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.

Q: What is the answer to CAPF 2021 Paper I Q84?

D. Correct — D, 1, 2, 3 and 4. Each statement holds. The soils are indeed poor: constant warmth and heavy rain leach the profile, so the nutrient stock sits in the living biomass and is recycled through a shallow root mat and rapid decay rather than stored in the ground, which is why cleared rain forest gives only two or three seasons of cropping before fertility collapses. The vegetation is evergreen because there is no cold or dry season to force leaf fall, so photosynthesis continues through the year. That combination of rich forest on poor ground is what earned the description 'deserts covered by trees' — a desert in the soil, hidden under the canopy. And the forests are the most productive land-based ecosystem there is, with net primary productivity above that of any other terrestrial biome, precisely because warmth, water and light never run short. (Provisional — pending the official UPSC key.)

(a)2 and 4 only
(b)1, 3 and 4 only
(c)1, 2 and 3 only
(d)1, 2, 3 and 4

Answer

Why

Correct — D, 1, 2, 3 and 4. Each statement holds. The soils are indeed poor: constant warmth and heavy rain leach the profile, so the nutrient stock sits in the living biomass and is recycled through a shallow root mat and rapid decay rather than stored in the ground, which is why cleared rain forest gives only two or three seasons of cropping before fertility collapses. The vegetation is evergreen because there is no cold or dry season to force leaf fall, so photosynthesis continues through the year. That combination of rich forest on poor ground is what earned the description 'deserts covered by trees' — a desert in the soil, hidden under the canopy. And the forests are the most productive land-based ecosystem there is, with net primary productivity above that of any other terrestrial biome, precisely because warmth, water and light never run short.

Why the others are wrong

(a)2 and 4 only — Drops the two statements about soil, both of which are true and are among the best-established facts about this biome. Poor soil under luxuriant forest is the paradox the ecosystem is famous for.
(b)1, 3 and 4 only — Drops the evergreen habit, which follows directly from the absence of a cold or dry season near the equator and is what allows productivity to run all year.
(c)1, 2 and 3 only — Drops the productivity claim. Tropical rain forest leads every terrestrial biome on net primary productivity, and the poverty of the soil does not contradict that, since the nutrients cycle through the vegetation instead of accumulating in the ground.

Concept

The tropical rain forest occupies the equatorial belt where temperature stays high and rainfall is heavy and well distributed. Its structure is layered — forest floor, understorey, canopy and emergents — and that vertical packing multiplies niches, which is why these forests hold the greatest species diversity on Earth. Nutrients move in a nearly closed loop: litter falls, decomposes within weeks in the warm damp conditions, and is taken straight back up by a dense mat of surface roots assisted by mycorrhizal fungi.

The closed loop is exactly why the ground beneath is poor. Rain percolating through a deeply weathered profile carries soluble bases away and leaves iron and aluminium oxides behind, giving the red, acidic, low-fertility soils of the humid tropics. Remove the forest and the loop breaks — the nutrient capital was in the trees, and once burnt and washed out there is little left, which is the ecological argument against converting these forests to permanent agriculture. Traditional shifting cultivation worked with that constraint by moving on and allowing regrowth; permanent clearance does not.

Key facts

Rain forest soils are deeply weathered, leached and low in nutrients; the nutrient stock is held in the biomass.
Litter decomposes within weeks and is taken up again by a shallow mat of roots and mycorrhizal fungi.
No cold or dry season means the vegetation is evergreen and photosynthesis continues year round.
Tropical rain forest has the highest net primary productivity of any terrestrial ecosystem.
The phrase 'deserts covered by trees' captures the contrast between rich vegetation and poor soil.

Testing the four statements

1. Soils are quite infertile→True — heavy leaching leaves oxisols and ultisols poor in bases; the nutrients are in the trees.

2. Evergreen, photosynthesis year round→True — there is no cold or dry season to force leaf fall near the equator.

3. 'Deserts covered by trees'→True — the standard description of rich forest standing on nutrient-poor ground.

4. Most productive land-based ecosystem→True — the highest net primary productivity of any terrestrial biome.

With statements 1, 2 and 4 all standing, only the option containing every statement can be right.

Study next

Net primary productivity across biomes and the position of the tropical rain forestNutrient cycling, litter decomposition and mycorrhizal associationsLaterite and the weathering profiles of the humid tropicsShifting cultivation, its ecology and its limits under population pressureThe latitudinal gradient of species diversity

Common traps

Assuming luxuriant vegetation implies fertile soil.
Reading the phrase 'deserts covered by trees' as a claim about rainfall rather than about nutrients.
Treating high productivity and poor soil as contradictory when they coexist through rapid recycling.

A four-statement item where the whole difficulty is the counter-intuitive pairing of poor soil with the most productive biome on land.

Related PYQs

UPSC_2011_GS1_Q232011Open & attempt →

If a tropical rain forest is removed, it does not regenerate quickly as compared to a tropical deciduous forest. This is because

(a) the soil of rain forest is deficient in nutrients
(b) propagules of the trees in a rain forest have poor viability
(c) the rain forest species are slow-growing
(d) exotic species invade the fertile soil of rain forest

Answer(a) the soil of rain forest is deficient in nutrients

The soil fact keyed on its own by UPSC. Poor soil under rain forest is not a marginal claim but the accepted reason these forests recover so slowly once cleared.

UPSC_2003_GS1_Q1102003Open & attempt →

Assertion (A): Unlike temperate forests, the tropical rain forests, if cleared, can yield productive farm-land that can support intensive agriculture for several years even without chemical fertilizers. Reason (R): The primary productivity of the tropical rain forest is very high when compared to that of the temperate forests.

(a) Both A and R are individually true and R is the correct explanation of A
(b) Both A and R are individually true but R is NOT the correct explanation of A
(c) A is true but R is false
(d) A is false but R is true

Answer(d) A is false but R is true

Both halves of the CAPF item settled in one row. Cleared rain forest does not sustain cultivation, because the soil is poor, while the forest's primary productivity really is the highest on land.

Practice

practice — not a real PYQ
The nutrients of a tropical rain forest ecosystem are held mainly in
- (a)the deeper soil horizons
- (b)the standing vegetation
- (c)the parent rock
- (d)groundwater
Answer(b) the standing vegetation — which is why clearance exhausts the site so quickly.
practice — not a real PYQ
Which terrestrial ecosystem has the highest net primary productivity?
- (a)Temperate deciduous forest
- (b)Tropical rain forest
- (c)Temperate grassland
- (d)Boreal coniferous forest
Answer(b) Tropical rain forest — warmth, moisture and light are available all year.