Variation in and natural selection for air pollution tolerances in trees

Abstract

An increasing number of forest ecosystems in North America and Europe are suffering from decline problems that have been related directly or indirectly to air pollution. It is our contention that air pollution has contributed to major changes in the genetic structure of many such forest ecosystems. Natural selection against sensitive species and individuals within species has long been known to occur near major point sources of air pollution such as ore smelters. Recently we have shown the first evidence for more subtle within-species population changes that have occurred with Populus tremuloides and Acer rubrum in eastern North America. These widespread changes, which result in a residual population consisting primarily of tolerant individuals, appear to be related to the impact of ozone and/or other regional air pollutants over large areas. Loss of a significant portion of the germplasm in natural forest ecosystems must be considered a significant problem, particularly if some genes are found exclusively in the sensitive genotypes as suggested by isozyme studies of Picea abies clones. Germplasm loss and the subsequent decrease in genetic diversity could be a more important air pollution impact in the long run than short-term economic losses.