Research on air pollution impacts on Indian forests.

Rapid technological and industrial advancement coupled with increases in population growth and urban expansions have triggered the deterioration of air quality in India. The problem of air pollution has assumed serious proportions in the metropolitan cities and around industrial areas. Research on air pollution impacts on forests mostly has been conducted around large point sources of pollution. The analysis of air quality data clearly shows SO2, NO2 and suspended particulate matter (SPM) as the major pollutants of concern. SPM concentration was found to be above the standards set by the Central Pollution Control Board (CPCB) at most of the monitoring stations throughout the country. SO2 and NO2 concentrations are highest in the northern region and lowest in the southern part of the country. High concentrations of the secondary pollutant O3 have also been reported around some cities. Studies conducted downwind of industrial sources have clearly shown the negative impact of air pollutants on stem diameter, leaf weight, flowering and fruiting of trees. Visible injury symptoms have also been observed on tree leaves near industrial areas. Reductions in chlorophyll and ascorbic acid contents, percentage photosynthetic area, leaf weight and concentrations of N and P, were common observations in plants growing in polluted areas. In contrast, foliar SO42--S and trace elements were higher in trees growing closer to polluting sources. Foliar SO42--S/organic S ratio was suggested to be a useful indicator of the health state of forest trees under S pollution stress. Air pollution has also been found to decrease the number of individuals and species richness of forests; the changes in species diversity increase the proportion of resistant species in the community. Litter decomposition rate was found to be reduced at polluted sites. Nutrient release was also low in F-contaminated litter compared with F-free litter. In general there are very few exposure studies on tree species conducted at known concentrations of pollution. These studies have also confirmed that SO2 has an adverse effect on trees on long-term exposure. The existing data clearly show that air pollution in India has great potential for affecting the forest vegetation negatively. Most of the forests in the country have not been studied in relation to air pollution stress. Therefore, a national programme is warranted to identify the areas of forests with varying risk potentials in relation to air pollution.