An undisturbed terrestrial ecosystem naturally develops into a sparse or dense forest. Factors like, humidity, temperature, rainfall and soil types etc. determine the nature and composition of the biotic community within a forest.
Those very factors of abiotic environment which influence and shape a forest are also in turn modified by the population of living organisms within the system. Deforestation involves removal of plant biomass which cripples the system. Various useful products such as firewood, timber, honey, fruits, nuts, resins and medicinal plants etc. are no longer available. A chain of events is set into motion the consequences of which can be summed up as follows:
1. Soil degradation and erosion
2. Changes in climatic conditions
3. Destruction of natural habitats.
4. Destruction of a valuable sink for environmental pollutants.
(1) Soil Degradation and Erosion:
Plants check rapid movement of air and water. Flowing waters stay in the area for a longer duration during which time nutrients are re-absorbed and as water percolates down, ground water table is recharged. Plant cover keeps the ground surface humid. Trees with the help of deep root systems are able to draw water from sub-surface water table. Humidity prevents excessive water loss and rapid desiccation. Plants contribute organic matter which upon decomposition adds humus to the soil. Porosity, water-holding capacity and productivity of the soil improve. Organic matter binds the soil particles in soil crumbs which make it more stable again forces of erosion.
Deforestation leaves the ground surface bare. In humid tropics a large portion of available mineral nutrients is taken away when the biomass is removed. Herbaceous plants and grasses are exposed to the action of sun, wind and rapidly flowing waters. There is further loss of mineral nutrients. Grazing may remove much of the organic matter with which there is further loss of nutrients. Where remaining vegetation is burned to clear the land and agriculture attempted loss of nutrients is even more rapid. Already poor tropical soil is made poorer. All this further reduces the cover of small plants and grasses as well.
Adequate plant cover keeps the soil temperature lower. At all depths up to 70 cm a higher temperature is observed in soil devoid of plant cover. Higher temperatures speed up mineralization of organic matter. This reduces the stability of soil crumb structure and the soil becomes easily erodible. It also loses its capacity to hold water, recycle mineral nutrients, nitrogen-fixing capacity etc. and turns into a dead mass of silt, clay and sand. With plant cover gone the battering action of wind and rains loosen the top soil which is thus carried along with water or air currents and deposited elsewhere.
The top soil which is thus lost is irreplaceable. Nature takes about 1000 years to produce 2.5 cms of top soil. It has been estimated that prior to man's influence on earth's crust, oceans received about 9-10 x 109 tons of sediments annually. Today about 25x10' tons of precious top soil flows into oceans as silt and sediments every year. India loses about 5300 million tons of soil every year. Of this enormous amount nearly 2000 million tons get deposited in stream and river beds, about 480 million tons get lodged in dams and reservoirs and the rest is flushed into the sea (Gurmel Singh qouted by Venkataramani, 1991).
Most of the multipurpose reservoirs in India are silting up at a faster rate - silting being about 146-874% faster than what was assumed earlier. This has curtailed the life-span of our multipurpose, multimillion crore rupee reservoirs drastically. The life of Ramganga reservoir in the Gangetic watershed has been curtailed to about one-fourth of its originally intended span of existence (Venkataramani, 1991).
Massive soil erosion aggravates flood situation in two ways. Firstly, the deposition of silt and sediments in river beds makes them shallow. Secondly, land devoid of forest cover loses its water holding capacity. About 10 million hectares of forested land can hold enough water to fill completely a reservoir as large as that of Bhakhra-Nangal dam. In absence of plant cover this water flows down in rapid torrents.
In streams and rivers it has to flow through shallow channels where it spills over its banks inundating low lying areas. Due to deforestation and extensive soil erosion in water sheds of almost all major rivers in India total area of land affected by floods has been rising steadily (Ehrich 1980).
(2) Changes in Climatic Conditions:
Forests shape our natural environment and local climatic conditions. They maintain humidity, regulate temperatures, break wind velocities and influence precipitation. The extent up to which forests influence our natural environment is a controversial subject. However, it is almost certain that dense growth of green plants has a moderating influence on local climatic conditions and the global environment in a number of ways:
1. Maintenance of Humidity:
Active transpiration by green plants keeps the environment humid. Roots of trees penetrate deep down to the sub-surface water table. They are able to draw water even when the surface is dry.
2. Regulation of Atmospheric Temperatures:
Average air temperatures under tree cover have been found to be appreciably lower than those measured in open fields. The type of plants also influences the temperatures in a forest.
