Indicator 3.3 - Proportion of watersheds with substantial stand-replacing disturbance in the last 20 years
supporting indicator


Stand-replacing disturbances such as fire, harvesting, and large-scale insect damage have substantial impacts on the water yield, timing, and peak flows in rivers and streams. Numerical estimates of the extent of these disturbances are described in Indicator 2.3 (Area of forest disturbed by fire, insects, disease, and timber harvest). Disturbances will have greater impacts if watersheds are small or disturbed within a short time period.

National measurements of water yield, timing, and peak flows are difficult to obtain because this task requires expensive monitoring equipment. Therefore, researchers are evaluating the proportion of watersheds affected by stand-replacing disturbances to estimate the potential hydrological impacts of these disturbances. To reach this goal, researchers will need to determine how best to measure this indicator, in particular the extent of disturbance in a watershed that constitutes a significant impact. Other issues, related to scale and definition, will also need to be resolved if this indicator is to reflect national trends. For instance, watersheds can be defined by area or by stream order and the appropriate scales must be determined. This is a complicated and expensive process, but research is underway in several jurisdictions to address these issues.

About 119 million ha of Canada's forest land are currently accessible and managed for timber production. Most harvesting is carried out by using variants of a clearcut, but other harvesting systems, usually involving the partial removal of the canopy, are increasingly being used. Disturbance from harvesting affects approximately 1 million ha annually and is more or less constant from year to year. The extent of disturbance from wildfire is variable and usually ranges from 1 to 7 million ha annually. Damage from insects and disease tends to be cyclical and also varies widely annually.

At present, provinces and territories are unable to provide estimates of the proportion of watersheds that have experienced substantial stand-replacing disturbances since the mid-1980s. However, research aimed at determining the proportion of watersheds that can be harvested without causing unacceptable damage is progressing. In addition, scientists are studying a range of forest values, particularly nontimber forest products and other socioeconomic values. This should lead to detailed watershed databases that will provide the foundation for planning tools to predict the hydrological impacts of forest harvesting and other disturbances across various spatial and time scales.

These research activities often take place locally or regionally such as within watersheds or on land under a forest management license. For instance, British Columbia is already augmenting its existing watershed database, which contains 19000 entries, to eventually include about 3 000 000 watersheds. The current database features data on substantial stand-replacing disturbances that took place from the mid-1970s to the mid-1990s.

In Ontario, work is underway to determine the impacts of forest harvesting on small watersheds. This will eventually lead to the development of indicators to determine harvesting impacts on water quality at various spatial scales across forest landscapes, and of predictive tools to help forest managers gauge the potential impacts of harvesting on watersheds.

In Quebec, significant work has been completed looking at the effect of forest harvesting on the peak output of rivers. These efforts have led to the development of a method for calculating the equivalent cut area (ECA) of a catchment in softwood-dominated forests. An acceptable maximum level of ECA per catchment has been established to protect aquatic habitats against potential significant changes in river output. Further studies have shown that under Quebec operating conditions, the acceptable maximum level of ECA is rarely exceeded in catchments of more than 10 000 ha and that aquatic ecosystems are not damaged. As a result, Quebec plans to concentrate its efforts to measure Indicator 3.3 on those catchments with rivers containing either Atlantic salmon or landlocked salmon because of the sensitivity and the socioeconomic importance of these species.

At this stage, the lack of data makes it impossible to reasonably assess the sustainability of current forest practices on the basis of this indicator. However, the research projects presented previously and similar initiatives underway across Canada are expected to begin yielding significant results in 2006 or soon after.