UPSC Prelims 2021 Q60: Identifying a Forest Biome from Its Description
"Leaf litter decomposes faster than in any other biome and as a result the soil surface is often almost bare. Apart from trees, the vegetation is largely composed of plant forms that reach up into the canopy vicariously, by climbing the trees or growing as epiphytes, rooted on the upper branches of trees." This is the most likely description of
- coniferous forest
- dry deciduous forest
- mangrove forest
- tropical rain forest
Answer: (d) tropical rain forest
Statement-wise explanation
- a: incorrect — Coniferous (taiga) forests grow in cold high-latitude or high-altitude climates where decomposition is slow. Needle litter builds up into a thick, acidic layer on the forest floor, so the soil surface is not bare. Their undergrowth is mostly mosses, lichens and low shrubs, not climbers and epiphytes reaching the canopy.
- b: incorrect — Dry deciduous forests grow where rainfall is low, about 70–100 cm a year in India, with a long dry season in which the trees shed their leaves. In the dry months the fallen leaves pile up and rot slowly, so the floor is not bare. The canopy is open and the undergrowth is grasses and shrubs, not epiphyte-laden branches.
- c: incorrect — Mangroves grow in the tidal zone of tropical coasts. Their defining features are salt tolerance, stilt roots, breathing roots (pneumatophores) that stick up from waterlogged mud, and seeds that sprout while still on the tree (vivipary). The passage says none of this. It describes no tides, salt or mud, only fast decay and a canopy full of climbers and epiphytes.
- d: correct — In tropical rain forests, constant heat and heavy rain all year keep decomposers active in every season. Leaf litter therefore breaks down faster than in any other biome, and the forest floor is often almost bare. The dense, multi-layered canopy lets little light reach the ground. Many plants reach the light by climbing trees (lianas) or by growing on high branches (epiphytes such as orchids, ferns and bromeliads), which is exactly what the passage describes.
How to eliminate
The passage gives two clues, and each one cuts the options down. The first is that decay is the fastest of any biome. Decay needs heat and moisture together. Coniferous forests are cold and dry deciduous forests have a long dry season, so both decay slowly and can be dropped. The second clue is that climbers and epiphytes reach into the canopy. That needs a tall, closed, multi-layered canopy, high humidity and year-round growth. Mangroves are short coastal forests whose defining traits (tides, salt, breathing roots) are never mentioned. Only the tropical rain forest fits both clues. Nothing here depends on a current fact. The thing to remember is that a hot, wet climate means fast decay and nutrient-poor soil, and a closed canopy means climbers and epiphytes.
Know more
Why rain forest soils are poor even though the forest is rich
Tropical rain forests (Koeppen Af) lie roughly within 10° of the equator. Every month is hot, with mean temperatures of 18°C or more, and rain falls all year. In India the matching forest is the tropical evergreen forest. It grows where annual rainfall is over 200 cm and the mean temperature is above 22°C: the Western Ghats, the north-east hills and the Andaman and Nicobar Islands.
With constant warmth and moisture, fungi, bacteria and termites recycle dead leaves within weeks. Roots take up the nutrients almost at once. Most of the nutrient stock sits in the living plants, not in the soil. Heavy rain also washes nutrients out of the soil, leaving red, leached, lateritic soils. That is why cleared rain forest land loses fertility within a few years.
The forest grows in layers: emergent trees, then the main canopy, then the understorey, then the dim forest floor. Light is the scarce resource. Plants have three main ways to reach it:
- Lianas, woody climbers rooted in the soil
- Epiphytes, such as orchids, ferns and bromeliads, which live on branches without taking food from the tree
- Stranglers, such as some figs, which start as epiphytes and later send roots to the ground
Big trees often have buttress roots because their roots are shallow.
| Feature | Tropical rain forest | Dry deciduous | Coniferous (taiga) | Mangrove |
|---|---|---|---|---|
| Climate | Hot, wet all year | Hot, long dry season | Cold, long winter | Hot, tidal coast |
| Leaf fall | No fixed season | Dry season | Needles kept | Evergreen |
| Litter decay | Fastest | Slow in dry months | Very slow, acidic layer | Slow in waterlogged mud |
| Typical feature | Lianas, epiphytes | Teak, open canopy | Needle leaves, cone shape | Breathing roots, vivipary |
NCERT Concepts to Revise
- Tropical evergreen forest: India's version of the biome in the passage; its rainfall and temperature limits explain why litter decays so fast there.
- Tropical humid climates (A): Group A Koeppen climates explain the year-round heat behind the decomposition rates the passage describes.
- Amazon rainforest: The world's largest example of the biome, useful for picturing the layered canopy, lianas and epiphytes.