Boreal Shield Ecozone

Changing Forest Landscape Conditions

Forest soil acidification: In the mid-latitude regions of Ontario and Quebec, relatively high rates of acid deposition have resulted in adverse impacts on soils associated with upland forests (Arp et al. 1996). The best evidence in support of pollutant-driven soil acidification comes from long-term watershed studies. Research from the Turkey Lakes Watershed in central Ontario shows that sulfate and nitrate deposition accelerates soil acidification by increased leaching from the soils of essential nutrients (calcium, potassium, and magnesium) (Morrison et al. 1995). On sites with inherently low nitrogen content, most of the nitrate deposition is absorbed by conifer and broadleaf foliage or is retained in the soil (Hazlett et al. 1995).

Major insects and diseases: In the Boreal Shield ecozone, insect defoliators are the greatest contributors to mortality and forest change. However, only a few species routinely infest large areas and are economically damaging to the commercial forest. The most significant insects attacking conifers are the spruce budworm (balsam fir and spruces), hemlock looper (balsam fir and eastern hemlock), and jack pine budworm (jack pine).

The forest tent caterpillar and the large aspen tortrix both attack poplars and are the most damaging insects of boreal hardwoods. Root rots (mostly Armillaria species) are chronic in older stands of both conifers and hardwoods.

Large spruce budworm outbreaks have fluctuated greatly in recent history. Changes in forest composition associated with harvesting and fire suppression are reasons suggested as influencing the frequency and duration of these outbreaks (Blais 1983). At its peak in 1975, the budworm defoliated 54 million ha at moderate to severe levels. Most recent figures suggest approximately 11 000 ha of defoliation within Quebec and 141 000 ha in Ontario in 1998 (Bordeleau 1998, Howse and Scarr 1998). By far, based on tree mortality, the most affected area was north-central and northwestern Ontario. Eastern Ontario had levels of infestation similar to that recorded for Quebec. The general trend of defoliation for both provinces has been downwards (Bordeleau 1998, Howse and Scarr 1998). Growth of balsam fir begins to be affected once 30% of the foliage is destroyed. Mortality results after 4-5 years of moderate to severe defoliation (Hall et al. 1998). The defoliation in Ontario resulted in 4 000 ha of tree mortality in 1998 for a total of 8.5 million ha since the early 1980s (Howse and Scarr 1998).

Historically, outbreaks of the jack pine budworm covered the range of jack pine throughout the western portion of the ecozone, only rarely extending into central Ontario. After 1967, outbreaks of the insect were detected regularly in central and eastern Ontario (Howse and Meating 1995). The reasons for the spread eastward are unknown. Outbreaks in the Prairie provinces have increased in size over time, likely as a result of improved fire control which means increasing the number of older stands that are susceptible to the insect (Volney 1988). This insect causes growth declines in jack pine and occasionally results in mortality.

Forest tent caterpillar defoliation has steadily increased in the Boreal Shield of Ontario since 1995 from 240 000 ha to nearly 3 million ha in 1998 (Howse and Scarr 1998). In Quebec, defoliation has decreased to minimal levels from a peak of 450 000 ha in 1988 (annual tree insect and disease reports for Quebec 1989-1996, Bordeleau 1998). Similarly, minimal defoliation by the Large Aspen Tortrix has been recorded within Quebec in recent years (Bordeleau 1998), whereas, in Ontario, some 198 000 ha of defoliation were mapped in 1998 (Howse and Scarr 1998). Repeated defoliation results in growth losses and several continuous years of defoliation result in mortality (Hall et al. 1998).

More: Native Forest Insects and Diseases

Infestation of balsam fir sawfly: The balsam fir sawfly is indigenous to mainland Canada but was accidently introduced to Newfoundland in the 1920s or 1930s. It attacks intensively managed young balsam fir stands aged 20 - 40 years. In 1996 and 1997, these stands were severely infested, with the area affected increasing sharply from approximately 1200 ha in 1995 to over 40 000 ha presently. These balsam fir stands are close to paper mills and represent a significant silvicultural investment to the companies involved. Similar levels of damage by the sawfly have not been observed in balsam fir forests not under intensive management (D. Ostaff, Canadian Forest Service, personal communication).

