G L Khajuria
Forest is biological community having complex social organisation of living communities at work. Trees play a crucial and prominent part in conserving soil, water and moisture. These take up water from soil through a process of ascent of Sap and release it into the air or atmosphere through transpiration and this process continues as a result of condensation.
The cycle of water absorption from the soil and condensation causes rainfall and this process also continues. At ecosystem level under such conditions these function as important sinks of air pollution. When exposed to intermediate dose, the individual tree species may be subtly and are affected by nutrient stress, impaired metabolism, predisposition to entomological or pathological stress as direct disease induction.
Exposure to high dosage may, however, induce acute morbidity or mortality of a specific tree species. For the survival of any living organism, clear air is vital. The earth’s atmosphere is a mixture of gases, water vapours and a variety of solid particles and the liquid particles. Pollution of air has been defined as the presence of solid, liquid and gases substances in the atmosphere in such concentrations as may be injurious to humans, plant and animal life or property.
In some of the cases, the composition of air is not confined to only to the cities. Volcanic eruptions, forest fires and dust stirred up by the storms, winds, pollen hair and other suspended particles and other living particles such as bacteria and viruses can do contaminate atmosphere.
Nearly 300 million tonnes of air pollutants are emitted onto the air which is much more than that can absorb into natural system safely. Air pollution from huma activity commenced when the human learnt to use fire.
But large scale of air pollution is relatively of recent development, mainly due to accelerated industrial activity. Further, the contaminants by the nature of activities that produce them are likely to be omitted into the air in thickly populated region. Therefore, the effect even if small or on global scale may be locally severe. The estimate level that increase of 6oC in the global mean temperature due to addition of enormous quality of carbon-di-oxide in to the atmosphere is likely to create climate condition that earth has never experienced for the last 70 million years. Even if the actual increase in the temperature is smaller than anticipated, it would be enough to meet large volume of polar ice and raise the sea level.
5 Meter. Similarly, the ozone in the stratosphere effectively blocks much of the harmful solar ultraviolet radiations that would otherwise reach to earth’s surface. The present calculation indicate around 18% of ozone layer will be adversely effected by the man-made chemicals.
Half the ozone layer destruction nearly 8% will occur in the next 35-40 years. Both industries and urban communities pollute and contaminate the air with substances that are dangerous to human and other living organisms in the environment. Such pollutants include radio-active dust, salt spray, herbicides, aerosols, combustion of coals, manufacture of bricks, ceramics, cement, glass, phosphate fertilisers, liquids droplets o acid matter, gasses and solid particles.
These substances can act alone to irritate all forms of life, or more dangerously, they join together to act adversely upon the environment. Nuclear Power Station of U.S.A. etc are warning signals both for the developed and developing countries.
Air pollution may be removed from the atmosphere by a variety of mechanism. The primary natural process of cleansing the environment are precipitation, chemical reaction, dry deposition or sedimentation and absorption.
However, the plant communities such as forests and other plantations play prominent part in mitigation environmental pollution. Plants or forests are efficient enough to sink for many gaseous pollutants. The interception and retention of atmospheric particles by plant is highly variable and primarily dependent on size shape, wetness and surface texture of the particles and as well as intercepting plant parts with micro and ultramicro climatic condition of the surrounding. Smaller leaves are generally more efficient particle collectors than large leaves. Particle deposition is heaviest at the leaf tip and long leaf margins. Leave with complex shapes and large circumstances are ratios collect particles more sufficiently.
The tree may be specially efficient filter of carbon particles because of large size, high surface to volume ratio of foliage, petioles, twigs and frequently hairy or rough leaf, twigs and bark surfaces, Because the interior portions of forest stands act still the air, mean wind speeds are reduced and particle deposition will be augmented,
When vegetation surfaces are wet or damp the pollutants removal rate may increase upto ten folds. Under damp conditions, the entire plant surface i.e. leaves, twigs, branches, stems are available for absorption. Light also plays a critical part in determining physiological activities of leaves and stomatal openings and as such exert great influence on foliar removal of pollutants.
Under conditions of adequate soil moisture, pollutants absorption of gaseous pollutants absorption by vegetation is constant throughout the day as the stomata are fully open. Moisture stress sufficient to limit stomatal opening and relatively common in urban environments would severely restrict absorption of gaseous pollutants diffusivity rates are greatest. Sulphur and nitrogen oxide are taken in respiring leaves in the dark, but uptake rates are greatly reduced as compared to the rates in the light.
Under certain environmental circumstances, especially where tree surfaces are wet and the leaves are metabolically active, biologically and medically significant reduction in ambient levels of Sulphur dioxide, Nitrogen dioxide ozone and hydrogen fluoride may be realised by stands of trees for extended periods as long as the atmospheric loading of the containment go is not excessive.
Apart from forest fires, which is one of the most sources of air pollution during the course of metabolism, a variety of woody plants release certain carbon, Sulphur, nitrogen oxide particulates and volatile hydrocarbon. From the point of view of human health, a huge quality of pollens are released during the reproductive phase by the forest trees which would be allergic some times.
High air pollution may result in severe perturbation in the ecosystem structure and function these situations involve the impact of gaseous pollutants such as release from industrial sources, on surrounding forests. In extreme situations, irrespective of specific pollutants, forest communities react first by loosing sensitive species, second by losing the tree stratum and third by maintaining cover of resistance shrubs and herbs wildly recognised as aerial succession species in less extreme situations, the losses of sensitive species followed by maintenance of tree stratum.
Field systems of air pollution are not highly specific mimicked by a wide variety of other tree stress factors and can be recognised only by edaphic, entomological, pathological and stress factors characteristic of a given flora in a given location.
(The author is former Deputy Conservator J&K Forest).