Mangrove trees are tolerant of saline soils, that is they are halophytes (Walsh 1974), but they are facultative rather than obligate in that they can also grow successfully in freshwater. This is demonstrated by the growth, fruiting and germination of Bruguiera sexangula, B. gymnorrhiza, and Sonneratia caseolaris (fig. 2.12) in the Botanic Gardens in Bogor (Ding Hou 1958). This is typical of many plants that might be regarded as restricted to growing in certain soil conditions. In fact, there are many organisms which exist not where they fare best, their 'fundamental niche', but where they grow most successfully in competition with other species, their 'realized niche'. Mangrove trees can sometimes be seen growing at the sides of rivers in seemingly freshwater conditions but in these cases there is generally a wedge of (heavier) salt water permanently or periodically near the bed of the river which maintains saline conditions for the tree roots. The occurrence of these trees in such locations may also be due to the flooding regime of the river (J. Davie pers. comm.).

      Mangrove forests, particularly those that are frequently flooded, differ markedly from dryland forests and from most inland swamp forest in the virtual absence of climbing and understorey plants (Ding Hou 1958). In essence, the only plants that grow in mature mangrove forests are trees whose crowns reach the canopy. The reason for this has yet to be confirmed, but it would seem that regular tidal (rather than seasonal) flooding is the most important factor, rather than a difference in salt tolerance between trees and smaller plants (Janzen 1985; Corlett 1986). Exactly how this flooding acts against the establishment of herbs has yet to be determined. Living in saline conditions is clearly costly in terms of energy, and this is supported by the observation that mangroves were killed after only a single spraying with defoliants in Vietnam, whereas nearby terrestrial trees had to be sprayed several times to achieve the same results (Janzen 1985).

      Most trees of the mangrove forest have developed peculiar root systems to allow for gaseous exchange above a water-logged and anoxic soil (Mann 1982) (fig. 2.13). Such 'breathing roots' are known as 'pneumatophores'. The stilt roots of Rhizophora may also be effective in preventing the growth of seedlings too close to a growing tree. These stilt roots are generally unbranched but secondary or tertiary branching can occur due to damage of the primary root tip by scolytid beetles (Docters van Leeuwen 1911) or by boring isopod crustaceans (Ribi 1981, 1982; Whitten et al. 1984) (fig. 2.14).

      The roots of Sonneratia and Avicennia are similar in gross structure and consist of a horizontal cable root held in place by anchor roots growing vertically downwards. The pneumatophores grow upwards from the cable roots and, small nutritive roots grow horizontal from these. As mud is deposited on the forest floor, so new nutritive roots are produced higher up the pneumatophores. In Bruguiera the cable root loops in and out of the soil and the exposed 'knee-roots' act as pneumatophores. Ceriops does not have special root adaptations but its bark has many large openings, or lenticels, to assist in gas exchange. If oil drifts into a mangrove forest the lenticels on the exposed parts of the pneumatophores become clogged and this is the primary reason that trees so afflicted will die. The dark oil also causes the water temperature to rise and the concentrations of dissolved oxygen to fall (Mathias 1977; Lugo et al. 1978; Getter et al. 1984). Mangrove trees that survive exhibit signs of chronic stress such as reduced productivity and gradual leaf loss (Lugo et al. 1978; Saenger et al. 1981). These effects can be quite local, however, as is evidence by the relatively small patches of dead mangrove trees around the natural oil seeps on the shore near Kabali, southwest of Luwuk.

      The feathery flowers of Sonneratia are superficially similar to those of the Myrtaceae (such as rose apples Eugenia spp., and eucalypts Eucalyptus) and in common with many of those, are pollinated by bats which may fly up to 40 km from their inland roost when Sonneratia is in flower (Start and Marshall 1975). The flowers of Rhizophoraceae have a range of mechanisms by which they effect pollination. For example, the anthers of Bruguiera open explosively when the flowers are visited by sunbirds Nectarinia (in the large flowered such as B. gymnorrhiza), or butterflies and other insects (in the smaller-flowered species such as B. paruiflora) in search of nectar. Ceriops tagal also has explosive anthers, triggered largely by moths. Rhizophora flowers are largely wind-pollinated but bees may also be involved (Ding Hou 1958; Tomlinson et al. 1979). Sunbirds can sometimes be seen visiting Rhizophora trees but this is largely to lick the sweet, sticky exudate from leaf buds or young flowers which have been slightly damaged by insects. Since the sunbirds also eat insects the birds help to reduce the damage to Rhizophora by scale insect Coccidae (Christensen and Wium-Anderson 1977; Primack and Tomlinson 1978; Wium-Anderson and Christensen 1978; Wium-Anderson 1981).

      Figure 2.13. Different types of roots in mangrove trees.

      Figure 2.14. Branching of Rhizophora mucronata roots caused by small scolytid beetles burrowing into the pith of the root tip.

      After Doctors van Leeuwen 1911

      Observations of fruiting and flowering of mangrove trees in Australia and Thailand showed that there was activity in every month but that most species flowered during the dry season, and dropped ripe fruits during periods of peak rainfall. This pattern is very similar to that often found in dry lowland forests (p. 366) and is probably related to insect abundance. The production of new leaves was depressed when fruit and flower production were maximal. The time it took for a flower

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