Caribbean Coastal Mangrove Forest

This ecological system represents the oceanward mangrove forest with a tidal flooding regime, distributed along the coasts of the Greater and Lesser Antilles and the Caribbean coast of Colombia and Venezuela. The mangrove forest structure and composition depend on the geomorphic and hydrological processes that characterize its specific location along the coast, with important differences between locations depending on total precipitation amounts, freshwater runoff, and wave action (Cintrón et al. 1978). Mangroves in drier climates and with less freshwater runoff exhibit simpler structure, less leaf fall and a lower rate of tree growth; the salinity in these mangroves approaches that of the seawater and has little change throughout the year. Mangroves on St. John, Virgin Islands, although containing the typical assemblage of species of Caribbean mangroves and associated halophytes, are poorly represented by a narrow strip of vegetation occurring in protected, shallow waters. These forests and, in general, fringe mangroves are dominated by Rhizophora mangle. The standard zonation of mangroves consists of Rhizophora mangle in the lower and middle intertidal zone, Avicennia germinans in the upper intertidal areas that are occasionally flooded, and Laguncularia racemosa in patches on higher elevations that are less frequently flooded. Dense mangrove forests do not typically have understory plant associations, except for mangrove seedlings (FNAI 1990).

Stands are dominated by Rhizophora mangle.

Fringe mangroves occur in close proximity to the ocean, are dominated by Rhizophora mangle, and may have leeward zones dominated by Avicennia germinans or Laguncularia racemosa. These tidal forests can reach 20 m (66 feet) high. Stands occur in frost-free zones, on soils that are permanently saturated with brackish water and which become inundated during high tides. The brackish environment tends to limit competition from other species. Mangroves are found on fine inorganic muds, muds with high organic content, peat, sand, rock, coral, shells, and some man-made surfaces if there are sufficient crevices for root attachment. Avicennia germinans grows best in soils of high salinity, Rhizophora mangle grows best in areas of estuarine salinity with regular flushing, and Laguncularia racemosa grows best in areas with freshwater input on sandy soils (FNAI 1990). Mangroves attain larger biomass in areas of low wave-energy shorelines, river deltas, and floodplains with depositional environments (Odum et al. 1982). Fluctuating tidal waters are important for transporting nutrients, controlling soil salinities, and dispersing propagules, but high wave energy prevents establishment and may destroy their shallow root systems (Odum and McIvor 1990).

Disturbance in mangrove forests may be caused by large-scale events such as hurricanes, or clearcutting, but also by small-scale events such as lightning, causing mangrove trees to die in small areas around lightning strikes, or attack by wood-boring beetles. The relative importance of these different types of disturbance vary with geography, with some localities more often subjected to the impact of hurricanes or lightning. Recovery from large-scale disturbance may be slow and may vary depending on species composition and intensity of stress factors subsequent to the disturbance event, with increases in solar exposure, soil temperature and/or salinity capable of inhibiting regeneration (McKee and Feller 1994, cited in Barbour and Billings 2000, Smith et al. 2009), or of influencing the establishment of the pioneer mangrove species, along with other factors such as presence of a seedling source, herbivory, and seed consumption.

Mangroves are considered pioneer species because of their ability to establish on otherwise unvegetated substrates. Once individuals begin to colonize a disturbed area, even-aged stands are established with little variance in the structure because new development of successive colonizers is arrested by the closed canopy. On shorter time scales, the pulses of the tides and freshwater runoff are very important factors in the dynamics of mangroves because these control the rates of sedimentation and vertical accretion and thus determine their intertidal position. Tidal flooding is also key for the distribution of soil nutrient resources in the coastal mangrove forest. The distribution of the different mangrove species and the mangrove community can experience fluctuations in structure and species composition as a result of changes affecting the hydrologic patterns.

Major threats to coastal mangroves are conversion for land uses such as shrimp farms, salt production, coastal development, deforestation for charcoal production, tannins, and timber uses. These impacts are responsible for the conversion of vast amounts of the original distribution of coastal mangroves (Valiela et al. 2001). In addition to direct conversion, the damming of rivers, the construction of channels and dikes, and in general any infrastructure that prevents or changes the natural water circulation can cause enough effects in the hydrologic regime to suppress the growth of mangroves, eventually leading to a dieback of the community. Studies in mangroves of arid regions of the Caribbean suggest that the main cause of die-offs of mangrove forest when freshwater inputs have been cut or altered is the increase in soil salinity (Cardona and Botero 1998).

Climate change also poses a threat for mangroves if they cannot keep pace with it because the rate of sea-level rise is higher than the mangrove's rate of sedimentation or change in elevation of the mangrove sediment surface. Based on available evidence, of all the climate change outcomes, relative sea-level rise may be the greatest threat to mangroves. Global sea-level rise is one of the more certain outcomes of global warming; it is already likely taking place (12-22 cm occurred during the 20th century), and several climate models project an accelerated rate of rise over coming decades (Gilman et al. 2008 and references therein). There are several processes that influence the elevation of mangroves' sediment surface. Besides the physical processes of sediment accretion and erosion there are biotic contributions, below-ground primary production, autocompaction, and fluctuations in water table levels. Changes in groundwater inputs, such as from long-term changes in precipitation levels resulting from climate change, would result in a long-term change in mangrove elevation. Short-term cyclical influences include variability in precipitation and tidal range. Research conducted to date has demonstrated the short-term effects of groundwater recharge on mangrove elevation, thus the combined effect of climate change and the suppression of freshwater inputs to the mangrove systems due to development-related alterations in the hydrology is a clear threat for existing mangroves.

This system is found in Colombia, the Greater Antilles, Puerto Rico, Virgin Islands, and Venezuela.