Page:Niger Delta Ecosystems- the ERA Handbook, 1998.djvu/84

The Freshwater Ecozone

The seasonal swamp soils have a higher clay content than the levee soils, with a thin peaty surface horizon; they are also relatively reduced.

Reduced soil conditions exclude many lowland tropical rainforest plants. However most palms, although not dependent on waterlogged conditions, are more tolerant of them than are the classic rainforest tree species. Palms cannot compete successfully for light in the LEM ecozones, but together with the bamboo they predominate in the swamps.

Bamboo. Members of the grass family, most bamboo have woody stems (although a few are soft herbs) enabling them to grow as tall as 40m. They grow in clumps, and their extensive underground 'roots' are actually a form of stem supporting both roots and shoots. They proliferate mainly via these rhizome-like systems, but will eventually fruit and then die.

However, the trade-off is one sided, because the number of plant species tolerating reduced conditions are limited. Swamp plant communities have low biodiversity.

Biomass in the swamps is also limited, partly because tree height is limited. Palms never grow to a great height, and other trees cannot develop sufficiently deep taproots in the waterlogged soil to support massive growth. On the other hand, bioactivity is high; above the actual water table, water is never a limiting factor.

6.4.3 THE FLOODPLAIN TROPICAL RAINFOREST SUB-ECOZONE

#The natural freshwater flood-plain tropical rainforest has the following Characteristics:

• An uneven canopy compared to the levee forest, as a result of the shorter life-span of the emergent species
• Trees with root systems that can tolerate high water-tables
• Soils that do not provide a very solid medium for growth
• Woody and herbaceous climbers
• The absence of ground vegetation except for mosses.

#Roots of the high water-table trees

Of special interest in the floodplain sub-ecozone is the variety of forms of tree roots. The high water table restricts their underground development, but the trees must still be anchored, and must still obtain oxygen. Various strategies have developed and include four main types:

• a 'floating mat' of palm tree roots
• buttresses, often very high, extending up to 5m
• stilt roots which may be as high as 3m high and act like divided stems
• Pneumatophore roots, which take in Oxygen above the surface and are of four types; lateral and serial knee-roots, root-knees, and peg-roots.

The ground of the flood plains is criss-crossed with small flood rivers, and rivulets that work their way around and sometimes underneath the tree roots.

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