Which of the following statements about tropical rain forests are correct? 1. The soils of tropical rain forests are quite infertile. 2. The vegetation is evergreen, enabling photosynthesis to take place year around. 3. They have been described as 'deserts covered by trees'. 4. They are most productive land-based ecosystem. Select the correct answer using the code given below.

The correct answer is D, 1, 2, 3 and 4. All four statements are true of tropical rain forests.

These forests grow near the Equator, in the Amazon, the Congo basin and South-East Asia, with heavy rain and high heat all year. Heavy rain leaches nutrients out of the soil, leaving it poor, often lateritic, and most nutrients are held in the living plants and the thin layer of litter. That is why the forests have been called 'deserts covered by trees': clear the trees and the land soon becomes barren. With no dry or cold season the trees stay evergreen and photosynthesise all year, which makes the rain forest the most productive ecosystem on land in net primary productivity.

A is wrong because it leaves out statements 1 and 3, which are correct. B is wrong because it leaves out statement 2, the evergreen nature of the forest. C is wrong because it leaves out statement 4; rain forests lead land ecosystems in productivity.

Exam tip: rain forest paradox: richest vegetation, poorest soil, because rain leaches nutrients and plants hold them.