Dendroecological applications in air pollution and environmental chemistry: research needs

Summary During the past two decades, dendrochronology has evolved in new dimensions that have helped address both the extent and causes of impacts of regional scale environmental pollution on the productivity and function of forest ecosystems. Initial focus on the magnitude and timing of alterations of baseline growth levels of individual forest trees has now broadened to include characterization of the geographic extent of effects, their distribution among species, and their relationship to soils and biogeochemical cycles. Increasingly dendrochronology has benefitted from and contributed to improved understanding of the physiological and biogeochemical basis of air pollution effects on forest ecological processes. In addition, the need to consider levels and types of remedial action has raised concerns about the relative roles of anthropogenic and natural causative factors. The subdisciplines of dendroecology and dendrochemistry have evolved in response to those needs. Such applications have extended the field from its initial primary focus on historical growth and climatic reconstruction to an emerging role as an exploratory research tool with the potential to address basic questions about the roles of air pollution in modifying relationships between the amount, timing, distribution, and quality of tree growth and biogeochemical and atmospheric processes. In this paper we focus on two regional scale air quality issues, acidic deposition and tropospheric ozone, as stressors. We evaluate past success, current limitations, and future potential of dendrochronology as an investigative tool for both quantifying and understanding the effects of these stressors on forests. Important issues related to the use of dendrochemistry to evaluate effects of acidic deposition include the role of natural vs anthropogenic processes in cation mobilization in soils; biological and geochemical significance of increases in potentially phytotoxic metals and depletion of essential base cations in stem wood; and quantitative vs qualitative interpretation of patterns of element changes in wood related to metal mobility and species differences in accumulation. Shifts in root growth, function, and distribution and increased sensitivity of tree growth to temperature stress are important indicators that cation depletion can alter forest function and the dendroclimatic signal. Critical challenges in evaluating forest responses to ozone, include defining the relative roles of episodic and chronic exposures in seasonal and annual growth cycles, and the quantifying impacts of ozone on the water relations of trees and stands. Here high-resolution measures of diurnal growth and water use patterns have the potential to identify critical features of both pollutant exposure and plant response. These insights should enhance our analytical capabilities in examining annual-scale measures of growth and provide needed understanding of changes in relationships of growth to climate. We conclude that dendrochronology, when coupled with mechanistic understanding of underlying ecological processes influencing growth, has been, and will continue to be, a valuable monitoring and investigative tool for exploring relationships between trees and their growing environment. We expect this role to become even more important in the future as better ways are sought to evaluate and predict forest growth and function in a changing global environment.