Air Pollution Interactions with Natural Stressors

A complete review of solid, liquid, and gaseous air pollutants and their impacts on plant and animal species, in combination with the full range of natural stresses, is clearly outside the scope of this chapter. We focus here on plants; on the four primary phytotoxic gases, ozone (O₃), nitric oxide (NO), nitrogen dioxide (NO₂), and sulfur dioxide (SO₂); and on the principal natural stresses due to mechanical loading, shortage of water, pests and diseases, nutrient deficiency, and low temperatures. The great majority of published material on air pollution responses is of course on single species or agricultural bicultures such as grass/clover mixtures. Even for those studies in which multiple natural species have been involved, such as forest ecosystem damage caused by smelter emissions in North America (e.g., Gordon and Gorham 1963), the pollutant load has often been so high, and the damage so acute, that the results cannot be extended to chronic pollutant effects, let alone to their interactions with other stresses. Consequently, our deductions on biodiversity impacts are based primarily on the interpretation of environmental and physiological data from rather simple biological systems. Superimposed on this already complex situation of pollutant exposure we have a set of relatively new concerns brought about by increasing atmospheric carbon dioxide and UV-B radiation, and by possible changes in the distribution of global climate patterns that we have previously considered stable. If we remember, as background to this discussion, that around one-half of all known species inhabit tropical rain forests about which we have virtually no information on air pollution or its effects (Wilson 1988), then the sheer scale of the problem becomes apparent.