A decade of forest tree monitoring in Canada: evidence of air pollution effects

Abstract

In Canada, the eastern tolerant hardwood forest is potentially at risk from air pollution. Surveys indicate that, overall, the condition of hardwood forests has either not changed or has marginally improved. However, stands growing on shallow, poorly buffered soils have a higher frequency and severity of decline symptoms, and some of these stands have continued to decline, in contrast to the general trend. In Ontario on the most sensitive sites (<6 mequiv. exchangeable bases/100 g), decline symptoms increased coinciding with a decrease in B horizon pH and base saturation and with an exchangeable Al increase. On these sites the chemical analysis of foliage, root, xylem, and soils indicate that nutrient deficiencies, nutrient imbalances, or metal/nutrient antagonism are related to stand decline. There is a consistent spatial relationship between white birch decline in the Bay of Fundy area and acidic sea fog. Acid fog injures and leaches foliage and hastens soil acidification, leaving the stands weakened and making them more sensitive to stress, particularly drought. Some recent studies suggest that forest growth rates are declining and that the relationship with climate has deteriorated, implying that the forests are responding to a regional, nonclimatic, nonpathogenic stress. This could be a response to altered forest nutrient cycling associated with acidic deposition and chronic O3 exposure. A study of sugar maple in Ontario estimated that after the mid-1960s growth has declined between 0.66 and 0.96 m3·ha-1·year-1, and that the decline was greatest on poorly buffered soils. Without enhanced forest management and more stringent air pollution regulations, the sustainability of forest productivity in Eastern Canada is likely to be difficult because of air pollution and soil fertility losses.Key words: dendroecology, acid deposition, ozone, forest decline, sugar maple, climate change.