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, but the modified light attenuation method gives the most reasonable estimate of total production, while litter fall plus incremental growth can give reasonable estimates of above-ground production (and excluding below-ground root production). The modified light attenuation method is most reasonable because it measures total net fixed carbon production and incorporates the most robust assumptions based on tree physiology and carbon balance. A number of recent studies have measured above-ground production using allometry (relationships of tree weight to stem diameter) coupled with litter fall or leaf turnover (Duarte et al. 1999; Coulter et al. 2001; Ross et al. 2001; Sherman, Fahey, and Martinez 2003).
The estimates of net primary production made using the modified light attenuation method include below-ground production but there is currently no clear understanding of how carbon is allocated to different parts of the tree. As pointed out by Clough (1998), it is not yet possible to construct a robust model of carbon balance for mangrove trees because of the lack of empirical data and the difficulty of measuring root processes and respiration of woody parts. However, some preliminary carbon data for mangroves suggest that roughly half of carbon incorporated into the tree is respired, an estimate that is in agreement with similar estimates for terrestrial trees (Barnes et al. 1998).
If we accept the data obtained using the modified light attenuation method as the most comprehensive estimate of net primary productivity of mangroves, the average rate of net primary production in New Guinea and surroundings (Table 5.4.3) is 51 tons dry weight per ha per yr. There is considerable range between values, but the figures do suggest that mangroves are significant primary producers. This is supported by empirical measurements of rates of leaf photosynthesis in mangroves (Clough and Sim 1989). Measuring gas exchange characteristics and water use efficiency for various mangrove species located on the Era, Wapo, and Ivi rivers along the Gulf of Papua, Galley Reach, and Motupore Island, Clough and Sim (1989) measured the most rapid rates of carbon dioxide uptake, stomatal conductance, and water-use efficiency yet measured. These high rates of CO 2 as-similation and other physiological attributes are a reflection of favorable climatic conditions.
Plotting all available data on mangrove productivity against latitude (Figure 5.4.6) gives a significant negative relationship, indicating that mangrove production declines away from the equator, mirroring the latitudinal decline in mangrove biomass (Figure 5.4.5) and litter fall (Saenger and Snedaker 1993). These graphs show that the mangroves of Papua are well placed geographically and climatically to grow to immense size and are as productive as any other tropical forests in the world.
Fauna
Mangrove forests in Papua support a wide diversity of biota, ranging in size from bacteria to crocodiles, and like most mangroves, house species originating from both land and sea. The fauna of the Papuan mangroves is poorly known. Most information comes from faunal surveys along the southern coast of Papua New Guinea and the west coast of Papua. Generally, there is a high level of similarity between the northern Australian and Papuan faunas (Macnae 1968); differences in species recorded are likely due to the lack of surveys in New Guinea rather than any real biogeographical anomalies.
Figure 5.4.6. Latitudinal changes in net primary production measured using a modified light interception method.
Source: Data from Gong, Ong, and Wong (1991); Gong, Ong, and Clough (1992); Atmadja and Soerojo (1991); Robertson, Daniel, and Dixon (1991); Sukardjo (1995); Clough, Ong, and Gong (1997); Clough (1998); Alongi and Dixon (2000); Alongi, Tirendi, and Clough (2000); Alongi et al. (2004).
The most comprehensive studies of the mangrove fauna of the island have been conducted in the Purari and Fly river deltas bordering the Gulf of Papua (White and White 1976; Liem and Haines 1977; Bayley 1980; Cragg 1983; Liem 1983; Pernetta 1983). The mangrove-associated fish fauna of Bintuni Bay in Papua has also been surveyed (Ecology Team 1984; Erftemeijer et al. 1989), but most of the species lists reflect attention to species of commercial or subsistence use and must be considered incomplete.
Of the mangrove vertebrates, 30 species of reptiles, 12 species of amphibians, 250 species of birds, 50 species of mammals, and 195 species of fish have been recorded on the island thus far (Appendix 8.3). Mangrove invertebrates have not been as well investigated, with the exception of commercially valuable groups, such as crabs and shrimps. The best described groups include the mollusks and insects, the most conspicuous of the latter being the Anopheles mosquito which is the vector for malaria and filariasis.
The mangrove forests of Papua support a rich molluscan and crustacean fauna consisting of approximately 95 and 80 species, respectively (Kartawinata et al. 1979; Sabar, Djajasamita, and Budiman 1979; Kastoro et al. 1991). Numerically, gastropods are the dominant group of mollusks, with Littorina scabra frequently found at the seaward margin in large numbers, with Monodonta labio a co-occurring species (Soemodihardjo 1987).
