Ecology of Indonesian Papua Part Two. Andrew J. Marshall
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 respiration, and represents the amount of carbon available for growth and tissue maintenance. Photosynthesis varies with many factors, especially light intensity, temperature, nutrient and water availability, salinity, tidal range, stand age, species composition, wave energy, and weather. Five methods have been used to measure mangrove forest primary production: litter fall and incremental growth of the stem, harvesting, gas exchange of leaves, light attenuation/gas exchange under the canopy; and demographic/allometric measurements of trees.
Figure 5.4.5. Mangrove forest biomass as a function of latitude. Nearly all data points between 2–8 S Latitude are from New Guinea.
Source: Modified from Alongi (2006).
Litter fall is by far the most common method used because it is inexpensive and easy to measure, but it only measures leaf production and not growth of the remainder of the tree. Two studies have measured mangrove litter fall in New Guinea, but unfortunately, both took place near Port Moresby where rainfall is less than on the rest of the island (Leach and Burgin 1985; Bunt 1995). Both sets of values indicate very high rates of litter fall (> 1,000 g dry weight per m2 per yr). Seasonally, as in other places, maximum litter fall is cued to the onset of the summer wet season (January–March), although different species flower at different times of the year.
Harvesting is labor intensive and slow, and accounts for only above-ground production. It often does not account for leaf production. Gas exchange is precise and rapid, although subject to error due to the problem of extrapolating from an individual tree to an entire stand. Moreover, relying solely on gas exchange measurements overestimates net production as it does not account for most tree respiration.
Combining measurements offers the best hope of accounting for production of all, or most, tree parts. Measuring litter fall and incremental growth of the trunk accounts for all above-ground production, but not below-ground production. Arguably one of the best methods is to measure light attenuation. The method relies on relating the amount of light absorbed by the mangrove canopy to the total canopy chlorophyll content. The early efforts (e.g., Bunt, Boto, and Boto 1979) provided rapid and relatively easy estimates of potential net primary production. The method, however, suffers from lack of actual photosynthesis measurements and a number of untested assumptions based on light attenuation models from temperate forests. Four workers subsequently modified the light attenuation method, combining measurement of light attenuation with a more robust method of calculation of photon flux density at the bottom of the canopy and empirical measurements of leaf photosynthesis (Gong, Ong, and Wong 1991; Gong, Ong, and Clough 1992; Clough 1997; Clough, Ong, and Gong 1997).
Litter fall underestimates, and gas exchange overestimates, net primary production,