Tree-ring analysis has frequently been used to assess the impact of pollution on tree growth around point sources. Its use in studies of the impact of air pollutants operating on a regional scale is more controversial. A variety of analysis procedures is available, and these should be carefully selected in relation to the problem under investigation. To assess the impact of regional air pollutants, nonstationary response functions, as developed using the Kalman filter technique, offer considerable potential. In the current spruce–fir debate in North America, tree-ring analysis has indicated that the current declines are unprecedented within the last 200 years in terms of the duration of depressed growth or the extent to which growth has been depressed. It has indicated that red spruce (Picearubens Sarg.) is particularly susceptible to certain climatic conditions (higher than average July and August temperatures and lower than average November and December temperatures). Since the onset of the decline, the growth of red spruce has been less than predicted from climatic factors, suggesting that the importance of some other effect on growth has increased or that there has been a change in the climatic factors influencing growth. In Europe, tree-ring analysis has helped to quantify the relationship between crown density and increment.
The extreme drought in the 1920s and its effect on tree growth deduced
from tree ring analysis: a case study in North China