Evapo-transpiration exercises profound cooling effect-air cools down by 2-5°C when it comes in contact with cool vegetation. A record of maximum temperature of Mussoorie (U.P.) bears ample testimony to the phenomenon. All around Mussoorie there were lush green forests earlier. Rapid deforestation has been taking place around the city since 1950 A.D. which was checked only by 1975 following demands of agitated inhabitants of the area.
The mean maximum temperature of the city rose by 6.5°C within a span of thirty years. A significant depression in the rising tree fan is observed from 1975 onwards as an extensive reforestation drive was undertaken to remix the situation.
3. Moderation of Wind Velocity:
Forests check wind velocity by obstructing its passage physically. The velocity of wind through a forest is profoundly affected by the density of vegetation. Higher wind velocities accelerate transpiration and evaporation which in turn speed up desiccation. Under drier conditions soil particles are loosened and transported by air currents resulting in a higher rate of soil erosion. High speed winds make man and animals uncomfortable, often uprooting weak structures and plants. The moderating effect of forests on wind velocity is, therefore, beneficial in many ways.
4. The Role of Forests in Enhancing Precipitation:
The effect of forests on annual precipitation has been one of the most debated topics among scientists. Experts have come up with varying assessments starting from nominal rise to an enhancement of about 12% in planes and almost twice as much in hilly areas. While studying the forest-climate link it is important to realize that every shower is not due to forests. Rains depend on a number of factors such as location, topography sea-surface temperatures etc. Of these forests could be one of the factors which promote rainfall.
An interesting example of deforestation affecting rains is provided by rain fall data from Chhota Nagpur Plateau of Bihar which had a considerable area under forest cover in the beginning of 20th century. There were plenty of afternoon showers during summer months. Though there is no appear reduction in monsoon rains over the plateau, these afternoon showers of summer months have declined.
This decline in pre-monsoon showers (April-May-June) is attributed to the extensive degenerate of forests in the area. Excessive summer heat caused the humid air under dense forest cover to be heated up during the earlier part of the day. It rose to considerable heights to cool down, condemn into water droplets and by the end of the day fall back as pleasant showers. Such rains are known as Convectional rains.
These pre-monsoon showers were a boon for tea-plantations which cameata crucial time, just when after a long spell of dry season water was badly needed. The number of rainy days excluding those of monsoon months, e.g., June, July and August, were 374 during the four-year period between 1870 A.D. to 1874 A.D. The number had gone down to 271 days during 1978-1982. The decline in the convectional rains has caused the tea-gardens of Chhotanagpur plateau to disappear (Mehr-Homji, 1991).
However, it is not easy to correlate precisely the large variations in rainfall with deforestation as a number of factors act cumulatively to produce a rain. Monsoon rains and the rains of cyclonic origin caused by differential heating of land and sea surface are hardly affected by a forest cover. However, it can be said with some certainty that forests do influence convectional rains or thunderstorms. The mechanism which is operative in forest-rains relationship can be examined as under:
Evaporation and transpiration recycle back about 50-70% rains to the atmosphere. This is not possible by evaporation alone. The total surface area from which evaporation can occur is enormous in forests due to the density of foliage. On bare land solar radiations heat up the soil and surrounding air making it drier.
The term albedo denotes the proportion of solar radiations reflected back into atmosphere to the total amount striking earth's surface. Lower albedo in regions under plant cover (about 15-25%) results in greater absorption of solar energy which sets into motion strong thermal up-currents. The moisture is taken high up into the atmosphere where it cools down and condenses into rain-drops. On bare soil albedo is higher (by about 30-35%) e.g., large amount of solar energy is reflected back into the atmosphere which heats up upper layers of air. This restricts upward movement of air bearing whatever moisture it happens to hold. The net result of which is reduction of rains of convectional type.
(c) The effect of mechanical friction:
Forests obstruct the path or air currents by raising the effective height of the land. While decreasing the velocity of air currents this obstruction redirects the air mass upwards, forcing it to rise. The moist air ascends and cools high up in the atmosphere, condenses and adds to the convectional rains.
(d) Pollen grains and other plant debris:
These act as effective nuclei on which water vapours condense. The aerosols of plant origin are lighter as compared to dust particles of the same size, which require a much lower temperature to act as a condensation loci for water vapours. Deforestation results in a sudden drop in the number of aerosols of plant origin in the atmosphere while those of dust particles rise as a consequence of soil erosion by wind. This has an adverse effect on convectional rains.
