How Do Trees and Other Long-Lived Plants Survive in Polluted Environments?

Most established vegetation within the vicinity or influence of industrial and urban areas is subjected to the regular or episodic impact of pollution. Complete decimation of the vegetation as a consequence of this is a relatively infrequent occurrence, although mining and industrial wastes containing metals or other toxins may remain virtually devoid of vegetation. The acute phytotoxic effects of many environmental contaminants are well known, but studying the possible chronic effects of lower concentrations of atmospheric contaminants on growth and vigour of plants is probably of more relevance, particularly to longlived plants. This has been a major focus of research in European and North American forests (Pitelka, 1988; Schulze, 1989) but much evidence is speculative due to the lack of long-term data (Blank et al., 1989). In many herbaceous plants the selection pressure of pollution has led to the natural evolutionary development of tolerant plant genotypes in response to a wide range of airand soil-borne pollutants and to herbicides (Bradshaw & McNeilly, 1981; MacNair, 1981; Murphy, 1983; Taylor, 1990). However, unlike the situation for many annual or colonizing plants, the long generation time of trees and other long-lived perennial and clonal plants prevents a rapid selection of tolerance. In this situation, as with other stresses which affect plants in a temporary or fluctuating manner, facultative adaptations (phenotypic plasticity) may be more appropriate (Bradshaw & Hardwick, 1989). This paper reviews the differing responses of plants to pollution, comparing the impact on individual plants, populations and more permanent plant communities, and considers how current knowledge explains the longevity of trees and clonal plants in polluted environments.