The Effect of Air Pollution on Western Larch as Detected by Tree-Ring Analysis

The growth response of western larch (Larix occidentalis Nutt.) to sulfur dioxide was studied in the Columbia River Valley near the lead-zinc smelter at Trail, British Columbia. A dendroecological (tree-ring) analysis was employed to assess the relative importance and longterm effects of sulfur dioxide on tree growth. Increasing sulfur emissions in the early 1900’s, reaching a maximum in 1930, resulted in a concomitant decrease in annual tree growth. After 1930, sulfur emissions decreased dramatically with the implementation of pollution abatement measures. However, the growth recovery of western larch to the decreased sulfur emissions was not immediate and reflected the residual nature of the sulfur dioxide effect on tree growth. Correlation and regression analyses were utilized to develop multivariate models for the larch sites sampled at varying distances from the Trail smelter. Climatic models developed for the control site (tree-ring) chronology were applied to site chronologies located within the sulfur dioxide affected area to remove the effects of climate on tree growth and examine the residual response of the system. The pattern of the residuals closely resembled the sulfur emissions from the smelter with the most negative residuals occurring when sulfur emissions were greatest. Further regression modelling identified the relative importance of sulfur emissions, prior growth, temperature, and precipitation to annual tree growth. In those sites closest to the smelter, sulfur emissions accounted for the greatest proportion of the variance calibrated by the regression models. As distance from the smelter increased, sulfur emissions became less important in the site models. In all the site models temperature, particularly summer temperature, appeared to be a primary limiting climatic factor. Prior growth also accounted for considerable variance in the models with precipitation variables appearing to be of lesser importance in explaining the variance in the site chronologies. The results of this study demonstrate the applicability of tree-ring analysis in identifying and quantifying the long-term effects of air pollution on forest communities. It also provides a basis for examining the interrelationships between air pollution, climate, and annual tree growth.