Page:Niger Delta Ecosystems- the ERA Handbook, 1998.djvu/92

The Brackish Ecozone

water: Rhizophora racemosa, R. harrisonii and R. mangle. The White mangrove species Avicennia germinans and A. nitida colonise more shallow waters, together with two of the Black mangrove family, Conocarpus erectus and Laguncularia racemosa.

However, the Nigerian mangrove forests are dominated by the Rhizophora genus, mainly the stilt-rooted R. racemosa (in association with Avicennia nitida), and the shrub mangrove, Laguncularia racemosa. Rhizophora tends to be the pioneer, while Avicennia grows on firmer established ground.

Only a few other plant types are associated with these trees. Most commonly found are the Fern Acrosticum aureum, Bamboo, the Raffia Palm Raphia vinifera (not the raffia palm from which wine is tapped), the purple branched climbing shrub Hibiscus tiliaceus (?) and the exotic Nipa Palm (Nypha fructicans), introduced around 1919 from Malaya.

The Nipa Palm: Nipa Palm (Nypha fructicans) is an exotic species to Nigeria, and its first introduction was probably from Malaya (now eastern Malaysia) in the 1910s, when there was a misguided attempt to provide an alternative to Oil Palms as a source of palm-wine. The palm colonises conditions similar to those colonised by the indigenous Rhizophora trees. On new alluvial deposits, or where preexisting Rhizophora trees have been severely damaged, the Nipa Palm outcompetes the indigenous species. Unfortunately the Nipa Palm does not create new land or stabilise riverbanks; on the contrary, it has a destabilising effect and yet has none of the economic uses to which the native Rhizophora is traditionally put.

Around towns and industrial installations in the Niger Delta, the relatively useless Nipa Palm is the tree of the future.

7.3.1 RHIZOPHORA RACEMOSA

Rhizophora racemosa is the predominant species; an arched tangle of R. racemosa stilt roots gives the Niger Delta mangrove forest its characteristic appearance.

An ordinary root system would be unable to support a large tree on such swampy ground. R. racemosa roots do not penetrate the soil to any great depth; instead, they divide into a mass of thick rootlets immediately below the surface. The tree therefore stands upon a system of arches entirely supported by a thick, felt-like raft of its own making.

Larger trees also put out rubbery adventitious roots which drop down over the water.

Adventitious: in biology the term generally refers to plant parts growing from unusual positions, for example roots developing from a stem, or buds developing from a root.

Mature R. racemosa can reach 2.4 metres in circumference and 45 metres in height above the stilt roots, but the more usual maximum height is 20 to 30 metres. Mature individuals can grow to this size if they are the pioneers on fresh deposits of soft mud on the riverine or lagoon-side margins of the sub-ecozone. However, later plants, developing from seedlings that have started out on mature mangrove sites, may not

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