Indicator 4.1.1 - Net change in forest ecosystem carbon
core indicator


Plants absorb and store carbon dioxide (CO₂), one of the main greenhouse gases present in the earth's atmosphere, to produce the carbohydrates they require for growth through photosynthesis. Because of their extensive distribution on earth and their large capacity to store carbon, forests have an important role in the global uptake of CO₂, its storage, and its release in the atmosphere. Carbon stock changes reflect the uptake of carbon through forest growth, the releases of carbon through tree respiration, decay, insects, diseases, and fire, and the storage of carbon in forest products after harvesting.

Disturbances like fire not only release carbon at the time of the disturbance, but also transfer large quantities of carbon to the dead organic matter, where carbon is released over subsequent decades through decomposition. Once forests begin growing again on disturbed sites, carbon storage will ensue. Carbon stock changes can therefore also be affected by the legacy of past disturbances as well as by the age-class structure of the forest. Not surprisingly, the potential impact of climate change on the frequency and intensity of forest disturbances related to fire and insects is a major concern with respect to carbon dynamics in Canadian forests (Kurz et al. 1992).

Canada's forests account for approximately 10% of the world's total forest area, so it is critical to understand their carbon budget. This improved understanding will add to the data on global forest carbon cycles and budgets, and allow Canada to better gauge the potential role of its forests in enabling the country to meet its Kyoto commitments. This indicator provides the status of overall carbon exchanges between the forest ecosystem and the atmosphere, and indicates whether Canada's forests are a sink for, or a source of, atmospheric carbon.

Researchers are currently refining and implementing an improved methodology for estimating the carbon stock changes of forests in Canada, and results were unfortunately unavailable when this report was drafted. A case study on carbon stock changes in the Boreal Plains of Manitoba and Saskatchewan, on the basis of the improved carbon model, provides an example of the carbon estimates that will soon be generated for all Canada's forest ecosystems. The 2000 C&I report, using an earlier version of the Carbon Budget Model of the Canadian Forest Sector (Kurz and Apps 1999), and an earlier biomass inventory (Bonnor 1985), noted that Canada's forests were, on average, a source of atmospheric carbon, releasing 44.6 Mt of carbon per year between 1990 and 1994.

CASE STUDY: Carbon stock changes and carbon storage in the Boreal Plains ecozone of Manitoba and Saskatchewan

This region, located in central Manitoba and Saskatchewan, contains 10.7 million ha of forest area. It is dominated by spruce, fir, and jack pine, with aspen and poplar stands found in regions situated close to the Prairie ecozone.

The net annual change in forest ecosystem carbon in this region was negative between 1990 and 2001, indicating that this forest was acting as a source of atmospheric carbon during that time (Figure 4.1a). However, declining emissions over the study period led to an almost neutral carbon balance in 2001. There is considerable variation in the net change in carbon stock over the study period, mainly due to natural disturbances, such as in 1995 when the forest ecosystem experienced severe fire activity (Figure 4.1b). Despite the pulses in emissions associated with natural disturbances, the overall capacity of the forest in this region to absorb carbon increased throughout the study period, likely due to increasing stands of younger, faster growing forest.



Figure 4.1a Estimated total carbon stock changes in forest carbon in the Boreal Plains terrestrial ecozone in Saskatchewan and Manitoba from 1990 to 2001. (Source: Canadian Forest Service)



Figure 4.1b Area burned in the Boreal Plains terrestrial ecozone in Saskatchewan and Manitoba from 1990 to 2001. (Source: Canadian Forest Service)

It is not unusual for regions that are carbon sources in some years to later become sinks, and vice versa. The overall carbon budget of Canada is expected to comprise regions experiencing carbon losses, and regions experiencing carbon gains, with fluctuation in the relative contribution of gains and losses over time.

Most (42%) of the forest ecosystem carbon in this ecozone is stored in hardwood forests, which account for 1208 Mt of carbon. Softwoods account for 936 Mt (33%) of the forest ecosystem carbon, while mixedwoods account for 698 Mt (25%).

The total forest ecosystem carbon stored in each age class in this study area is closely related to the area in each age class (Figure 4.1c). However, the total forest ecosystem carbon in a region is influenced not only by the area of each age class but also by the combination of stand types, growth rates, and soil carbon in each stand. In other ecozones, with different forest growing conditions, age-class structures, and disturbance histories, the distribution of carbon relative to the age-class distribution will differ.



Figure 4.1c Forest age-class structure and carbon stocks (biomass and dead organic matter) for the Boreal Plains ecozone in Saskatchewan and Manitoba.

Because of the variation in conditions between ecozones, results from this case study should not be considered representative of Canada's forests as a whole.