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Mangrove Ecosystems: Definitions, Distribution, Zonation, Forest Structure, Trophic Structure, and Ecological Significance
By Karen L. McKee
Forest Structure
In addition to zonation, mangrove forests are also characterized by attributes such as species richness, canopy height, basal area, tree density, age/size class distribution, and understory development. Lugo & Snedaker (1974) described six mangrove forest types based on size, productivity, and composition in Florida: riverine, overwash, fringe, basin, scrub, and hammock. These forest types reflect differences in geomorphology and hydrology and are generally applicable to forests in the Caribbean Region. Species richness appears to be influenced by temperature, tidal amplitude, rainfall, catchment area, freshwater seepage, and frequency of cyclones (Tomlinson 1986; Lugo & Snedaker 1974).
Another structural characteristic of mangrove forests is the frequent absence of understory species, which are usually found in other forest systems (Janzen 1985). Shrubs, grasses, lianas, and other herbaceous plant species do not usually occur under the closed canopy in the mangrove forests. The lack of understory is probably related to the combination of salinity and flooding stresses and low light levels, which exceeds the tolerance limits of plants (Lugo 1986). An understory may develop, for example, where the canopy is open (allowing light penetration to the forest floor) or where rainfall or freshwater runoff lowers salinity levels. There may also be a second layer composed of mangrove seedlings and juveniles, but densities are greatest in light gaps.
Although mangrove forests are usually described as having an even-aged size class structure, the data supporting this viewpoint are not extensive. It is thought that large-scale disturbances such as hurricanes kill large areas of forest, which then regenerate at about the same time from seedlings or previously established juveniles. This view of even-aged structure in mangrove forests does not agree with what is known about the dynamic processes in other forest ecosystems. Forest systems are now viewed as mosaics of patches that vary in size and contain individuals of different ages or stages of development. These patches have resulted from natural disturbance events that create openings or gaps in the forest. Preliminary work indicates that gap dynamics may be an important process structuring mangrove forests.
Natural disturbances such as lightening strikes, wind damage, and frost are often associated with the development of patches or gaps in mangrove forests. Recent work in Belize, however, demonstrates that wood-boring insects are primarily responsible for the creation of gaps in red-mangrove forests (Feller & McKee 2001).
Forest structural characteristics such as canopy height, tree density, and biomass accumulation may be influenced primarily by climatic factors such as rainfall and by nutrient input (Golley et al. 1975; Smith 1992). Areas characterized by high rainfall typically have tall canopies, high basal areas, and low tree densities. Similarly, larger, more productive trees typify mangrove forests receiving high inputs of nutrients—for example, those areas used as bird rookeries.
Source :
MANGROVE ECOLOGY WORKSHOP MANUAL
Edited by IIka C. Feller & Marsha Sitnik |
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