Pollution-induced injury in North American forests: facts and suspicions.

Toxic gases are the only airborne chemicals that have been demonstrated to cause detrimental effects on forest trees in North America. Ozone is the only regionally dispersed pollutant known to have injured foliage, decreased radial growth, decreased resistance to insects and diseases, and increased mortality of sensitive tree species over a wide geographic range. All of these effects have been observed in eastern white pine (Pinus strobus L.) throughout its natural range, in some conifers and hardwood trees in southern California, and in a few tree species in the central Appalachian Mountains. Very little research has been done to determine effects of airborne chemicals on forests as a whole. At present, there is no conclusive evidence that acid deposition or other regionally dispersed pollutants are affecting the productivity of any forest in North America.