ABSTRACT
Terrestrial ecosystems of the Northern Hemisphere contain large pools of biospheric carbon (C), and are known to play a very dynamic role in the global carbon cycle (Apps et al., 1993). Canadian forests account for approximately 25% of the C in the boreal zone and 10% of the world’s forested area. The net budget of C-fluxes between Canadian forests and the atmosphere is therefore an important component of the global C-cycle (Schindler, 1998). The boreal forest is the most extensive forest biome in Canada and is estimated to contain 40% of the total biotic C in Canada (Price and Apps, 1993). Short growing seasons, low temperatures, and high moisture content are important factors that limit the decomposition of organic matter in these ecosystems (Kimmins, 1996). Consequently, the soils of Canada’s boreal region contain significant C reservoirs that have accumulated over thousands of years and play an active role in the source/sink relationships for terrestrial C (Apps et al., 1999a). An increase in large-scale, stand-replacing disturbances since ca. 1970 has been reported for Canadian forests (Kurz et al., 1995b). This has resulted in transitory decreases in net ecosystem productivity and increases in the pools of decomposing organic matter, causing the Canadian boreal forest to become a small net C-source rather than a C-sink (Kurz and Apps, 1996). The change in disturbance regimes for these northern ecosystems may be a response to the larger scale phenomenon of global change, which results from human-induced changes in the physical climate system, land-use changes, and atmospheric pollution (IGBP, 1998; Woodwell et al., 1998). Changes in the disturbance regime, and the resulting forest response, are the consequence of both direct (e.g., forestry operations) and indirect (e.g., climate change) effects of human activity. As global change alters weather patterns, the frequency and severity of natural disturbances such as wildfire and insect outbreaks are affected, leading to changes in ecosystem dynamics (Figure 1). Under present projections of climate change, the rate of natural disturbance will likely increase, resulting in an increased proportion of younger age stands in the forests (Kurz and Apps, 1999). Other vegetative and soil processes are also expected to change in response to changes in temperature, precipitation, ambient C 0 2 concentration, and other global climatic factors. The responses may also be influenced by changes in land-use practice and atmospheric deposition or pollution which also vary strongly over time and with local conditions. Understanding the complex relationship between global change and forest ecosystem processes is necessary to predict both the feedbacks to global change and the future forest resource. To achieve this understanding the appropriate use of spatial and temporal scale is essential (Houghton et al., 1998; Apps, 1993; Holling, 1992).
