The visible and quantifiable effects of air pollution include tree injury and crop damage, with examples occurring nationwide (Mackenzie and El-Ashry 1989). Many influences shape the overall health and growth of trees and crops. Some of these influences are natural: competition among species, changes in precipitation, temperature fluctuations, insects, and disease. Others result from air pollution, use of pesticides and herbicides, logging, land-use practices, and other human activities. With so many possible stresses, determining which are responsible when trees or crops are damaged is difficult. Crop failures are usually easier to diagnose than widespread tree declines. By nature, agricultural systems are highly managed and ecologically simpler than forests. Also, larger resources have been devoted to developing and understanding agricultural systems than natural forests. Figure 5.1.11 shows the states in the contiguous United States where air pollution can affect trees or crops (Mackenzie and El-Ashry 1989).
The air pollutants of greatest national concern to agriculture are ozone (O3), sulfur dioxide (SO2), nitrogen dioxide (NO2), sulfates, and nitrates. Of these, ozone is of greatest concern; the potential role of acid deposition at ambient levels has not been determined. At present deposition rates, most studies indicate that acid deposition does no identifiable harm to foliage. However, at lower-than-ambient
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