Tropical forest: distribution, climate and biomass

Overview

Temperature characteristics of a tropical forest climate, rainfall regime of tropical forest and explain what causes it and layered structure of tropical forest vegetation.

What you need to know

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Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Where in the world is tropical forest found?

   Model answer

   In a discontinuous belt within about 5-10 degrees of the equator, where the ITCZ is present for most of the year. The three largest blocks are the Amazon basin in South America, the Congo basin in central Africa, and the islands and peninsulas of South East Asia, with smaller areas in Central America, West Africa, Madagascar and north-east Australia. The belt is broken where continents are absent, and it extends further from the equator on east-facing coasts where onshore winds and warm currents bring extra rainfall.

2. 2

   What are the temperature characteristics of a tropical forest climate?

   Model answer

   Mean monthly temperatures are consistently high, around 26-28 degrees Celsius, and the annual range is remarkably small, typically only 2-3 degrees. This is because day length and the angle of the sun barely change through the year at the equator, so insolation is almost constant. The daily range, around 8-10 degrees, is therefore greater than the annual range, which is why the climate is sometimes described as having no seasons, only days and nights.

3. 3

   Describe the rainfall regime of tropical forest and explain what causes it.

   Model answer

   Rainfall is high, generally above 2000 mm a year and often well over 2500 mm, and it falls in every month, with no month reliably below about 60 mm. It is convectional: strong morning heating and very high humidity produce rapid evaporation and convection, cumulonimbus towers build during the day, and heavy downpours with thunder and lightning fall in the afternoon or early evening, often in short, intense bursts. High evapotranspiration from the forest itself recycles moisture back into the atmosphere and helps to sustain the rainfall.

4. 4

   Why is humidity so persistently high in tropical forest?

   Model answer

   Temperatures are high, so evaporation from wet ground, from standing water and from the leaf surfaces of a dense, permanently active vegetation cover is rapid and continuous. Transpiration from an enormous leaf area adds to it. Because there is no cold season and no dry season to interrupt the process, the air is kept close to saturation, with relative humidity commonly around 80-90 per cent and often higher at night.

5. 5

   Describe the layered structure of tropical forest vegetation.

   Model answer

   There are usually four or five recognisable layers. Emergents are isolated giant trees rising to 45-50 m above the general canopy, exposed to full sun and wind. The canopy is a continuous, closed layer of crowns at roughly 30-45 m that intercepts most of the light and most of the rainfall. Beneath it is a discontinuous under-canopy of younger and smaller trees at about 15-25 m. A sparse shrub layer occupies the lowest few metres, and the ground layer is dark, humid and surprisingly open, with little more than seedlings, ferns and a thin litter of decaying leaves.

6. 6

   If tropical forest is so productive, why is the forest floor relatively bare?

   Model answer

   Because the canopy intercepts as much as 95 per cent of the incoming light, so almost nothing at ground level receives enough energy for photosynthesis. The image of impenetrable jungle applies only where light reaches the ground, along river banks, at forest edges and in clearings where a large tree has fallen, and that secondary growth is dense precisely because the light gap is temporary.

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