Terrestrial Ecosystem- 1

Introduction

The interrelations between organisms and their land environment constitute terrestrial ecology. Variation in topography - valleys, mountains, slopes - together with latitudinal and altitudinal gradients produce differences in climate, soil and biological communities. These differences are reflected in both abiotic (temperature, moisture, soil) and biotic (plants, animals, microbes) diversity, causing the large biosphere to be partitioned into distinct terrestrial ecosystems.

• Altitudinal and latitudinal variations produce shifts in climatic patterns (temperature, rainfall, seasonality) that determine the composition and distribution of terrestrial vegetation and associated fauna.
• The major limiting factors for terrestrial ecosystems are temperature and moisture, although soils, light availability and disturbance regimes (fires, grazing) also shape communities.
• Terrestrial ecosystems are hierarchical and may be described as biomes (broad global zones) containing many local ecosystems adapted to local soil, climate and topography.

Tundra

Tundra refers to cold, treeless regions where environmental conditions are severe and vegetation remains low and sparse. There are two main types: Arctic tundra and alpine tundra.

• Distribution: Arctic tundra forms a continuous belt immediately below the polar ice cap and above the tree line in the northern hemisphere; it occurs across northern Canada, Alaska, northern Europe, European Russia and Siberia, and on Arctic islands. Antarctica has only very limited tundra because most of the continent is ice-covered.
• Alpine tundra occurs above the treeline on high mountains at many latitudes. Because it is found at high elevation rather than high latitude, alpine tundra experiences large diurnal (day-night) temperature variations.
• Soils and permafrost: Soils in arctic tundra commonly contain permafrost - a permanently frozen subsoil layer - with a shallow active layer that thaws each summer. Soil nutrients are limited and decomposition rates are slow due to cold conditions.
• Flora: Vegetation is low-stature and adapted to cold, wind and short growing seasons: cotton grass, sedges, dwarf heaths, dwarf willows and birches, mosses, lichens and cushion plants. Many tundra plants have thick cuticles and epidermal hairs for insulation and reduced water loss.
• Fauna and adaptations: Typical animals include reindeer (caribou), musk ox, arctic hare, lemmings and ground squirrels. Mammals often show adaptations that reduce heat loss: large body mass relative to surface area, small tails and ears, and dense fur. Insects complete short life cycles in the brief summer.
• Some tundra plants are long-lived; for example, arctic willow and certain prostrate shrubs may live for many decades or centuries (reported lifespans for some arctic woody plants range from decades to a few centuries).

What is a Forest Ecosystem?

A forest ecosystem is a complex assemblage of plant, animal, fungal and microbial communities interacting with their physical environment over an area dominated by trees. Abiotic conditions such as temperature, moisture, soil type and topography determine which forest types establish and their internal structure.

• Forest vegetation may be evergreen or deciduous, and leaf form may be broad-leaved or needle-leaved (coniferous).
• Major global forest biomes are often grouped as coniferous, temperate and tropical forests; these occur along gradients of latitude and altitude.
• Forest ecosystems provide structural complexity (multiple vertical layers), high biomass, and extensive interactions between producers, consumers and decomposers.

Major forest categories

1. Coniferous forest (Boreal forest)

• Also known as the taiga, boreal coniferous forests occur in cold regions with seasonal climates: long winters and short summers. They form a broad belt across high northern latitudes.
• Vegetation is dominated by evergreen needle-leaved species such as spruce, fir and pine.
• Typical animals include lynx, wolf, bear, red fox, porcupine and squirrels; amphibians such as species of Hyla and Rana occur in favourable microhabitats.
• Soils are typically thin, acidic and nutrient-poor - commonly podzols - because cold temperatures slow rock weathering and decomposition of conifer needle litter. Extensive leaching of soluble nutrients occurs, leaving soils deficient in calcium, potassium and other bases.
• Overall productivity and community stability of boreal forests are lower than many temperate and tropical forests because of short growing seasons and poor soils.

2. Temperate deciduous forest

• Found in regions with moderate climate and distinct seasons; dominated by broad-leaved deciduous trees that lose leaves in autumn and regrow in spring.
• Precipitation tends to be fairly uniform through the year.
• Soils are often deeper and more fertile (podzolic or rendzina in some areas) than boreal soils, supporting a rich understorey and diverse fauna.

