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 spathacea, and Heritiera littoralis.
A ‘‘deltaic’’ pattern (where muddy soils and quiescent conditions predominate) has been described for a variety of river deltas and sheltered embayments, such as the Purari and Fly deltas discharging into the Gulf of Papua (Cragg 1983; Robertson, Daniel, and Dixon 1991), Bintuni Bay on the sheltered west coast of Papua (Erftemeijer et al. 1989) and on the banks of the Ajkwa and Tipoeka estuaries in southwestern Papua (Ellison 2005).
The mangroves of Bintuni Bay are the most developed and extensive mangrove forests of Papua, covering an area of 618,500 hectares. The most seaward stands are dominated by seedlings and saplings of Avicennia marina and Sonneratia alba. Further upstream, the vegetation is dominated by stands of Rhizophora apiculata, Bruguiera parviflora, and Bruguiera gymnorrhiza. Overwash islands, colonized mostly by Rhizophora apiculata and, to a lesser extent, by Bruguiera parviflora and Bruguiera gymnorrhiza, abound within the embayment. In the Ajkwa and Tipoeka estuaries, Ellison (2005) identified five major mangrove forest types, recording extensive Bruguiera- dominated forests, consisting of Bruguiera cylindrica, Bruguiera parviflora, and Xylocarpus mekongensis, mostly north of the main Ajkwa River mouth. Nypa fruticans and mixed mangrove-floodplain forest dominated areas landward in both lower salinities and at higher elevation. At the seaward margins were found Rhizophora -dominated forests, mostly composed of R. stylosa, R. apiculata, and R. mucronata, whereas accreting mudbanks were colonized by pioneering stands of Avicennia marina and Sonneratia caseolaris. Within river channels, high structural diversity was found, apparently in relation to microscale topography. Ellison (2005) noted that tree heights for Bruguiera and Rhizophora often exceeded 25 m.
In the Fly Delta of Papua New Guinea, mangroves cover 87,400 hectares mostly on the delta islands (Robertson, Daniel, and Dixon 1991). Twenty-three species of mangroves were recorded, classified into three major forest types: Rhizophora apiculata-Bruguiera parviflora (salinities > 10); Nypa fruticans (salinities 1–10); and Sonneratia lanceolata-Avicenna marina (accreting banks). On accreting banks in very low salinity areas, S. lanceolata was found in large monospecific stands.
In the Purari delta further east of the Fly delta, Cragg (1983) recognized three major types of mangrove forest: fringing, main, and transitional. He also classified mangrove associations for the southern coast of New Guinea and identified groups related primarily to salinity regime (Table 5.4.2). Sonneratia lanceolata is the dominant mangrove found in fringing stands, ranging from the seaward edge to many kilometers inland. In lower salinity, Sonneratia alba, Avicennia eucalyptifolia, and Aegiceras corniculatum are major members of fringing forests, while palms (Pandanus sp. and Nypa fruticans) dominate fringes below a salinity of 2. The main mangrove species is Rhizophora apiculata, followed closely by Bruguiera parviflora and Bruguiera sexangula. Greatest diversity is encountered in the transitional areas between zones, where true mangrove species, mangrove associates, terrestrial intruders, epiphytes, and climbing plants coexist. The most common mangrove and mangrove associates in this zone are Bruguiera sexangula, Camptostemon schultzii, Dolichandrone spathacea, Diospyros spp., Excoecaria agallocha, Heritiera littoralis,
R. apiculata, and Xylocarpus granatum. Several freshwater swamp species invade this zone and frequently develop root structures similar to mangroves. These species include Calophyllum sp., Intsia bijuga, Myristica hollrungii, and Amoora cucullata. In this zone, an understory of Barringtonia, Brownlowia, Inocarpus, Hibiscus, and Cerbera with scattered small palms Areca, Arenga, Metroxylon, and Nypa is often formed.
There are marine macroalgae associated with mangroves, particularly with stilt roots of Rhizophora and pneumatophores of Avicennia and Sonneratia (Coppejans and Meinesz 1988; King 1990). In Bintuni Bay, the red alga Gracillaria crassa is very common on the pneumatophores of Sonnertia alba. In the Madang region of Papua New Guinea, 25 species of macroalgae have been recorded, including a "Bostrychia-Caloglossa" association and the genera Caulerpa, Halimeda, Neomeris, Chnoospora, Cutleria, Dictyota, Padina, Catenella, Laurencia, Murrayella, Peyssonnelia, Polysiphonia, and Stictosiphonia (King 1990). Further information is fragmentary, but it appears that macroalgae associated with mangroves in New Guinea are derived from inshore reefs (Tanaka and Chihara 1988).
Forest Biomass and Production
The mangrove forests of New Guinea are among the largest on earth, rivaling the height and mass of even the largest tropical rainforests. Figure 5.4.5 provides best estimates of the above-ground biomass of the world’s mangrove forests, including the few data from New Guinea (nearly all of the values between 2˚ and 8˚ S Latitude). Mangrove forest biomass ranges from 48 to 580 metric tons dry weight per hectare, with most mature forests being between 100–400 metric tons/ha in weight. The mean weight of all New Guinea mangroves is 285 metric tons/ha. Arguably the New Guinea mangroves are the largest stands yet recorded.
Critical to our ability to estimate the role of mangroves in fisheries and wood yield is an accurate estimation of net primary production. This is because primary producers and the carbon they fix via photosynthesis are the crux of mangrove food chains. About 2% of the radiant energy reaching the earth’s surface is used by plants to assimilate atmospheric CO2 into organic compounds used to construct new leaf, stem, branches, and root tissue, as well as to maintain existing tissue, create storage reserves, and provide chemical defense against insects, pathogens and herbivores.
Net production is the balance between gross photosynthesis and leaf dark
