Page:Niger Delta Ecosystems- the ERA Handbook, 1998.djvu/67

The Lowland Equatorial Monsoon Ecozone #

Which means literally 'without light'. Here the prolific nature of the euphotic layer means that the underlying Oligophotic layer is becomes increasingly dark towards the ground. There is great competition for the light that does reach the Oligophotic layer, and shade-tolerant plants, called the Sciophiles, grow here. As little as 1% of the available sunlight may reach the forest floor, which is therefore much less bioactive.


 * Within the Oligophotic layer a further three subdivisions can be defined:

A Dim Phase, where there is uniform shade, and which is the common condition for natural tropical rainforests;

A Light Phase, where light can reach the lower forest through small gaps made by fallen trees or big branches and where new tree seedlings may germinate; and

A Dark Phase beneath tangles of dead branches and living vines that have fallen with them, where there is no light.

#

This is another source of light within the Oligophotic layer. Flecks of sunlight may reach through the canopy directly, or be reflected down by the vegetation itself. The movement of the sun changes sunfleck position and intensity, but over short periods it may follow a daily pattern. Many plants in the Oligophotic layer depend on sunflecks to set off flowering and seed formation.

5.5.2 MICRO-CLIMATE: TEMPERATURE

We have discussed the seasonal and diurnal (daily) temperature cycles within the macroclimate of the tropical rainforest biome as a whole. However, micro-climatic temperature changes also occur.

The surface temperature of any living thing affects its activity. In the direct midday sunlight of the tropics, surfaces of human skin, naked soil and leaves can reach temperatures in the high thirties Celsius. When this happens, we humans can move into the shade; leaves may close their stomata and/or the whole leaf structure, wilt or turn away from the direct sunlight.

If naked soil reaches these temperatures, its microbiological life is killed. However, the forest floor is protected from such fierce heat, as we notice when we enter its shade during the day. In fact the temperature can be as much as 4 degrees lower than at the canopy, and oligophotic plants have adapted accordingly. However, this does mean that when forest is damaged or disturbed it is less well able to withstand the temperature stress than the euphotic plants. Lianes and other climbing euphotic plants tend to take over the lower forest stories when this happens, and create a real 'jungle'—thereby restricting the development of more economically useful tree species.

5.5.3 MICRO-CLIMATE: WATER

Water plays an important role in the microclimate of the tropical rainforest. While forests in the brackish and freshwater alluvial ecozones (the BAM and FAM) also receive 65