Pollutants may affect ecosystems at various levels: accumulation in the plants, soil, or ground water; damage to consumers; altering competition; disrupting biogeochemical cycles; disrupting stability; breakdown of stands and associations; and expanding denuded zones. These changes may originate as consequence of pollutant exposures for periods ranging from a few hours to a year or more. In the last decades the character of air pollution influences on ecosystems has been significantly altered. While in time past the impact of high concentrations of pollutants was limited to small surrounding areas in the vicinity of the pollution sources, today large areas with low concentrations of pollutants at great distances from the emitting sources can be found; there the character of pollutant influence is partially changed, and the impact of acid precipitation takes places in a more expressive way. Conifer forests provide the most seriously affected examples. Great differences exist between various ecosystems mainly because of the different sensitivity of plant species, as well as of individuals and clones. Also, at low concentrations of pollutants, the total biomass production of natural ecosystems can be altered. If the main compartment of the ecosystem is a very sensitive one, e.g. Norway Spruce, its biomass production, reduced on account of pollution, can be replaced by that of one of a more resistant tree species or plant species of the ground vegetation. But even in this case the reduction in the production of, for example, economically significant forest products, might be a serious and continuing impact on forest management. Besides, the damaging of the natural ecosystems, especially forests, over large areas, might also seriously affect the other functions of the forests in the landscape, for example, their ability to protect the soil, their effect on water-flow from the forest watersheds, and their suitability for recreation purposes.