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 above sea level and several kilometers inland in southern New Guinea; in the Fly Delta, mangroves can be found 500 km upstream (Saenger 2002) and in Bintuni Bay on the west coast of Papua, mangroves can be found 30 km inland. Often there is little or no discontinuity from the sea to upland forests; the coastal vegetation progressively changes from mangroves to inland freshwater swamp and terrestrial forest (Taylor 1959). Generally, the mangrove forests along the southern and western coasts of New Guinea are more expansive than on the northern and eastern coastlines.
Mangroves thus develop best in areas associated with high rainfall. It is in the largest river deltas, where high rainfall and subsequent runoff transports and deposits mud, that the most luxuriant mangrove forests develop. In dry regions of the island, such as near Port Moresby, mangrove forests are reduced in height and are of lower species diversity (Frodin and Huxley 1975).
Figure 5.4.2. Map of New Guinea showing the major river systems and mangrove forests (blackened areas).
Forest Structure and Zonation
The physical settings of mangrove forests are based on the dominance of key physical characteristics: rivers, tides, waves, and sediment type and origin (Wood-ruffe 1992). The majority of mangrove forests in New Guinea inhabit river- and tide-dominated settings (Cragg 1987), but a great variety of composites of these settings are known (Johnstone and Frodin 1982).
Mangroves are typically distributed from mean sea level to highest spring tide with the most conspicuous feature being the sequential change in species either perpendicular or parallel to shore. Mangroves in New Guinea often consist of narrow crowned trees that can attain 30–40 meters in height, although emergents are common in Nypa palm stands. Rhizophora stylosa and Rhizophora apiculata are emergents in Bruguiera cylindrica and Bruguiera exaristata forests (Figure 5.4.3), with a dense ground layer or understory of Nypa proximate to the river bank. Similarly, Avicennia species are emergents in Ceriops tagal forests. The understory, when present, most often consists of Acanthus ilicifolius, Acrostichum speciosum, Excoecaria agallocha, Dalbergia candenatensis, and Maytenus emarginata, and various lianas and scrambling vines (Johnstone and Frodin 1982).
There are subtle and complex patterns of species distribution across the inter-tidal seascape and upstream-downstream, relating to individual species tolerances to abiotic factors (e.g., soil salinity, nutrient status, degree of anoxia [lack of oxygen], degree of soil wetness) and to biotic factors (e.g., competition, predation). Some of these factors come into play over different temporal and spatial scales to control the distribution of tree species, prohibiting generalizations about the mechanisms governing zonation. Many such physical and ecological variations are often expressed within a single estuary (Duke, Ball, and Ellison 1998). For an individual tree, several factors operate to regulate tree growth, including temperature, nutrients, solar radiation, oxygen, and water.
Figure 5.4.3. A mature (> 30 m tall) mixed Bruguiera forest with a dense understory close to the river bank, Fly Delta, Papua New Guinea.
Photo: D. M. Alongi.
Mangrove forests in New Guinea are often naturally disturbed by storms, lightning, tidal surges, and floods, and may take decades to recover (Johns 1986). For instance, many river deltas in Papua and in Papua New Guinea experience tidal bores which are powerful tidal surges that can sweep up a river to destroy entire forests (Figure 5.4.4). Other natural events, such as disease and pests, may not immediately kill trees but can cause stunted growth, slow death, or the replacement of a species. Dieback of mangrove stands has been observed in Papua New Guinea (Arentz 1988) and attributed to either lightning strikes or periods of drought. Johns (1986) similarly reported the death of stands of mangrove forest in New Guinea, attributed to lightning strikes. Regeneration of mangrove seedlings was recorded, but analysis of aerial photographs suggested that mangrove forests affected by previous events had required over 200–300 years to recover fully.
In the river deltas of New Guinea, the zonation and distribution of some man-grove forests corresponds to the ‘‘classical’’ zonation parallel to shore, but most do not, as various zonation schemes for mangroves have been overemphasized. Brass (1938), Percival and Womersley (1975), Floyd (1977), Paijmans (1976), Paij-mans and Rollet (1977), Green and Sander (1979), Spenceley (1981), Johnstone (1983), and Gylstra (1996) have described zonation of mangroves in various sites around New Guinea and adjacent islands (e.g., the Aru Archipelago). For the mangroves in New Guinea, Johnstone and Frodin (1982) presented a more realistic depiction of patterns of zonation based on the following factors: tidal range and inundation frequency, degree of wave action, drainage, salinity, substrate type, and composition of biota. Zonation patterns are inconspicuous or absent in flat areas, but become more obvious with increasing ground slope, as water depth and frequency of tidal inundation control the seaward limit of mangroves. A good example of the importance of this factor is the mangrove flora of Galley Reach on the southern coast of New Guinea (Paijmans and Rollet 1977) where there are two large-scale zones of ‘‘true’’ mangroves and ‘‘transitional’’ mangroves. Many species occur in both zones, but some species are restricted to either zone: Bruguiera cylindrica, Bruguiera gymnorrhiza, Rhizophora mucronata, Sonneratia alba, Sonneratia caseolaris, and Xylocarpus granatum in the true mangrove zone and Avicennia rumphiana, Exocoecaria agallocha, Heritiera littoralis, Lumnitzera racemosa, Acrostichum aureum, and Acanthus ilicifolius in the transitional zone. The transition
