grow above 4 metres, as competition and impoverished soils prevent them from fully developing their root systems. It is therefore usual to find a vigorous outer fringe of mangrove forest where new silt is being deposited, with a lower shrubby tangle of smaller trees on the older soil inland.
The seeds of R. racemosa develop singly and are remarkable because they germinate and develop into a seedling, or propagule (see fig. II), while still hanging on the parent tree. The propagule is a complete young plant, with a strong stem about 30 cm in length and a hard, pointed and heavy root tip. When the propagule drops from the parent tree, the remaining fruit protects the growing point and allows the plantlet to float until the root tip catches on mud. If conditions are right, it will then sprout roots; the fruit detaches and leaf growth starts. The plant will survive if it is sufficiently well anchored to avoid being dislodged by currents or wave action, and upon depth (it must not be entirely submerged by the tide for long periods).
7.3.2 AVICENNIA
Growing in the most shallow inner zone of the mangrove forest, Avicennia species are also characterised by root type. Their stubby and vertical pneumatophore or 'breathing' roots grow upwards from submerged lateral roots; they rise above the reduced mud and water to obtain oxygen. Mature stands of Avicennia trees are widely spaced and straight, rising above a carpet of pneumatophores. Avicennia typically grow on the small tidal brackish-water creeks that penetrate the sand barrier islands.
Avicennia seeds also differ from those of Rhizophora; they develop in bunches and resemble soft dicotyledonous seeds. However they do also germinate on the parent tree, forming the first pair of leaves and a small pre-root before dropping and floating away until taking root in the next available mud patch. Although they are less hardy than the seedlings of Rhizophora, more of them are produced so that sufficient numbers will survive.
7.4 THE DYNAMICS OF MANGROVE ECOLOGY
Mangrove forests are dynamic over space and time, developing in continual succession. They serve as transitory communities, colonising and stabilising new ground before being replaced by more permanent vegetation. A typical mangrove forest could be described as follows:
- On the sides of the main rivers and lagoons, where fresh silt is being deposited, a belt of tall Rhizophora racemosa trees grow on the young 'Cat Clay' soils. Their adventitious roots drop downwards from stems hanging over the open water.
- On the more established new ground behind these tall trees is a tangle of much smaller R. racemosa. A Chicoco soil is developing, and adventitious roots are noticeably absent.
- Further inland, the trees become more tangled and shrublike; R. mangle individuals begin to appear.
- Where the soil has risen to such a degree that the long Rhizophora propagules are unable to float across it during the shallow tidal inundation, there tend to be only a few sparsely spaced shrubby trees. Avicennia are found here, because their small pre-germinated seeds are more successful in the shallow water and there is room for them to spread out their lateral roots.
