The Use and Limitations of Dendrochronology in Studying Effects of Air Pollution on Forests

Abstract

The annual ringwidths of trees can be used to search for hypothesized air pollution effects on forests. This search is extremely complicated by the inherent statistical properties of ringwidth data and the high level of uncertainty regarding the sources of variance observed in the ringwidths. A linear aggregate model for ringwidths is described which highlights the general classes of variance which may be found in a tree-ring series. Dendrochronological principles and techniques are described which can be used to create a tree-ring chronology that is suitable for rigorous statistical analysis and hypothesis testing. The need to model climatic influences on tree growth prior to the search for pollution effects is necessary and a method for achieving this is described. Only after the variance due to age trends, stand dynamics effects and climatic influences has been accounted for can any confidence be placed on inferred pollution effects. An analysis of a red spruce tree -ring chronology indicates that a decline in ringwidths since 1968 cannot be explained by a linear temperature response model using monthly climatic variables. However, threshold responses to climate that could be responsible for the decline need to be considered before the anomalous decline can be attributed to non-climatic influences such as pollution.