Biomes, Forest Types and Ecological Succession: Biogeography Notes Part 2

These biogeography notes for OPSC OAS Part 2 cover the environmental controls on vegetation, forest structure and ecological succession, the concept and classification of biomes, and the major world biomes — tropical rainforest, tropical deciduous forest, desert, savanna, temperate grassland, Mediterranean and taiga — along with mangroves and the forests of Odisha. Soil formation, processes, profile and classification are covered in Part 1.

Environmental Controls on Vegetation

Vegetation is the visible expression of climate, soil and geomorphic setting, and it feeds back into all three. Four groups of controls operate together.

Leaf size responds to both temperature and moisture, which is why the largest leaves occur in the warm, wet tropics and the smallest in cold or arid zones. Species richness rises from the poles towards the equator — the latitudinal biodiversity gradient. Plants of tropical and subtropical regions are classified as megatherms, followed by mesotherms (temperate), microtherms (cold temperate) and hekistotherms (arctic-alpine).

CDS 2020 asked which geomorphic factors influence plant and animal distribution — slope angle, slope aspect and relative relief together.

Forest Structure and Ecological Succession

A natural forest is layered vertically. From top to bottom the sequence is canopy, shrub layer, herb layer and mosses or litter. Light, humidity and temperature change sharply down these layers, which is why each stratum has its own fauna.

Ecological succession is the orderly replacement of one community by another until a stable climax is reached. The full sequence is a sere; each step in it is a seral stage; the first coloniser of bare ground is the pioneer community; and the end state is the climax.

Interspecific competition drives each replacement: early species modify the site, later species outcompete them, and the community converges on the climax. Disturbance before climax yields a sub-climax community.

The Biome Concept

Plant communities dominate the character of a biome and define its boundaries, though a biome includes the associated animals, soil and climate. Biomes are divided into regions on the basis of climatic variation. A biochore is a still larger biogeographic unit, and an anthropogenic biome is one created and maintained by human activity, of which cropland is the standard example.

UPPSC 2018 asked which is an anthropogenic biome — cropland.

Tropical Rainforest Biome

The tropical evergreen rainforest lies roughly between 10°N and 10°S, with rainfall above about 200 cm spread through the year and uniformly high temperature. It is the megathermal biome, and it holds the greatest plant biomass and the largest number of species of any terrestrial biome.

• Structure: multi-storeyed canopy with emergents, dense undergrowth, buttress roots, lianas, epiphytes and aerial roots.
• Leaf habit: evergreen, with leaf fall staggered so the canopy is never bare.
• Soils: deeply weathered, nutrient-poor oxisols and laterites; nutrients are held in the biomass and recycled rapidly.
• Distribution: Amazon basin, Congo basin and Southeast Asia — not California or the north-western African coast.

Because conditions are optimal, the biome is described as an optimum biome, which is why species richness is highest there. Its main threats are deforestation for timber, cattle and plantations, and fire.

Astt. Comm. 2017 asked which statements about the equatorial region are correct — that it is megathermal and a region of aerial streamlets (epiphytes and aerial roots).

Tropical Deciduous Forest Biome

Tropical deciduous forests occupy the monsoon belt with a marked dry season and rainfall of roughly 100–200 cm. Trees shed their leaves in the dry season to reduce transpiration, so the biome has two distinct seasons and a bare canopy for part of the year.

Species include sal, teak, tendu, mahua and bamboo; climbers are present but fewer than in the rainforest; and species richness is lower than in the evergreen forest. This is one of the most human-disturbed biomes, having been cleared extensively for agriculture and settlement. Northern Australia, parts of India and Southeast Asia and the drier margins of the Amazon carry this formation.

The tropical deciduous forest is not evergreen. Seasonal leaf fall, two distinct seasons and monsoon climate are its defining features.

Desert Biome and Xerophytic Adaptation

The world's great hot deserts lie on the western margins of continents in the trade wind belt — Sahara, Arabian, Thar, Kalahari, Atacama, Namib, Sonoran and Australian. Their aridity results from descending dry air of the subtropical high, offshore trade winds, and in coastal deserts, cold ocean currents that stabilise the air and suppress rainfall.

