Ecology of Sulawesi. Tony Whitten
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form a ripe fruit varied both between and within species but was about 1-2 months for Nypa, 2-6 months for Avicennia and about 15 months for Ceriops (Chris-tensen and Wium-Anderson, 1977; Wium-Anderson and Christensen 1978; Wium-Anderson 1981; Duke et al. 1984).
A few species of mangrove trees have evolved an unusual, though not unique, form of reproduction. Generally speaking, fruit develops on a plant and, when it is ripe or fully developed, the fruit or the seed inside it is then dispersed; the seed germinates when, or if, it comes to rest in suitable conditions. In most of the Rhizophoraceae such as Rhizophora and Bruguiera, however, the fruits ripen and then, before leaving the parent tree, the seeds germinate inside the fruit, possibly absorbing food from the tree. The hypocotyl (embryonic root) of the seedling pierces the wall of the fruit and then grows downwards. The cotyledons (first leaves) remain inside the fruit. Eventually, in Rhizophora mucronata for example, the root may reach a length of 45 cm. The seedling then drops off by separating it self from the cotyledon tube, the scar of which forms a ring around the top of the fallen seedling, and the small leaf-bud can be seen above this scar (fig. 2.15). Bruguiera behaves similarly, but the break occurs at the stalk of the fruit. This form of behaviour presumably allows the rapid establishment of the young plant. These types of fruit are described in more detail elsewhere (MacNae 1968).
Figure 2.15. The propagule of Rhizophora mucronata showing the root (often mistaken for part of the fruit) and the top of the seedling that detaches itself from the parent plant.
Zonation
Mangrove tree species tend to grow in zones or belts (figs. 2.16, 2.17 and 2.18). On gently sloping accreting shores (where sediment is being actively deposited), the forest nearest the sea is dominated by Avicennia and Sonneratia, the latter usually growing on deep mud rich in organic matter (Troll and Dragendorf 1931). On firm clay sediment A. marina is more common whereas on softer muds A. alba predominates (Ding Hou 1958). Behind these zones Bruguiera cylindrica can form almost pure stands on firm clays which are only rarely inundated by the tide. Further inland B. cylindrica becomes mixed with Rhizophora apiculata, R. mucronata, B. panriflora and Xylocarpus granatum (the canopy of which can reach 35-40 m). The mangrove forest furthest from the sea is often a pure stand of B. gymnorrhiza. Seedlings and saplings of this species are tolerant of shade but only under larger trees of other species; they are unable to grow under the canopy of their parents. This is presumably due to some chemical interaction. The boundary zone between mangrove forest and inland forest is marked by the occurrence of Lumnitzera racemosa,4 Xylocarpus moluccensis, Intsia bijuga (fig. 2.19), Ficus retusa, rattans, pandans, the stemless palm Nypa fruticans, and the tall spiny-trunked palm Oncosperma tigillaria. Where the mangrove forest has been opened, the most common undergrowth plant is the fern Acrostichum aureurn. Relatively steep-sided creeks, bays and lagoons are generally fringed with Rhizophora trees.
Figure 2.16. Zones of mangrove forest observed in part of Malangke.
After Anon. 1980a
Figure 2.17. Changing abundance of adults (above) and juveniles (below) of four tree species along a transect inland from the seaward edge of mangrove forest at Lainea, Kendari. Vertical bars represent 10 trees or seedlings/10 m2.
After EoS team
Recognizable zones may arise for two different reasons where neighbouring vegetation associations have little or no floristic affinity despite growing in the same environmental conditions (continuous variation), and where neighbouring environmental conditions are different enough to result in sudden changes between vegetation associations (discontinuous variation) (Bunt and Williams 1981). Thus vegetational changes can be continuous, discontinuous, or a combination of both. This is why it is crucial to consider scale before attempting an analysis of mangrove forest and why, in the absence of such consideration, data from isolated transects, even from the same area, are so hard to interpret. Comparisons of transect data from different sites are useful in compiling inventories and in noting similarities, but are not a basis for a discussion of zonation which should be based instead on air photos of, and ground surveys over, parts of a number of forests.
Figure 2.18. Profile diagram through somewhat disturbed mangrove forest at Lapangga, Morowali National Park. Note: the general predominance of R. apiculata and discrete occurrence of the other species.
After Darnaedi and Budiman 1984
The tendency of mangrove forests to occur in distinct zones has been interpreted variously by different authors as a consequence of plant succession, geomorphology, physiological ecology, differential dispersal of propagules and seed predation. Each of these is considered next.
Figure 2.19. Intsia bijuga. Scale bar indicates 1 cm.
After Soewanda (n.d.)
Plant Succession. Plant succession is a classic ecological concept and is defined as being the progressive replacement of one plant community with another of more complex structure (p. 366). Much of the early work on mangrove forests focused on its supposed land-building role and it seemed clear from this that one species colonized an exposed bank of mud and, as conditions changed (such as an increase in the organic debris of the mud), so other species took over. For example the colonization of a new shallow or exposed substrate by Avicennia or Sonneratia trees, such as on the banks of the Rongkong River delta at the north of Bone Bay, produces a network of erect pneumatophores which have three indirect functions:
Figure 2.20. Succession in mangrove forests.
After Chapman 1970 in Walsh 1974
they protect the young trees and germinating seeds from wave damage;
• they entangle floating vegetation which decays and becomes incorporated in the soil; and
• they provide a habitat for burrowing crabs which help to aerate the soil (Chambers 1980).
These changes, and subsequent additional sedimentation, lead to a succession of species or communities of species over a period of time (fig. 2.20). It is clear, however, that the stages of succession are not always consistent and different local environmental conditions and man's impact on those have an influence (Steup 1941; Anon 1980a).
A total of 20 characteristics have been noted for secondary succession in tropical forests (Boudowski 1963). If the characteristics are examined in relation to the succession of mangrove forests, only seven of the characters apply, nine do not, and four are inconclusive. This suggests that attributing the apparent zonation to succession is not the whole story (Snedaker 1982b).
Geomorphological Change. As stated above, early workers on mangrove felt that it was mangroves that 'built land'. However, it is clear from observations in the huge deltas of the Ganges, Indus and Irrawaddy on the coast of the Indian sub-continent, that it is the process of sediment deposition that builds land. It is now generally agreed that mangroves do not have any influence on the initial development of the land forms. Mangroves may accelerate land extension but they do not cause it (Ding Hou 1951).
From a geomorphological perspective, it is the shape, topography and history of the coastal zone that determine the types and distributions of mangrove trees in the resulting habitats. The position of species relative to tidal levels (and thus soil type) is obviously important and the pattern of tidal inundation and drainage has been considered to be the major factor in mangrove zonation (Watson 1928). This idea has been developed to include variation in the salinity of the tidal water and the direction of its flow into and out of the forest (Lugo and Snedaker 1974). Although good correlations exist