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5.4. Mangrove Forests of Papua
DANIEL M. ALONGI
TANGROVE FORESTS are one of the major ecosystems within the coastal zone of Indonesia. They develop best where low wave energy and shelter foster the deposition of fine sediments, and are the only woody plants living at the confluence of land and sea. Evidence of their success is the fact that the standing crop of mangrove forests is, on average, greater than any other aquatic ecosystem.
Unlike other tropical forests, mangroves are architecturally simple, being composed of relatively few tree species and often lacking an understory of shrubs and ferns (Figure 5.4.1). Mangrove trees possess morphological and physiological characteristics that make them uniquely adapted to the tidal zone, including aerial roots, salt-excreting leaves, and viviparous water-dispersed young seedlings (i.e., seeds that germinate while still on the parent tree).
Mangroves forests are heavily used traditionally for food, shelter, timber, fuel, and medicine. These tidal forests occupy a crucial niche along the Indonesian coast, as they are a valuable ecological and economic resource. Mangroves provide important nursery grounds and breeding sites for fishes, reptiles, birds, crustaceans, shellfish, and mammals; accumulation sites for sediment, contaminants, carbon, and nutrients; protection against coastal erosion; and a renewable source of wood (Alongi 2002).
Figure 5.4.1. A mature, mixed Rhizophora-Bruguiera forest in the Fly Delta, Papua New Guinea.
Photo: P. Dixon.
This chapter describes the mangrove forests and associated ecosystems in Papua. As the mangroves of Papua are not structurally and functionally different from those in Papua New Guinea, the mangroves of the entire (800,000 km2) oceanic island of New Guinea will be reviewed here. Information about mangroves and their ecology on the other islands of Indonesia can be found in chapters of the other volumes of this series (e.g., Chapter 19 in Tomascik et al. 1997).
Distribution
The island of New Guinea has large tracts of mangrove forest with the greatest species diversity of mangroves in the world due to its location bordering the Australasian and Indo-Malesian centers of diversity (Duke 1992). There may be as many as 43 species in New Guinea (Table 5.4.1) with fewer species on the north coast than on the south coast. This disparity of species richness is indicative of a floral discontinuity between the northern and southern sides of the island. Duke (1992) maintains that this is convincing evidence of a fusion of boundaries between two previously isolated and different mangrove floras. Similar floristic discontinuities have also been described for upland plants on the island (Heads 2001).
These discontinuities are associated with tectonic events of the collision between the Pacific and Australian plates. The southern coast of New Guinea is a part of the stable Australian plate and has been subjected to alternating episodes of submergence and emergence as a result of glaciation that last took place around 18,000 years ago, when sea level was about 100–150 m lower than at present. In contrast, the north coast of New Guinea lies at the northern edge of the Australian Plate, which has remained submerged. Saenger (2002) notes that the mangroves along the northern shore of the island represent more ancient forests than those along the southern coast; the northern flora is derived from the Indo-Malesian mangroves but the southern flora is largely derived from northern Australia. The geographical isolation of the mangrove flora of the southern and northern coasts of the island is maintained by the high mountain ranges which form the backbone of New Guinea (Milliman 1995).
The island of New Guinea contains approximately 34,739 km2 of mangrove forest, of which 13,820 km2 are in Papua and 5,399 km2 are in Papua New Guinea (Darsidi 1984; Soemodihardjo 1986; Soemodihardjo et al. 1993; Spalding, Blasco, and Field 1997; Tomascik et al. 1997). The area of mangroves on the island is subject to considerable uncertainty; the area of Papua mangroves is a crude estimate only, as figures range from 13,000–26,000 km2 (Tomascik et al. 1997). What it is certain is that the mangroves of Papua constitute by far the largest area of mangroves (69–80%) in Indonesia.
Mangrove forests in New Guinea are situated in the deltas of large rivers and along the banks of 253 small and medium rivers (Figure 5.4.2). The large rivers on the island are the Mamberamo, Sepik, Ramu, Markham, Purari, Kikori, Bamu, Fly, Digul, and Palau-Palau rivers, which cumulatively discharge 1.7 billion metric tons of sediment to the adjacent coastal ocean (Milliman 1995). This high fluvial discharge is a result of high rainfall on the island and facilitates the development of large river deltas colonized by extensive inland freshwater and estuarine mangrove forests that can often penetrate quite deeply inland. For example, Sonneratia caseolaris occurs 75 m