4.2.3 Forest Disturbance Regimes
| Box 4.3. The Reduced Impact Logging Project, Carbon Sequestration
Through Reduced Impact Logging
The RIL (reduced impact logging) project developed by Innoprise, a Malaysian
company with forestry activities, and the New England Power Company, USA,
aims to save CO2 already stored in forest biomass by reducing
damage to vegetation and soils during harvesting. The hope is to reduce
damage by 50% compared to that of conventional harvesting. The techniques
employed are modifications of conventional bulldozer harvesting techniques;
including pre-felling climber cutting, directional felling, skid trail
design, and post harvest operations such as rehabilitation of log landings.
Today the total project area amounts to 2,400 ha. The pilot research project
has quantified the carbon implications and costs on 1,415ha. They found
that avoided emissions amounted to 65–90MgC/ha and that the associated
costs were US$3.55/MgC (Wan Razali and Tay, 2000).
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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.
Disturbances, both human-induced and natural, are major driving forces that determine
the transition of forest stands, landscapes, and regions from carbon sink to source
and back. The current pattern of forest vegetation and its role in carbon cycling
reflects the combined effects of anthropogenic and natural disturbances over a
range of time scales. For C stocks with very slow turnover rates (such as soils
and peat) the effects of past disturbances on carbon cycling may reverberate for
centuries and millennia (Figure 4.5). For example,
carbon continues to accumulate in young soils (such as those associated with the
isostatic uplift following deglaciation in Canada and Finland), which appear to
be actively accruing carbon (Harden et al., 1992). In these soils, losses from
decomposition of accumulated organic matter are exceeded by the inputs of fresh
organic debris (Liski et al., 1999). Human influences on the disturbance regime
include both direct effects, such as harvesting or inducing and/or suppressing
natural disturbances (fires, insects, flooding, etc.), and indirect influences
from altering the forest environment. Indirect influences include both climate
change and atmospheric pollution, and their effects on tree health and survival.
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Figure 4.5: Conceptual model of soil organic matter decomposition
and accumulation following disturbance (after Johnson, 1995; IPCC, 2000a).
At steady state (I), carbon (C) inputs from litter (L) equal C losses
via decomposition (D) (i.e., L/D = 1). After a disturbance, D often exceeds
L resulting in loss of C (II), until a new, lower steady state is reached
(III). Adoption of new management, where L exceeds D results in a re-accumulation
of C (IV) until a new, higher steady state is reached (V). The eventual
steady state (A, B, or C) depends on the new management adopted.
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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.
