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Canadian Smog Science Assessment Highlights and Key Messages

Effects on Ecosystem Health

In Canada, quantification of the effects of smog on ecosystems is presently limited to impacts of ozone (O3) on selected plant species. O3 is taken up by plants through pores, or stomates, in their leaves. Once inside the plant, O3 can cause direct physical damage leading to premature senescence (aging), reduced uptake of carbon dioxide (CO2) and reduced primary productivity. O3 can also affect a plant indirectly by diverting energy use from important physiological processes to the detoxification of O3.These effects on individual plant health can lead to ecosystem changes, as plant species that are more resistant to O3 can become more dominant than those that are less resistant. A plant’s response to O3 is a function of both duration and concentration of exposure, varies with species and plant development stages, and is modified by environmental factors, such as soil moisture content and humidity. 

Currently, exposure-based metrics are the most appropriate tools for quantifying concentration exposure-response relationships and are well correlated with the metric used to calculate the Canada-wide Standard for O3. This indicates that measures aimed at reducing the 8-hour average O3 concentration would also reduce vegetation exposure to O3.

The impacts of particulate matter (PM) on vegetation are dependent upon the chemical constituents and the particle size, as these define the plant-specific PM phytotoxicity levels. Plant response to PM is largely due to the resultant changes in soil chemistry (i.e., acidic PM deposition can leach out soil nutrients into surface waters) rather than direct deposition on the plant. Various PM constituents taken up by the plant from the soil can interfere with photosynthesis, thereby reducing plant growth and productivity. PM can also cause physical damage to plant surfaces via abrasion. Although beyond the scope of this assessment, PM deposition has also been shown to affect surface water chemistry and species diversity10.

The impacts of PM and O3 on wildlife is an area of emerging concern in understanding impacts to ecosystems. Research has focused on indirect effects of PM and O3 on wildlife species through influences on vegetation, soil conditions and changes to habitat on which they depend. O3 animal exposure studies (e.g., mice, rats, guinea pigs, rhesus monkeys), which are designed to extrapolate results for assessing exposure effects on humans, indicate species are variable in their responses and in their sensitivity to exposure; thus, the same can be reasonably assumed for wildlife species. At this time, however, there is no research available to extend the results of these laboratory studies to assess the impacts on wildlife species or to identify which species are more sensitive to exposure.


10. Environment Canada, 2005. The 2004 Canadian Acid Deposition Science Assessment. Environment Canada, Gatineau, Quebec.