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The Lowland Equatorial Monsoon Ecozone Alluvial Equatorial Monsoon ecozones (the BAM and FAM), and then the Lowland Equatorial Monsoon or LEM. Where the Adamawa massif juts into Nigeria, and in the south-facing valleys of the Yoruba uplands, there are islands of seasonal tropical rainforest, where there is a marked dry season. The northernmost fringe of the Nigerian tropical rainforest biome is a forest-savannah mosaic.

Another source of variation in the West African rainforests is age. Between 15 and 18 thousand years ago, an ice age in higher latitudes caused a squeezing of the climatic bands towards the equator. The West African rainforest shrank back until it was restricted to refuge areas that correspond roughly to the present-day areas of very high rainfall shown in Map 1. These areas, including the Niger Delta and rainforest to the East, are therefore substantially older than those to the West—perhaps by hundreds of thousands of years.

5.5 MICRO-CLIMATE OF THE LEM RAINFOREST

Interactions between tropical rainforests and their environment are far more complex than those of temperate forests, and this also complicates their study. The most important interactions are between roots and soil, and between leaves and the atmosphere. These are relatively straightforward in temperate forest, where the former occurs below ground and the latter above ground. In rainforest, however, it is not so simple.

For example, the nature and location of soil itself is difficult to define because of the spatial confusion of the various soil and semi-soil components: litter, detritus, humus, inorganic particles, organic compounds, and the soil-water and soil-atmosphere interfaces. Apart from the parasitic plants, all plants need soil of some sort and since rainforest trees give physical support all sorts of other plants—in branch forks, in holes made by animals, on stems and branches, and in any nook or cranny—there is soil in places other than on the ground.

Similarly, in the rainforest we find bodies of water other than in the ground, often high in the crown of trees. Whole aquatic ecosystems will live in these water tanks: some, such as tree frogs, never coming into contact with the ground in their entire life cycle.

The microclimate within a rainforest has evolved with the plants themselves; upon it they and their associated animals depend for survival. There are four prime aspects of the tropical rainforest microclimate.

5.5.1 MICRO-CLIMATE: LIGHT

Light intensity can be defined as bright, dim and dark. Photosynthesis, and therefore bioactivity, is at its maximum in the areas of bright light.

Two main vegetative layers can also be defined within tropical rainforest according to the light intensity they receive:

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Which receives light, made up of light-demanding or 'Heliophilic' plants. This layer includes the tree canopy with its associated animal life, Heliophilic epiphytes such as orchids, and lichens. The Euphobic layer receives between 25 and 100% of the sunlight falling on the forest, so that bio-activity is prolific and there is a dense pack of leaves, flowers and seeds. 64