Rainfall as a 'limiting factor' is also complex. Research in Amazon rainforests has shown that they reach their maximum height, not in the wettest zones, but in zones adjacent to forest savannah (where limited rainfall reduces tree population to a degree that grasses dominate ground cover). Excess rainfall seems to limit the growth of rainforest trees especially where drainage is limited, as seen in the river flood plains and swamps throughout the Niger Delta: rainforest plants, like most others, prefer dry feet.
There are essentially three major ecozones of the tropical rainforest biome: Lowland, Alluvial and Montane. Lowland and Alluvial rainforest would be the general case in the Delta, but there is no land of sufficient height for Montane tropical rainforest.
Montane Tropical Rainforest
In Africa, these are found on the great African mountains such as Uhuru (Kilimanjaro) and Cameroon. At higher altitudes, temperatures are lower. The main effect on tropical rainforests is that in comparison with the lowland forests, tree sizes are reduced, undergrowth is more abundant (often including tree ferns and small palms), the ground is rich in herbs and mosses, and lianes (vines) become more apparent. Bamboo forest is sometimes present. The forest is often in the clouds so that the shorter distorted trees are loaded with epiphytes (tree ferns, orchids and mosses), while the floor is covered in mosses, liverworts and herbaceous ferns. If the mountain is high enough the forest may become ericaceous (ericas are tough evergreen shrubs able to withstand very cold conditions); still higher, there may be alpine belt ecosystems similar to those of the European Alps.
Epiphytes: plants which grow upon other plants and use them for physical support, but which do not rely on them for food or water.
Tropical rainforests are not easy to study, which is why so much less is known about their ecosystems than about many others. Three reasons for this express the complexity of the rainforest.
Firstly, the sheer dimensions of tropical rainforest are a problem. A meaningful sample area must cover square kilometres; vertically, access must be gained to intermediate levels and to the canopy which may be over 30m high, then root systems must be followed as far as 5m down through a soil that is hard and crammed with vegetable life.
Secondly, the life span of the dominant life forms (mainly trees) makes for observation periods that exceed human life spans.
And thirdly, the high diversity of plants and animals is daunting; a square kilometre of natural Nigerian rainforest will contain thousands of plant species and countless more animal species.
5.3 CORRECTING SOME MISCONCEPTIONS ABOUT THE RAINFOREST
These points will help in understanding the descriptions and ideas that follow.
5.3.1 THE TROPICAL RAINFOREST IS NOT AN IMPENETRABLE JUNGLE
Rarely is the natural tropical rainforest a nightmare 'jungle' of vines and thick undergrowth. More often the dense tree canopy above ensures that light intensity on the
