Biological Diversity Ecosystem Condition and Productivity Soil and Water Role in Global Ecological Cycles Economic and Social Benefits Society's Responsibility
Indicator 2.1 Total growing stock of both merchantable and nonmerchantable tree species on forest land Indicator 2.2 Additions and deletions of forest area by cause Indicator 2.3 Area of forest disturbed by fire, insects, disease, and timber harvest Indicator 2.4 Area of forest with impaired function due to ozone and acid rain Indicator 2.5 Proportion of timber harvest area successfully regenerated
Indicator 2.4 - Area of forest with impaired function due to ozone and acid rain
core indicator


Ground-level (tropospheric) ozone and acid rain are two major air pollutants that can have significant impacts on forest health and productivity. Ozone is a highly reactive gas that damages living cells and interferes with the normal healthy functioning of living organisms, including trees, other plants, and humans. Acid rain carries acidifying compounds, most notably sulphate and nitrate, that lead to the loss of important nutrients from forest soils. Both pollutants are caused primarily by anthropogenic emissions.

Ground-level ozone concentrations have generally increased over the last century. Today, typical "clean" ambient air contains about 30-40 parts per billion (ppb) of ozone compared with approximately 10-15 ppb a century ago. However, concentrations appear to be stabilizing or decreasing. Trends in the annual fourth highest daily maximum 8-hour ozone concentration (ppb) for 1993-2002, averaged across western and eastern Canada, show little overall change or slight decreases in some areas, with values ranging between about 52 and 53 ppb in western Canada and 68 and 77 ppb in eastern Canada (Environment Canada 2004).

Governments are taking action to reduce the impacts of ozone. In June 2000, the federal, provincial, and territorial governments, except Quebec, signed the Canada-wide Standard for Ozone. This standard commits government to significantly reduce groundlevel ozone to 65 ppb by 2010, as measured using the annual fourth highest daily maximum 8-hour ozone concentration averaged over three consecutive years.

The 2004 Canada-United States Air Quality Progress Report provides a map of Canada depicting the average annual fourth highest daily maximum 8-hour ozone concentration for 2000-2002 (Figure 2.4a). This map is a very general depiction of ozone concentrations and, due to scale, misses some sensitive regional and local pockets of high ozone concentrations. However, in general, three major areas in Canada have consistent episodic ozone events every summer: southern British Columbia, the Windsor- Québec corridor, and southern Atlantic Canada.

Figure 2.4a

Figure 2.4a Ozone concentrations (ppb) in the Canada-United States border regions: average annual fourth highest daily maximum 8-hour ozone concentration, 2000-2002. (Source: Canada-United States Air Quality Committee 2004)

The Canada-wide Standard for Ozone is not specific to forest ecosystems but serves as a surrogate benchmark. Elevated levels of ozone above 65 ppb are considered potentially harmful to the health of sensitive forest tree species.

Information on impacts of episodic ozone events on forest ecosystems is sparse, particularly for southern British Columbia and the Windsor-Québec corridor. Most of the thousands of studies carried out on the response of plants to ozone have been limited to single, or very few, young, immature trees. Measuring the impact of ozone on ecosystem structure and function will require larger scale, longer term, and much more expensive controlled-exposure studies. Therefore, Natural Resources Canada, Canadian Forest Service has partnered with the United States Forest Service, several universities, the governments of the United Kingdom and Finland, and others in the Aspen FACE (Free-Air Carbon Dioxide Enrichment) Experiment.

Established in 1997 in northern Wisconsin on 32 ha of forest land, Aspen FACE is the first open-air study on the long-term response of forest trees to the two major greenhouse gases, carbon dioxide and groundlevel ozone, affecting global forests.

