Forest Stress: Symptomatic Foliar Damage Caused by Air Contaminants

Under conditions of sufficient dose, air pollutants directly cause visible injury to forest trees. The accumulation of particulate contaminants on leaf surfaces or the continued uptake of gaseous pollutants through leaf stomata will eventually result in cell and tissue damage that will be manifest in foliar symptoms obvious to the trained, but unaided eye. This direct induction of disease in trees by air pollutants is the most dramatic and obvious individual tree response of all Class II interactions. It is the only Class II interaction that can be detected in the field by causal observation. Unlike altered reproductive strategy, nutrient cycling, tree metabolism, or insect and disease relationships; the degree of foliar symptoms induced by air pollutants can be relatively easily observed, inventoried, and quantified. In the presence of sufficient dose, tree damage may be of sufficient severity to cause mortality. Tree death directly induced by ambient air pollution exposure is considered a Class III interaction and is treated in Chapter 16.