Some facts about FORESTS

Overview

• Forests are areas with a high density of trees
• Forests cover approximately 9.4% of the Earth’s surface i.e. about 30% of total land area, although they used to cover as much as 50% of land area
• Forests are differentiated from woodland by the extent of canopy coverage: in forests, the foliage of trees meet and interlock while in woodlands there is enough gap between trees allowing sunlight to penetrate to the ground
• Forests are one of the most important aspects of the Earth’s biosphere
• The functions of forests include
  • Habitat for organisms
  • Hydrologic flow modulation
  • Soil conservation
• Human factors affecting forest sustenance include logging, urban sprawl, agriculture, industries, human-induced forest fires etc. Natural factors affecting forests include forest fires, insects, diseases, weather etc
• Only about 20% of the world’s original forests remain in undisturbed forest. Of this, 75% are in Russia, Canada and Brazil

Distribution of forests

• In general, forests can be found in all regions capable of sustaining tree growth (at altitudes up to the tree line), except where natural disturbance is too high or human activity has altered the environment
• The areas between latitudes 10 N and 10 S are mostly covered in tropical rainforests, and between 53N and 67N have boreal forests (taiga)
• Forests can contain many species in a small area (like rainforests) or relatively few species in a large area (like taiga and montane coniferous forests)
• Forests have higher biomass per unit area compared to other vegetation types. Much of the forest biomass occurs below the ground in root systems and partially decomposed detritus
• The major types of forest systems are
  • Rainforests (both tropical and temperate)
  • Taiga
  • Temperate broadleaf forests
  • Tropical dry forests

Temperate rainforest in Washington, USA (northwestern USA)

Old growth forests

• Old growth forests (also called primary forests, ancient forests) are forests that contain trees which have attained great age
• Old growth forests typically contain large and old live trees, large dead trees and large logs
• Death of individual trees creates gaps in the canopy layer allowing light to penetrate and create favourable conditions for undergrowth
• Old growth forests are often home to rare and threatened species, making them ecologically significant. For instance, the Northern Spotted Owl is reliant on old growth forest
• The importance of old growth forests include
  • They contain rich communities of plants and animals due to the long period of forest stability
  • They serve as a reservoir for species that cannot thrive or regenerate in younger forests
  • They store large amounts of carbon both above and below the ground (either as humus or in wet soils as peat)
• Forests that are regenerated after disruptions must wait several centuries to millennia before they can reach the stable equilibrium that signifies old growth forests
• Due to increased human activity, old growth forests have been substantially destroyed over the last century. Of the old growth forests that still remain, 35% are in Latin America (Brazil), 28% in North America (mainly Canada) and 19% in northern Asia (Siberia)

Second growth forests

• Second growth forests (secondary forests) are forests that have re-grown after a major disturbance such as fire, insect infestation, logging, windthrow etc
• Second growth forests tend to have trees closer spaced than primary forests and have more undergrowth
• Second growth forests usually have less biodiversity than old growth forests, since the former have had lesser time to develop and reach stable equilibrium
• Secondary forests are common in areas under shifting agriculture, areas with forest fires, and forests that are recovering from harvesting and agriculture
• Secondary forests can several generations of trees (centuries) to resemble the original old growth forests. However, in some areas, secondary forests do not succeed due to soil nutrient loss and erosion (especially in tropical rainforests)
• Most of the forests of eastern North America and of Europe are secondary forests

Distribution of tropical rainforests in the world

Overview

• Rainforests are forests characterised by high rainfall, with minimum annual rainfall as high as 1700-2000 mm
• Rainforests are responsible for 28% of the world’s oxygen turnover. However, rainforests do not contribute much to the net oxygen additions to the atmosphere. Instead, they are vital in storing carbon in bio sequestration
• The Intertropical Convergence Zone (ITCZ), the area near the equator where winds originating in the northern and southern hemispheres meet, plays a significant role in creating the rainforests
• Despite the growth of vegetation, soil quality in a rainforest is poor. Most trees have roots near the surface due to lack of nutrients below the ground
• More than half the world’s species of plants and animals are found in rainforests