Results from this study show that the primary impact of elevated ozone is on photosynthesis that was reduced by 20-30% in aspen and aspen/birch stands. Decreased photosynthesis can create cascading ecosystem impacts leading to detrimental changes in gene expression, foliar and root biochemistry, and to reductions in volume growth (20-26%), biomass (both above and below ground), fine root longevity, forest floor respiration (20% late in season), and net primary productivity (16%). Other consequences include increased incidence of foliar rust on some tree species, and of some important hardwood leaf feeders (e.g., forest tent caterpillar), as well as reduced "natural enemy" populations (parasites, predators), and even improved escape mechanisms in aphids. Eventually, these ecosystem level responses may result in changes to soil microorganisms and soil fauna.

Acid rain has long been acknowledged as a threat to forest health, and governments have taken numerous steps to reduce the area affected by acid deposition. In 1998, the federal, provincial, and territorial ministers of Environment and Energy signed the Canada-Wide Acid Rain Strategy. This strategy established a long-term goal of remaining below certain critical loads of acidifying compounds.

Critical load is defined as the highest deposition of acidifying compounds that will not cause chemical changes leading to long-term harmful effects on the overall structure or function of an ecosystem. Critical loads of acid deposition have been developed for certain Canadian forest soils (Arp et al. 2001), reflecting the inherent capacity of soils to buffer incoming acidity. When combined amounts of sulphur and nitrogen deposition are below these loads, forest ecosystems are buffered against adverse effects. However, if critical loads are exceeded for long periods, essential nutrients for tree growth and vigor are leached from the soil. Continued loss of soil nutrients leads to a decline in forest productivity, through reduced tree growth, and to increased vulnerability of the tree to insect and disease attack. Reduced tree vigor also increases potential adverse impacts from climatic variation, such as drought or extreme temperatures.

Figure 2.4b shows the extent of exceedance of critical loads associated with soils for eastern Canada. The map depicts exceedances given a no harvest scenario. In some cases, exceedances would be greater if nutrient depletions associated with harvesting were also considered. On average, areas with exceedance cover almost 52% of eastern Canada. The lowest percentage area of exceedance is in Prince Edward Island (3.5% of mapped area), while highest exceedances occur in eastern Ontario and southern Quebec. Overall, preliminary estimates indicate that more than 48% of the upland forest area in Ontario and Quebec, and over 35% of the upland forest of Nova Scotia and insular Newfoundland, receive acid deposition in excess of the critical load.

Figure 2.4b

Figure 2.4b Critical load exceedance of acidifying compounds for forest soils in eastern Canada (no harvest scenario). (Source: Forest Mapping Working Group of the New England Governors/Eastern Canadian Premiers Secretariat in cooperation with Trent University, Ontario; Environment Canada; and Natural Resources Canada, Canadian Forest Service)

Efforts are underway to improve the accuracy of critical load estimates and exceedances by using better estimates of dry deposition and harvesting removals, and by investigating the linkage between exceedance of the critical load and adverse biological effects.

For example, models applied to south central Ontario predict that soil acidification will continue, even with proposed reductions in sulphur emissions. Approximately 40 million ha of Ontario's forests receive deposition of sulphur and nitrogen in excess of the critical load. If nutrient removals through forest harvesting are taken into account, the area of exceedance of critical load increases to approximately 45 million ha and the magnitude of the exceedance also increases (Watmough et al. 2004).

In Quebec, researchers have found that areas subjected to critical load exceedance experience a 30% reduction in forest growth. Most of the research plots where deposition has exceeded critical loads are located in nutrient-poor sites in the Laurentian Mountains of the Canadian Shield and in the Appalachian range of southeastern Quebec. The researchers concluded that further reductions in national and international sulphate and nitrate emissions rates should be undertaken to protect Quebec forests from excessive soil acidification (Ouimet et al. 2001).

Much of the southeastern Canadian forest landscape is affected by both acid rain and elevated levels of ozone. For these areas, the cumulative effects on forest health may be more severe than impacts based solely on either pollutant. Furthermore, interactions between the two pollutants and their chemical precursors make it difficult to predict the extent to which emission reductions will reduce the level of either pollutant.