3. Temperate evergreen forest (Mediterranean type)

• Characteristic of regions with a Mediterranean climate: hot dry summers and cool moist winters.
• Vegetation is often low, sclerophyllous (hard-leaved) evergreen shrubs and trees adapted to drought.
• Fire is a frequent disturbance; many species show adaptations for rapid post-fire regeneration.

4. Temperate rain forests

• Occur where temperatures are moderate and rainfall is very high (often with persistent fog). Examples include parts of the Pacific northwest of North America and some oceanic islands.
• Fog can be a significant water source for vegetation.
• Biotic diversity in temperate rain forests is greater than in other temperate forests but lower than in tropical rainforests.

5. Tropical rain forests

• Located near the equator, tropical rain forests are among the most diverse and productive terrestrial communities on Earth.
• Temperature and humidity are high and relatively constant year-round.
• Annual rainfall typically exceeds 200 cm and is often well distributed through the year.
• Vegetation is vertically stratified: emergent trees, a continuous canopy layer, an understory of smaller trees and shrubs, and a forest floor with herbs, ferns and seedlings.
• Epiphytes, lianas, creepers, orchids and bromeliads are abundant on tree trunks and branches.
• Soils are often deeply weathered latosols (red/yellow soils) and are poor in available nutrients because intense leaching removes soluble minerals. Nutrient cycling in the fast-decomposing litter and biomass is crucial to sustain productivity.
• Understorey growth may be limited by low light levels at ground level despite high overall productivity.

6. Tropical seasonal forests (Monsoon forests)

• Also called monsoon forests; found where total annual rainfall is high but seasonally concentrated with a pronounced dry season.
• Seasonal leaf shedding (deciduous behaviour) is common as an adaptation to water stress during the dry period.
• Distributed across South-East Asia, parts of Central and South America, northern Australia, western Africa and parts of India and its islands.

7. Subtropical rain forests

• Broad-leaved evergreen subtropical rain forests occur where rainfall is high but seasonal temperature differences are smaller than in temperate zones.
• Epiphytes are common and animal communities resemble those of tropical rainforests.

Indian Forest Types

India contains a wide range of forest types - from tropical rainforests of the south to alpine pastures of the north - determined principally by climate, soil, topography and elevation. Forest composition varies regionally and with human influence such as shifting cultivation and fire.

Champion and Seth classification of forests

The classification by Champion and Seth divides Indian forests into sixteen major types; it is widely used for ecological, management and policy purposes.

1. Tropical wet evergreen forests

• Found along the Western Ghats, Andaman and Nicobar Islands and parts of the north-east.
• Tall, straight, evergreen trees with a multilayered canopy. Common tree species include jackfruit, betel nut palm, jamun, mango and hollock (Shorea spp.).
• The understorey includes shrubs, ferns and numerous epiphytes such as orchids. These are among the most biodiverse forest types in India.

2. Tropical semi-evergreen forests

• Occur in the Western Ghats, Andaman and Nicobar Islands, and Eastern Himalayas.
• These forests are a mixture of wet evergreen and moist deciduous elements and are structurally dense with a high diversity of tree species.

3. Tropical moist deciduous forests

• Extensive across much of peninsular India except in the arid west and north-west.
• Tall broad-trunked trees dominate; many species shed leaves during the dry season. Common species include sal (Shorea robusta), teak, bamboo, mango and rosewood.
• These forests typically have a well-developed understorey of shrubs and regenerate well where disturbances are moderate.

4. Littoral and swamp forests

• Found along the Andaman and Nicobar Islands and deltaic regions of the Ganga and Brahmaputra; includes mangrove and tidal swamp vegetation.
• Plants often have specialised aerial or breathing roots and other adaptations to waterlogging and salinity.

5. Tropical dry deciduous forests

• Found across much of northern and central India and parts of peninsular India (Madhya Pradesh, Gujarat, Andhra Pradesh, Karnataka, Tamil Nadu).
• Canopy heights are generally lower (rarely exceeding ~25 metres). Common trees include sal, various Acacia species and bamboo.