Tropical Rainforests

• Tropical rainforests are rainforests in the tropics, near the equator between the Tropic of Cancer and the Tropic of Capricorn
• Tropical rainforests are found in South America (Brazil), Central America (Yucatan Peninsula), Sub-Saharan African (Congo), Northeast India, Southeast Asia (Indo-Malaya, Indonesia, Papua New Guinea)
• Tropical rainforests are called ‘world’s largest pharmacy’, since over 25% of modern medicines originate from these plants
• Tropical rainforests are home to half of all the plant and animal species on earth
• Tropical rainforests are characterised by heavy rainfall, resulting in poor soil due to leaching of nutrients
• Temperatures range 15 C to 50 C. Rainfall ranges from 1250 mm to 6600 mm annually

Temperate Rainforests

• Temperate rainforests are rainforests that occur in the temperate zone and receive high rainfall
• Temperature range 4 to 12 C. Rainfall minimum is around 1400 mm annually
• Temperate rainforests are found in close proximity to oceans, and usually occur in coastal mountains. This is because temperate rainforests depend on the proximity to oceans to moderate seasons, creating milder winters and cooler summers. Coastal mountains increase rainfall on the ocean facing slopes
• Wildfires are uncommon in temperate forests due to the high moisture content in the forest
• Mosses are abundant in temperate rainforests
• Temperate rainforests sustain the highest levels of biomass of any terrestrial ecosystem
• Temperate rainforests are notable for trees of massive proportions, including coast redwood, coast douglas fir, sikta spruce etc
• Temperate rainforests are found in western North America, south-western South America, Norway, northern Spain, south-eastern Australia and New Zealand

Distribution of temperate rainforests in the world

• Rainforests are typically divided into four layers, each with different plants and animals adapted for life in that environment
• Emergent layer
  • The highest layer, formed by a small number of very tall trees that grow above the general canopy
  • They reach heights of 45-55 m, occasionally even 70-80 m
  • Need to be able to withstand high temperatures and strong winds
  • Eagles, butterflies, bats and some monkeys inhabit this layer
• Canopy layer
  • The canopy layer consists of the largest number of tall trees, which provides a more or less continuous cover of foliage by adjacent treetops
  • Usually reach heights of around 30-45 m
  • The canopy layer is the densest area of biodiversity in a rainforest. It is estimated that the canopy layer is home to about 50% of all plant species and 25% of all insect species
• Understory
  • The understory layer lies between the canopy and the forest floor
  • Leaves are much larger at this level
  • Only about 5% of sunlight incident on the rainforest reaches the understory layer
  • This layer is home to a number of birds, snakes, lizards, and predators like jaguar, boa constrictors etc
• Forest floor
  • The forest floor is the bottom most layer
  • The forest floor receives only about 2% of the sunlight. Only plants adapted to low light can grow in this region
  • Due to low sunlight penetration, forest floor is relatively clear of vegetation. This makes it possible to walk through a rainforest

Effect on global climate

• Rainforests emit and absorb massive quantities of carbon dioxide. Undisturbed rainforests usually have no net impact on atmospheric carbon dioxide levels
• However, rainforests play a vital role in other climatic effects such as cloud formation and water vapour recycling
• Deforestation caused by human activities and drought can cause rainforests to release massive amounts of carbon dioxide into the atmosphere

TROPICAL DRY FORESTS

Overview

• Tropical dry forests are located in the tropical and subtropical latitudes
• These forests occur in areas that are warm and receive plentiful rainfall (several hundred centimetres) but experience long dry seasons which last several months. These seasonal droughts have great impact on the forest
• Deciduous trees dominate in these forests
• Tropical dry forests are less biologically diverse than rainforests
• However, they are home to a wide variety of wildlife including monkeys, deer, parrots, large cats etc. Mammalian biomass tends to be higher in dry forests than in rainforests.

