Ecology of Indonesian Papua Part Two. Andrew J. Marshall
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 is very distinct, suggesting that floral discontinuity is related to the tides.
Figure 5.4.4. The destructive power of tidal bores in the Fly Delta, Papua New Guinea.
Photo: D. M. Alongi.
Drainage and substrate type appear to also be important factors controlling mangrove distribution. The well-drained banks of mangrove creeks are often inhabited by Rhizophora mucronata and Avicennia officinalis, whereas Sonneratia caseolaris is common on poorly drained banks. On coarse-grained and rocky substrates, Aegialtis annulata and Osbornia octodonta are common. Heritiera littoralis, Acrostichum speciosum, and Acanthus ilicifolius frequently occur on biogenic structures such as callianassid lobster (Thalassina anomala) mounds that can approach one to two meters in height. In sandier habitats, Avicennia marina, Heritiera littoralis, Ceriops tagal, Ceriops decandra, Lumnitzera racemosa, Lumnitzera littorea, Avicennia rumphiana, and Xylocarpus mekongensis are frequently found, although the latter species often occurs in mud.
Salinity is one of the major factors regulating community composition of Papuan mangroves. Along vast expanses of river banks of low salinity, the mangrove palm Nypa fructicans and, to a lesser extent, Sonneratia caseolaris, dominate the vegetation. In high salinity areas where rainfall is low (e.g., near Port Moresby), Ceriops tagal is usually the last mangrove species to be found before the transition to open ground (Frodin and Huxley 1975).
Despite that fact that no single factor or simple set of factors regulate the distribution and zonation of Papuan mangroves, large-scale patterns have been defined in specific locations around New Guinea. An ‘‘open coast’’ pattern (where wave action is significant) has been described for the northwest side of Hood Lagoon (Johnstone and Frodin 1982) and in the Raja Ampat Islands in far western Papua (Takeuchi 2003). From the sea to the land, the discernible assemblages in Hood Lagoon are: a beach fringe of Avicennia marina and Sonneratia alba followed by denser stands of Rhizophora stylosa, Rhizophora apiculata, Bruguiera cylindrica, and Bruguiera gymnorrhiza. Further inland, Ceriops tagal and stunted A. marina are common. In the Raja Ampat Islands, mangroves are sparse and species-poor compared with mangroves on the main island of New Guinea, and consist of Bruguiera gymnorrhiza-Rhizophora mucronata associations along the banks of the Gam and Kasin rivers, and a well-developed upstream sequence of Rhizophora mucronata-Ceriops tagal, Bruguiera gymnorrhiza, and Nypa fruticans with a brackish-freshwater zone composed of Xylocarpus granatum, Dolichandrone