Changing Biodiversity

Changing tree species mix: In Ontario, a permanent shift from softwood to hardwood cover is occurring in areas where harvesting has replaced fire as the dominant disturbance (Hearndon et al. 1992). Clearcutting, because it does not mimic the ecological effects of fire, sometimes results in fundamental ecological shifts in fire-dominated ecosystems (Carleton and MacLellan 1994). The combination of clearcutting and fire suppression favors establishment of such species as trembling aspen and white birch to the detriment of conifer species such as black and white spruce and jack pine. Carleton and MacLellan (1994) found that spruce regeneration declined by 77% and aspen and birch increased more than threefold. In Ontario, the proportion of spruce within the actively managed forest has diminished to 4% from an historical 21% (Hearndon et al. 1992).

Plants under pressure: The globally rare arboreal lichen Erioderma pedicellatum has been eliminated from Europe and its only known significant remaining populations are in the Newfoundland portion of the Boreal Shield (Ringius 1997). Vulnerable to timber extraction, it is also highly sensitive to air pollution.

Red pine and white pine have suffered serious declines in Newfoundland; special genetic reserves have been created to protect remnant populations. Red pine is a species that likely experienced a "genetic bottleneck" in its evolutionary past, and has limited genetic diversity today compared to most other boreal conifers (Mosseler et al. 1991, 1992). Both pine species have been greatly reduced in the continental portions of their ranges. In the case of white pine this demise is attributable to excessive harvesting and white pine blister rust, an introduced disease. Red pine decline may be related to the reduced incidence of wildfire.

Changing Atmospheric Environment

Acid rain - a continuing problem: Forests of the southeastern Boreal Shield, Mixedwood Plains, and Atlantic Maritime ecozones are exposed to frequent acid rain (Boulet 1990). In addition, much of the forest is growing on nutrient-poor, naturally acidic, shallow soils (Bernier and Brazeau 1988; Camiré and Ouimet 1994; Li 1985). These factors combine to make these forests particularly susceptible to adverse effects of acid deposition (Arp et al. 1996). A hardwood forest health survey for the period 1986 to 1995 revealed a generally healthy hardwood forest in Ontario, except in pollution-sensitive soils where a trend towards forest deterioration is occurring. Bowers and Hopkin (1997) and Lachance et al. (1995) also state that the forest is generally healthy. Where deterioration is occurring, pollution is probably a contributing factor, but cause/effect relationships are inconclusive as climatic conditions, such as drought or other influences (insect attack), certainly affect the condition of sugar maple and other trees (McLaughlin et al. 1996, Ryan et al. 1994).

Acid rain and boreal lakes: The effects of acid runoff from forest soils on boreal lake ecosystems have significantly reduced fish populations (Jeffries 1997). Fish damage estimates for eastern Canadian lakes were compiled by taking estimates of the original pH of the lakes and relating them to historic populations of fish. Current lake acidity served to estimate current populations of fish in these lakes. Conservative estimates suggest that at least 162 000 populations of fish have disappeared from the lakes of the eastern Boreal Shield and Atlantic Maritime ecozones since the start of anthropogenic acidification of these lakes (Jeffries 1997). A `population' comprises all the fish of a given species for a specific lake. A lake may contain several populations of fish. Because the food web of many of these acidified lakes has been altered, birds (such as loons and ducks) have been forced to search for new, perhaps less suitable, habitat.

More: Acid Rain: Impacts and Risks for Forest Ecosystems

Ground-level ozone: No damage to trees or other forest species have been documented from ground-level ozone. However, forests in the south-central portion of the ecozone are being exposed to damaging levels of ground-level ozone concentrations (Thomson 1992). Trends noted in ozone monitoring data indicate increasing levels between 0.1 and 0.5% per year. Native boreal tree species range from ozone sensitive to ozone intolerant, although some commercially important conifers have not been evaluated. Reductions in growth, early needle fall, alterations in carbon allocation, and predisposition to insects and diseases are expected following several seasons of exposure to current levels (Pearson and Percy 1997).