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Mangrove vegetation includes trees, shrubs, palms and ground greeneries that develop within the intertidal areas of waterfront and estuarine edges (Benfield, 2002). There are about 20 families of plants that have individuals in mangrove area and they share several specialized features that enable them to adapt with the stressors related with regular inundation by the tide (Duke et al, 1998). These adaptations include pneumatophores (exposed breathing roots) to permit gas exchange in what are frequently anaerobic sediments, supporting prop salt roots for support in the shallow and unstable sediment, buoyant and viviparous propagules that allows dispersion of the seed in an aquatic environment and salt excretion glands to manage balance within the plant in what is frequently a saline environment. Mangroves develop at the interface between land and sea in tropical and sub-tropical latitudes where they exist in conditions of high salinity, extreme tides, strong winds, high temperatures and muddy, anaerobic soils. They grow in highly humid to extremely arid conditions and in several substrate types including clay, peat, sand, coral rubble and bare rock (Cardona and Botero 1998 and English et al, 1997).
Mangroves provide humans with many useful products and ecological services. Due to their coastal location, mangroves maintain coastal water quality (English et al, 1997) form a barrier for coastal protection from wave storm and flood damage (Yoshiro et al, 1997). Mangroves are also important breeding grounds for juvenile fish that stock many offshore adult populations including fishery species (Nagelkerken et al, 2000), and can also sustain fisheries through their nutrient export (Robertson et al, 1991). There may also be fisheries that involve taking directly from the mangrove (e.g. Wolfe et al, 2000) and they can be used as filters for aquaculture effluent, namely from shrimp ponds (Robertson and Phillips, 1995). Mangroves are also used by many indigenous peoples for traditional uses including as a source of medicine (see Bandaranayake, 1998 for a review) as well as for building materials (Walters, 2005). Although mangroves cover vast areas of tropical and sub-tropical coasts in many of these areas they are found to be under threat from a variety of anthropogenic activities. Coastal communities traditionally cut mangroves for charcoal, housing materials, boat construction, and medicine. The growing population among coastal villages has increased the demands for these materials. This was observed as a threat in Samar Island, Philippines (Mendoza and Alura, 2001). In the face of climate change, many of the regulating services of mangroves are actually becoming more necessary and valuable, especially their buffering capacity against storms and flooding.
Alongi (2002) reports that approximately one-third of the worlds mangrove forests have been lost. Large areas of mangrove forests may often be cleared to make way for shrimp aquaculture in developing countries (Primavera, 1993, Adeel and Pomeroy, 2002). Other impacts include population pressure, wood extraction (Walters, 2005), conversion to agriculture, salt production, tin mining, coastal industrialization and urbanization(Ong, 1995; Macintosh, 1996). Oil spills may also cause large-scale damage to mangrove ecosystems (Duke et al, 1997). As well as direct impacts from human activity mangroves may also be under threat from global warming (Field, 1995) this is especially significant for mangroves, their intertidal location means they are likely to be one of the first habitats to be affected by a rise in sea level. Alongi (2002) describes the greatest hope for the future of mangrove forests as a reduction in the human population.
Mangrove forest is also known as the “rainforest of the sea”. It grows well in tropical countries, including the Philippines. The Philippines is considered as a megadiverse country in terms of marine flora and fauna. Biological diversity of coastal plants in the country is one of the richest in the world (Calumpong and Menez, 1997). There are estimated forty species, came from sixteen families, considered as true mangroves (Primavera et al 2004). It is no doubt that mangrove forests are one of the world’s most threatened tropical ecosystems. In fact, 11 true mangrove species (Table 3) qualified for the IUCN Red List categories of threat including two critically endangered, three endangered, and six vulnerable species (Polidoro et al. 2010). For these reasons, many tropical countries have considered the sustainable management of mangroves as major priorities in biodiversity conservation (Macintosh and Ashton 2002). In addition, several countries have already come up with their local mangroves Red List of threatened species.
A number of efforts on mangrove conservation and rehabilitation have been completed in the country. Some were successful, some were not. Primavera and Esteban (2008) reviewed eight mangrove rehabilitation projects in the Philippines and found out that despite heavy funding in the hundreds of millions of dollars to rehabilitate thousands of hectares of mangroves over the last two decades, the longterm survival rates of mangroves are generally low at 10–20 %. Two of the main reasons cited are inappropriate species and sites because the ideal sites have been converted to brackish water fishponds.
The structure of a mangrove forest at any one point in time will be a function of its stage in succession, the species present, zonation, growth and primary productivity, mortality and survival, propagule dispersal, establishment and survival as well as seedling and sapling growth and survival.
There are obviously many factors to consider and to analyze each one of the above components and relate them to mangrove forest structure (and each other) would turn into a complicated exercise in ecological modeling (such attempts have been made (e.g. Schaeffer-Novelli et al, 2005; Twilley and Rivera-Monroy 2005 and Twilley et al, 1998)). This review instead of investigating all possible influences on mangrove forest structure will look at more broad topics e.g. factors affecting propagules or succession in mangrove forests and attempt to bring together the relevant literature to see what controls these and how these effect mangrove forest structure.
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