The concept of “forest disturbance” refers to events such as forest fire, harvesting, wind-throw, insect and disease outbreak (epidemics), and forest flooding that cause large pulses of CO₂ to be released into the atmosphere through combustion or decomposition of resulting dead organic matter. Stand-replacing disturbances, such as crown fires and wind-throw, are associated with the sudden death of large cohorts of trees near one another (Pickett and White, 1985; Kurz et al., 1995a, 1995b; Kurz and Apps, 1999, see Box 4.2). Some disturbance agents, such as pollution and some insects and disease outbreaks, may result in large areas with productivity decline but only local mortality (Hall and Moody, 1994). Disturbances play an important natural part in the lifecycle and succession dynamics of many forest systems. In boreal systems large-scale, natural, stochastic forces tend to dominate the ecosystem dynamics, even when direct human influences are considered (Kurz et al., 1995b). The return interval of these disturbances, their intensity, and their specific impacts are referred to as the disturbance regime (Weber and Flanigan, 1997). Kurz et al. (1995b) and Price et al. (1998) (having compiled insect, fire, and harvest data) showed that the disturbance regime of Canadian forests changed over the last quarter of the 20th century from about 2.5Mha/yr prior to 1970 to 4Mha/yr between 1970 and 1990. Using these data, Kurz and Apps (1999) showed that these changes in the disturbance regime resulted in a switch of Canadian forests from being a net sink of C to a small net source of C to the atmosphere.

The different types of disturbances are often linked. For example, in some forest types the probability of fire may increase following insect outbreaks because of increases in available fuel (litter). In some cases salvage logging (recovering the usable timber following a disturbance) can reduce the total area of living forest that is disturbed in a given year by all agents combined. It is common to try to replace natural disturbances (such as wildfires) with commercial harvesting, using a combination of protection and scheduled logging. In Sweden and Finland, for example, logging has become the main disturbance type; and large-scale natural disturbances resulting from wildfire, insect outbreaks, or storms have been almost non-existent for half a century (Lähde et al., 1999).

Disturbances affect the carbon stocks of all components of forested ecosystems. During and following a disturbance, carbon is transferred from living material, above and below ground, to the dead organic matter pools (Figure 4.2). In the case of a forest fire, part of the ecosystem carbon is released immediately into the atmosphere as combustion products. Disturbed forest stands continue to release carbon into the atmosphere as the enlarged pools of dead organic matter tend towards a new steady-state condition (Bhatti et al., 2001). Regrowth follows, but maximum uptake may not be achieved for some time (decades or more), and during much of this period decomposition of dead organic matter may exceed vegetative uptake. The corresponding re-sequestration of carbon through regrowth can last 50 to 200 years or more.

Management of natural disturbance regimes can provide significant C mitigation opportunities, e.g., through activities to prevent or suppress disturbances. Such measures can significantly enhance the strength of C sinks (Kurz et al., 1995a; Apps et al., 2000; Bhatti et al., 2001) and maintain existing C stocks, but only as long as the programmes are maintained. Other factors being equal, during periods of reduced disturbance (e.g., with increasing suppression effort), C stocks tend to increase as biomass accumulates and litter production (in all forms) increases: forests act as a sink for atmospheric C (Bhatti et al., 2001). In contrast, with increasing disturbance (e.g., with reduction in suppression effort), the net losses of C from forest ecosystems can exceed inputs from photosynthesis (Figures 4.2 and 4.3) and the forests could become a net source of C. We note that all forms of disturbance, not just highly visible fires, play a role in these dynamics. In a changing climate, the control of new pathogens and immigrant herbivores (especially insects and disease), to which local forest ecosystems may be maladapted, may be critical to avoid emissions and maintain existing forest C stocks.

Disturbances affect the carbon stocks in vegetation, in soil, and in dead organic matter. All these stocks vary over time as a function of the history of disturbances (MacLaren, 1996; Bhatti et al., 2001; Kurz and Apps, 1999). With an increase of widespread disturbance events the carbon stocks of living vegetation decrease and the age-class distribution of the forest shifts to younger stands containing less carbon. If forests are disturbed at regular intervals (i.e., an unchanging, disturbance regime), the carbon stock of large tracts of forest can be relatively stable.

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