both the black and white mangrove trees.

Zonation - mangrove species distribution perpendicular to the coast.