Seed Predation. Experiments conducted in Australia have shown that about 75% of the propagules of five species of mangrove trees were consumed by predators, primarily grapsid crabs, and there were significant differences among species and between forest types. Rhizophora stylosa was preyed upon the least and Avicennia marina the most. As might have been expected, predation was generally highest where a particular tree did not have neighbouring trees of the same species. Predation rates seem to be associated with chemical composition because A. marine, selected by predators over all the other species, had the highest concentrations of protein and sugars and the lowest concentrations of fibre and tannins. The propagules with the highest tannin content appeared to be preyed upon only by a single, specialist crabs (Smith 1986). These differences will influence the pattern of seedling establishment in a mangrove forest, such that establishment is most likely to occur in areas where seed predators are least likely to occur. In the case of A. marina this may mean establishment in the areas most frequently inundated.

      Geomorphology, physiology and seed predation thus appear to be the most relevant forest. The impact of human activities, so ubiquitous in coastal regions, plays a major role in modifying species composition and physical conditions, however, and so should be considered first in any 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).