Gases such as ozone (O₃), sulfur dioxide (SO₂), and nitrogen oxides (NO_x) have always been part of the environment, occurring only locally or in trace concentrations (e.g., O₃ formation under high solar radiation, SO₂ set free by volcanic activity, and NO_x by N cycling in ecosystems). Over the past 50-100 years, concentrations of these and other constituents of the atmosphere have increased. Scientists now recognize that this increase represents anthropogenic pollution, which can have both local and widespread distributions with concentrations above natural levels, and has the capacity to affect vegetation directly and indirectly. There is evidence that some species with short generation times may be evolving in response to these relatively new selective forces, as air pollution alters plant productivity and, ultimately, fitness (Taylor et al., 1991). However, the postindustrial appearance of air pollutants has occurred within such a small part of the lives of long-lived perennial plants that some species may not be able to adapt to this new, anthropogenic stress. Nevertheless, resource acquisition, allocation, and organ differentiation proceed in the presence of, and are affected by, air pollutants. A number of texts have reviewed responses of vegetation to SO₂ (Winner et al., 1985), O₃ (Guderian, 1985), and acid deposition (Schulze et al., 1989), as well as the effects of air pollutants on forests (Smith, 1990). [...]