Characteristics

• During the drought season a leafless season occurs. The shedding of leaves allows trees like teak and ebony to conserve water during these dry periods
• When the trees enter the dry leafless season, the canopy layer opens up allowing sunlight to reach the ground, thereby enabling growth of thick undergrowth
• However, certain areas of tropical dry forests can have evergreen trees. This happens especially when the forests are on moisture sites or have access to groundwater
• Three tropical dry forest regions have evergreen forests:
  • East Deccan dry evergreen forests (India)
  • Sri Lanka dry zone evergreen forests (Sri Lanka)
  • Southeastern Indochina dry evergreen forests (Cambodia, Laos, Thailand, Vietnam)
• The forests of central India are tropical dry forests
• Dry forests are extremely sensitive to forest fires, overgrazing and deforestation. Restoration of dry forests is possible, but challenging

Distribution

• Dry forests tend to exist north and south of the equatorial rainforest belt, and south and north of the subtropical deserts
• They usually occur in two bands: one between 10 and 20 N latitudes and the other between 10 and 20 S latitudes
• The most diverse dry forests of the world are found in southern Mexico and Bolivia
• The dry forests of central India and Indochina are notable for their diverse and large vertebrate fauna
• Other tropical dry forests are found in New Caledonia,     Madagascar, south eastern Africa and the Pacific coast of South America

TEMPERATE BROADLEAF AND MIXED FORESTS

Overview

• Temperate broadleaf and mixed forests are a temperate and humid biome (ecological system)
• These forests typically have four layers
  • Canopy layer: contains mature trees 100-200 ft high
  • Understory: shade tolerant layer of trees that grow to about 30-50 feet shorter than the canopy
  • Shrub layer: low growing woody plants
  • Herbaceous layer: this is the ground cover, most diverse layer
• Characteristic broadleaf trees these forests include oaks, birches, beeches and maples. Mixed trees are basically coniferous trees such as pines, firs and spruces
• Areas of temperate broadleaf and mixed forests include northeast USA, northern India, eastern Australia, New Zealand, southwest China

Distribution

• Temperate broadleaf and mixed forests occur in areas with distinct warm and cool seasons, with moderate annual average temperature (5-15 C)
• They usually occur in moderately warm and rainy climates, sometimes with a distinct dry season
• Annual rainfall is typically over 600 mm and sometimes over 1500 mm

The Taiga, the world's largest terrestrial biome, is found throughout the high nothern latitudes

Overview

• Taiga is a biome characterised by coniferous forests
• The Taiga is the world’s largest terrestrial biome
• The taiga experiences relatively low precipitation (250mm – 750 mm), mostly in the form of fog, snow and summer rain. However, since evaporation is also low, there is enough moisture to enable dense vegetation growth
• Taiga soils tend to be young and nutrient-poor. The soil tends to be acidic and hence the forest floor only has lichens and mosses growing

Characteristics

• The taiga has harsh continental climate and large range of temperatures: -54 C to 27 C
• Except for the tundra and permanent ice caps, the taiga is the coldest biome on earth
• There are two main types of taiga:
  • Closed forest: closely spaced trees with mossy ground cover
  • Lichen woodland: trees that are farther apart and have lichen ground cover. More common in colder areas
• In the northern taiga areas, forest cover is not only sparse but also stunted
• The forests of taiga are mainly coniferous consisting of larch, spruce, fir and pine
• Taiga trees tend to have shallow roots to take advantage of thin soils.
• Since the sun is low on the horizon most of the time, it is difficult to photosynthesise. Pine and spruce do not lose their leaves in winter and can photosynthesise using their older leaves
• The adaptation of evergreen needles (on pines) limits water lost to transpiration and the dark green colour increases sunlight absorption

Distribution

• The taiga covers most of Canada, Alaska, Sweden, Finland, Norway, the Scottish Highlands and Russia. It is also found in parts of northern USA, northern Kazakhstan, northern Mongolia and northern Japan
• Large areas of Siberia’s taiga have been destroyed in recent years
• In Canada, less than 8% is protected development and more than 50% has been allocated for logging

Taiga in Alaska, USA

• The taiga is home to a large number herbivorous mammals and smaller rodents
• Some of the animals, like bears, eat in summer and hibernate in winter. Others have evolved layers of fur to insulate them from the cold
• Due to the climate, carnivorous diets are inefficient for obtaining energy.
• A significant number of birds like Siberian thrush, white throated sparrow, black throated green warbler migrate to the taiga to take advantage of long summer days

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