The role of air pollution in forest decline : a literature review and survey of white ash trees in northern Indiana
An observed mortality and reduction in the growth rate of trees in forests across the United States and Europe led to the proposal of a number of possible causes for the forest decline. Connecting all the observed symptoms to a single proposed cause has proven difficult.The term "decline" describes complex diseases that result in a progressive weakening of trees and lead to dieback, the death of portions of the foliated canopy (21). Gradual loss of vigor involving reduced growth rate and increased susceptibility to secondary stresses typically ensues. Death may occur unless the stress is removed.Among the factors that could have an effect on the growth rate of plants are climate conditions, soil moisture and nutrient availability, competition, insects, fungi, disease, and air pollution. Research focusing on the role of air pollutants in triggering the forest decline is complicated by the complex nature of the problem evident in the large number of variables introduced by other influencing factors. A single physiological effect may result from one or many causes. In turn, that symptom may become a factor that predisposes, incites, or contributes to still other physiological effects.Declines have been widespread in European and North American forests in recent decades. Economic and ecological impacts are great enough to cause much concern among scientists and in the forest industry.Indiana is not exempt from the problem of forest decline. White ash trees in northern Indiana forests have experienced alarming canopy dieback and mortality in recent years. This research paper will have as its purposes a review of the pertinent literature and a presentation of data describing the growth rate of white ash trees in Indiana.