6. Tropical thorn forests

• Occur in drier regions with black soils and low rainfall: parts of north, west, central and south India.
• Vegetation is xerophytic with small trees and shrubs (spines, thorny species such as Euphorbia, caper, cactus).

7. Tropical dry evergreen forests

• Found along parts of the coromandel coast (Tamil Nadu, Andhra Pradesh, Karnataka).
• Characterised by hard-leaved evergreen trees with aromatic flowers and some seasonal deciduous elements; adapted to a long dry season.

8. Subtropical broad-leaved forests

• Occur in the Eastern Himalayas and moist pockets of the Western Ghats (for example, Silent Valley).
• Flora varies regionally: in Silent Valley one finds species such as poonspar, cinnamon, rhododendron and fragrant grasses; in the Eastern Himalayas evergreen wet forests are rich in oaks, alder, chestnut, birch and cherry trees, along with many orchids, bamboos and creepers.
• These forests are often impacted by shifting cultivation and forest fires in some areas.

9. Subtropical pine forests

• Found on dry, steep slopes in the Shivalik hills, Western and Central Himalayas and certain north-eastern hill regions.
• Species include chir pine, along with oak, rhododendron and other broad-leaved trees in lower elevations.

10. Subtropical dry evergreen forests

• Found in the Shivalik hills and Himalayan foothills up to about 1000 metres.
• Characterised by evergreen trees with glossy leaves adapted to periodic dryness and cooler winters.

11. Montane wet temperate forests

• In the north, these occur east of Nepal into Arunachal Pradesh and receive over 2000 mm of rainfall in places.
• Three vertical layers may be evident: upper coniferous layer, mid-layer of broad-leaved deciduous trees (e.g. oak) and a lower zone dominated by rhododendron and Champa in suitable localities.
• In the southern montane blocks (Nilgiri Hills) similar wet temperate communities occur but are generally less dense.

12. Himalayan moist temperate forests

• Spread from the western to the eastern Himalayas. Western sectors have broad-leaved oaks, walnut, rhododendron and other species; the eastern Himalayas receive heavier rainfall and support denser, lusher vegetation with many ferns and bamboo.
• Coniferous species may be mixed with broad-leaved trees depending on elevation and exposure.

13. Himalayan dry temperate forests

• Found in regions such as Lahaul, Kinnaur and parts of Sikkim and other drier Himalayan valleys.
• Dominated by drought-tolerant conifers and broad-leaved trees such as oak, maple and ash; at higher elevations fir, juniper, deodar and chilgoza (pine nut) may appear.

14. Subalpine forest

• Extends across the Himalaya roughly between 2,900 and 3,500 metres elevation.
• Western Himalaya: juniper, rhododendron, willow and black currant are common. Eastern parts have red fir, birch, larch and higher timberlines due to greater precipitation; rhododendrons are abundant.

15. Moist alpine scrub

• Occurs along the Himalayas and in higher hills adjoining Myanmar; characterised by low, dense evergreen scrub - mainly rhododendron and mountain birch - with mosses and ferns on the ground. This zone receives heavy snowfall in winter.

16. Dry alpine scrub

• Found approximately from 3,000 to 4,900 metres elevation in drier trans-Himalayan and highland zones.
• Dwarf shrubs predominate: black juniper, drooping juniper, honeysuckle and various alpine willows adapted to extreme cold and wind.

Importance of Forests

Forests provide multiple ecological, economic and social services essential to human well-being and to the functioning of the biosphere.

• Forests help maintain natural ecological balance by regulating climate and hydrological cycles.
• Forests purify air by absorbing carbon dioxide and releasing oxygen through photosynthesis, and by filtering pollutants.
• Forests create and maintain local microclimates, moderating temperature extremes and humidity.
• Forests influence precipitation patterns at regional scales by aiding evapotranspiration and cloud formation.
• Forests reduce flood risk by absorbing and slowing surface runoff during heavy rain.
• Forests prevent soil erosion by binding soil with roots and reducing surface runoff velocity.
• Forests are a source of medicinal plants and traditional remedies.
• Forests supply fuelwood, timber and non-timber forest products vital for rural livelihoods.
• Forests provide raw materials for many industries (timber, fibres, resins) and support biodiversity that underpins ecosystem services.