Desert vegetation must tolerate aridity, saline soils and humus deficiency at the same time. Xerophytes adapt through succulence (cacti, euphorbias), thick stems for water storage, reduced or spiny leaves to cut transpiration, sunken stomata, waxy cuticles and deep or widely spreading roots. Small leaf size in dry regions is primarily a device to reduce transpiration. Dryness also slows decomposition, so litter persists.

Other useful terms: halophytes tolerate salinity, mesophytes need moderate moisture and hydrophytes live in water.

Savanna Biome

The savanna lies roughly between 10° and 20° latitude in both hemispheres and is tropical mixed vegetation dominated by grasses with scattered trees. It has a sharp alternation of a hot rainy season and a cool dry season, which is why water availability controls tree growth and why grasses dominate.

Savanna trees such as acacia and baobab are pyrophytic — thick corky bark and resprouting ability let them survive periodic fires. Fire is not incidental but structural: regular burning prevents closed forest from establishing and maintains the grass-tree balance. The African savanna, with its great herds and predators, is called the Land of Big Games.

UPPSC 2019 asked which region is called the Land of Big Games — the tropical savanna.

Temperate Grasslands

Temperate grasslands occupy continental interiors in the mid-latitudes, where rainfall is moderate and the annual temperature range is large. Their grasses are shorter but more nutritious than tropical grasses, which is why these regions became the world's great wheat belts.

Their soils are the dark, base-rich mollisols and chernozems, among the most fertile in the world. Southern Hemisphere grasslands such as the Pampas have a more moderate climate than the continental steppes, which show a high annual temperature range.

UPPSC 2018 asked where the Pampas are situated — Argentina.

Mediterranean Biome

The Mediterranean biome lies on the western margins of continents between about 30° and 45° latitude, on the poleward edge of the subtropical high. Its most noticeable characteristic is a hot, dry summer under the subtropical high, followed by a mild, wet winter under the westerlies. In Koppen terms it is Csa.

The vegetation is sclerophyllous — hard, leathery, waxy leaves and deep roots adapted to summer drought — and ranges from mixed evergreen woodland to scrub. The scrub has different names across the biome.

The biome supports olives, grapes and citrus, and the dry ripening season is exactly what makes citrus fruit production successful. It does not occur in the Pampas, which is temperate grassland.

CDS 2019 asked the most noticeable characteristic of the Mediterranean climate — dry summer.

Temperate Coniferous Forest (Taiga)

The boreal coniferous forest, or taiga, is the world's largest continuous forest formation by area. It is dominated by evergreen conifers — spruce, fir, pine and larch — with needle or scale-like leaves and a cone-bearing habit.

• Climate: long, severe winter and short summer, with a growing period of only about 50–100 days.
• Temperature: a large annual range because of continentality, not a small one.
• Structure: very little undergrowth; the closed canopy and acidic litter suppress ground vegetation.
• Soils: acidic, leached podzols.
• Species richness: low compared with tropical forests, though the area is vast.

Logging and fire are the main pressures, and the taiga plays a major role in the global carbon cycle.

Mangroves and Coastal Vegetation

Mangroves are halophytes adapted to saline, waterlogged, oxygen-poor tidal mud. Their characteristic adaptations are pneumatophores or breathing roots, stilt roots, salt-excreting leaves and vivipary, in which seeds germinate while still attached to the parent tree. They stabilise coastlines, attenuate storm surges and are important nurseries for fish and crustaceans.

The Sundarbans in West Bengal is the world's largest mangrove formation, and Bhitarkanika in Kendrapara district is India's second largest. Mangroves are not confined to the equatorial belt; they extend into subtropical latitudes on suitable coasts.

Odisha Angle: Forest Types and Protected Areas

Odisha's forest cover is roughly one-third of its geographical area, and its forests range from tropical semi-evergreen pockets in high-rainfall areas to dry deciduous forests in the west, with mangroves along the coast.

Similipal in Mayurbhanj district is a sal-dominated tropical moist deciduous forest, declared a biosphere reserve in 1994 and added to UNESCO's World Network of Biosphere Reserves in 2009; it is also a tiger reserve and an elephant habitat. Bhitarkanika was declared a Ramsar site in 2002 and is India's second largest mangrove ecosystem after the Sundarbans. Chilika, a brackish lagoon, was among India's first Ramsar sites in 1981 and supports extensive submerged and fringing vegetation.

Practice Questions