(e) Horizontal precipitation:
Plants growing on tropical mountains and coastal zones are capable of absorbing moisture from moist and humid air. This water is sufficient for the plant growth as well as for maintaining the surroundings humid and hospitable. No water can be harnessed if land is rendered devoid of plant cover.
5. Forest Cover and Global Warming:
An enormously large quantity of Carbon is trapped in forests and forest soils of the world. Coniferous forests possess about 117.6 billion metric tons, forests and woodlands of temperate zone about 188.6 billion metric tons while the tropical seasonal and rain forests possess about 502.25 billion metric tons of carbon. Needless to say, the oxidation of this carbon shall yield a huge quantity of carbon dioxide which will be added to the atmosphere.
Apart from being the basic raw material for photosynthesis Carbon dioxide is an important green house gas. Rising concentrations of this gas in atmosphere could cause excess warming up of our planet just as a thicker blanket makes us too much warm and uncomfortable. A rise in global temperatures, howsoever mild, could cause serious problems for the mankind.
About 81.50 billion metric tons of carbons are added into the atmosphere annually out of which about 3.5 billion metric tons are contributed by combustion of fossil fuels, organic matter, forest fires, deforestation and other human activities.
A major part of this Carbon dioxide, almost 70 billion metric tons out of 81.5 billion metric tons are absorbed by green plants and is converted to organic matter. Of the remaining 11.5 billion metric tons, some C02 dissolves in rain water and is brought down to earth's surface while a continuous exchange of this gas occurs between water and air above at the air-water interface.
Deforestation substitutes lush green forests with agriculture, grass land or herbs and shrubs with low productivity and little biomass. Enormous quantities of carbon dioxide are set free while loss of plant cover reduces the overall photosynthetic efficiency of the system. Thus while the input of carbon dioxide to the atmosphere is increased its output decreases. Many scientists believe that deforestation has been contributing significant amounts of carbon dioxide to the global atmosphere and thereby to the greenhouse effect or global warming.
(3) Destruction of Natural Habitats and Reduction in Biodiversity:
In forests the shaded area under trees provides protection, tolerable temperatures, adequate humidity etc. to other smaller plants, animals and microbes The living organisms co-exist interact with one another and with plants, animals and microbes. The living organisms co-exist, interact with one another and with abiotic components maintain a state of dynamic equilibrium. It is these interactions which provide nutrition, water and shelter - a suitable habitat for the diverse flora and tauna to live and thrive in it.
Each species has developed through countless million years as an unique specimen in perfect harmony with its surrounding environment. No doubt the existence and well being of man is intimately linked with other life forms which provide the basic raw materials for his existence.
However, deforestation caused by man himself tends to disturb or eliminate completely the very habitats of millions of species. The bio-diversity is collapsing at an alarming rate. The greatest threat comes from the tropical regions where we are losing forests at an equally alarming rate of about 1.7 million sq. kms per year. The home of almost half of the living species on this planet is being destroyed and almost one species of mammal birds or plants is condemned to extinction per day.
(4) Destruction of an Important Sink for Pollutants of the Environment:
Forest soils and vegetation have a large capacity to absorb, transform and accumulate various pollutants of environment. Vegetation acts as an effective sink for a number of undesirable constituents of the environment. Deforestation not only destroys this sink but also reduces soil's capacity to eliminate pollutants. Removal of plant cover leaves the soil bare. Its organic matter content is quickly mineralized. Without organic matter microbes fail to survive and the soil is turned into a lifeless mass of sand silt and clay. In absence of microbial machinery the soil is unable to perform the biochemical activity involved in absorption, accumulation or transformation of pollutants.
On global scale an enormous quantity of carbon dioxide, carbon monoxide, sulphur dioxide, oxides of nitrogen, reactive hydrocarbons, ozone and other particulates are cleared by vegetation and the soil (Table no. 9.4). These pollutants may serve as fertilizer to the plant community.
As much as 1 -13 kg per hectare per year of Nitrogen, 6.1 kg per hectare per year of sulphates and about 20-40 kg per hectare per year of calcium and potassium may be provided to natural ecosystem through atmosphere in the shape of dry or wet deposition (Likens et al, 1977, Boremann and Likens 1979, Swank 1984).
No doubt many of the contaminants eliminated by green cover and the soil originate naturally from plants and the soil microorganisms. However, there is an efficient natural sink to eliminate them. The natural system is also capable of making adjustments for limited inputs of pollutants added by human activity. If we destroy this natural sink while introducing more and more pollution to the system, it is bound to fail with grave consequences for the human race.