study of zonation.
Biomass and Productivity
'Biomass' is a term for the weight of living material usually expressed as dry weight, in all or part of an organism, population or community. It is commonly expressed as the 'biomass density' or 'biomass per unit area'. Plant biomass5 is the total dry weight of all living plant parts and for convenience is sometimes divided into above-ground plant biomass (leaves, branches, boughs, trunk) and below-ground plant biomass (roots). It appears that no study of mangrove biomass has yet been conducted in Sulawesi but several studies have been conducted in Peninsular Malaysia (Ong et al., 1980a, b; 1985). In one undisturbed forest the biomass was found to be between 122 t/ha and 245 t/ha, but in another, which has been exploited and managed for timber on a sustained basis for 80 years, the biomass of trees was 300 t/ha (Ong et al. 1980b). As is explained below, the higher biomass in managed forest is not unexpected. Above-ground biomass in Australian mangrove forests has been found to correlate with parameters of soil quality such as extractable phosphorus, redox potential, and salinity (Boto et al. 1984).
Biomass is a useful and a relatively easy-to-obtain measure but it gives no indication of the dynamics of an ecosystem. Ecologists are interested in productivity because, if the dry weight of a community can be determined at a moment, and the rate of change in dry weight measured, then the rate of energy flow through an ecosystem can be calculated. Using this information different ecosystems can be compared, and their relative efficiencies of converting solar radiation into organic matter can be calculated.
Plant biomass increases because plants secure carbon dioxide from the atmosphere and convert this into organic matter through the process of photosynthesis. Thus, unlike animals, plants make their own food. The rate at which a plant assimilates organic matter is called the 'gross primary productivity'. This depends on the leaf area exposed, amount of solar radiation, temperature and upon the characteristics of individual plant species (Whitmore 1984). Plants, like all other living organisms, respire and use up a proportion of the organic matter produced through photosynthesis. What is left after respiratory loss is called 'net primary productivity' and the accumulation over a period of time is termed 'net primary production'. Net primary productivity is obviously greatest in a young forest which is growing and it should be remembered that a dense, tall forest with a high biomass does not necessarily have a high net primary productivity. Large trees may have virtually stopped growing. Indeed in an old 'overmature' forest, the death of parts of the trees and attacks by animals and fungi may even reduce the total plant biomass while net primary productivity remains more or less constant. The major aim of silvicultural management in forests of timber plantations is to maximize productivity and so the trees are usually harvested while they are still growing fast and before the net primary productivity begins to decrease too much (fig. 2.21).
One means of assessing net primary production is to measure the rate at which litter is produced. The production of litter appears to be very similar between the sites examined in the Indo-Australian region, being about 7-8 t/ha/yr for leaf litter, and 1-1.2 t/ha/yr for all small litter (mainly leaves, but with twigs, flowers and fruit) (Ong et al. 1985; Woodroffe 1985). The total litter production of mangrove forests in Peninsular Malaysia and Papua New Guinea has been found to be about 14 t/ha/yr (Sasekumar and Loi 1983; Leach and Burgin 1985), which is similar to results obtained in Queensland (Duke et al. 1981), and therefore probably similar to that found in Sulawesi. Interestingly, these figures are similar to or higher than those obtained in lowland forest (p. 365) and support the contention that mangroves grow, reproduce, and die fast (Jimenez et al. 1985), similar to dry lowland forest on young terraces (p. 361).
Figure 2.21. Changes in: a - biomass, b - mean annual increment, c - litter, and d - net productivity in four even-aged stands of Rhizophora apiculata in Peninsular Malaysia. In the forest from which these data were collected, the trees are harvested on a 30-year rotation.
After Ong et al. 1985
Variation in net primary production in mangrove forests in northeast Australia and Papua New Guinea has been ascribed to availability of phosphorus (Boto et al. 1984), which is consistent with the view that nitrogen and phosphorus are limiting in coastal marine environments (Rhyther and Dunstan 1971). The approximate annual accumulation of litter on the mangrove forest floor at Merbok was calculated to be 0.33 t/ha for leaf litter and 1.13 t/ha for total litter. An experiment at the same site revealed that 40%-90% of fallen leaves were lost after 20 days on the forest floor. The major agents in the disappearance were probably crabs which either bury them or eat them, later to be excreted as detritus. The importance of the crabs is seen when they are prevented from reaching the leaves, in which case the time needed for total decomposition was 4-6 months (Ong et al. 1980a).
The detritus becomes rich in nitrogen and phosphorous because of the fungi, bacteria and algae growing on and within it and is therefore an important food source for many 'detritivore' animals such as zooplankton, other small invertebrates, prawns, crabs, and fish. These detritivores are eaten in turn by carnivores which are dependent to varying degrees on these organisms. It is probable that most of the micro- and macro-fauna in the mangroves and surrounding coastal areas are dependent on the productivity of litter from mangrove forests (Ong et al. 1980a, b). A major initiative to study the important issue of transport of material in mangrove estuaries in the Indo-Australian region is currently underway (Ong et al. 1985).
Mangrove forests are highly productive ecosystems but only about 7% of their living leaves are eaten by herbivores (Johnstone 1981), and most of the mangrove forest production enters the energy system as detritus or dead organic matter (fig. 2.22). This detritus plays an extremely important role in the productivity of the mangrove ecosystems as a whole and of other coastal ecosystems (Lugo and Snedaker 1974: Ong et al. 1980a, b; Saenger et al. 1981; Mann 1982). Its importance to offshore ecosystems is not clear (Nixon et al. 1980).
The high productivity of mangroves and the physical structure and shading they provide forms a valuable habitat for many organisms, some of which are of commercial importance. At present the most valuable mangrove-related species in Indonesia are the penaeid prawns. The juvenile stages of several of these prawn species live in mangrove and adjacent vegetation, while the adults offshore (Soegiarto and Polunin 1980).
The influence of mangroves extends far beyond the prawn fisheries (p. 187). For example, carbon from mangrove trees has been found in the tissues of commercially important bivalves such as the cockle Anadara granosa, oyster Crassostrea, shrimps such as Acetes, used in the making of belacan paste, crabs such as Scylla serrata, and many fish (Rodelli et al. 1984) such as mullet Mugil, milk fish Chanos and barramundi or giant perch Lates (MacNae 1968; Moore 1982; Polunin 1983).
Figure 2.22. Major pathways of energy flow in a mangrove-fringed estuary.
After Saenger et al. 1981
OTHER COASTAL VEGETATION
There are three main types of beach vegetation: the pes-caprae formation, the Barringtonia formation and the vegetation of rocky shores. The removal through development of the vegetation on a sandy beach may not be regarded as a particularly serious loss in itself, but its ability to hold together a loose sandy substrate means that in its absence more or less continuous coastal erosion occurs. This, and its resultant impact on human settlements is most damaging during severe storms, because the power of the wind and waves is no longer countered by deep-rooted vegetation.
Pes-caprae Formation
The pes-caprae formation is found along sandy beaches which are actively accreting; that is, where sand is being deposed, or on already-developed beaches that are now being eroded. Its name is derived from the conspicuous, purple-flowered creeper with two-lobed
