Effects of forest cover change on ecosystem services and society

The ecological changes caused by fire have important societal consequences, which can be described in terms of ecosystem services (the benefits that society derives from ecosystems). Some of these services affect society globally through changes in the climate system and others affect society more locally through changes in local conditions.

Fire-induced changes in tree cover and carbon sequestration in boreal forest can have large effects on the global climate system through changes in energy exchange (Bonan et al, 1992; Foley et al, 1994), carbon emissions to the atmosphere (Kurz and Apps, 1999; Kasischke and Stocks, 2000; McGuire and Chapin, 2006), and other processes such as soot deposition on glaciers and changes in cloudiness or ozone concentrations of the atmosphere (Randerson et al, 2006). The largest of these climate effects are changes in carbon sequestration and energy exchange. The increased areal extent and severity of burning described earlier result in greater CO2 release to the atmosphere, which acts as a positive feedback to global warming. Because CO2 has a relatively long lifetime in the atmosphere, this warming effect is distributed globally by atmospheric mixing (McGuire and Chapin, 2006).

Fire also increases albedo (short-wave reflectance), which reduces the amount of solar energy absorbed by the land surface and transferred to the atmosphere. This occurs in spring due to removal of the tree canopy, exposing the more reflective snow-covered ground, and in the summer due to replacement of the dark complex black spruce canopy by the more reflective, less complex canopy of herbs, grasses and deciduous shrubs and trees (Chapin et al, 2000, Chambers and Chapin, 2002, McGuire and Chapin, 2006). Fire thus has a net cooling effect on climate through this change in energy exchange. Most of this cooling occurs locally, due to the short residence time of heat in the atmosphere (Chapin et al, 2000).

As a result of these counterbalancing effects of fire on climate from trace gas emissions (a heating effect) and energy exchange (a cooling effect), the net effect of fire on climate appears to be a modest cooling of climate, with this effect being concentrated near areas that burned (Randerson et al, 2006). This balance between heating and cooling is sensitive to burn severity (the amount of carbon released) and vegetation trajectory (the time required for late successional conifers to return to dominance). If fire severity or the areal extent of wildfire continue to increase, as projected, we expect this to increase both the positive feedbacks to warming (greater carbon emissions) and the negative feedbacks (longer time before return to the low-albedo conifer vegetation), with the net effect on climate feedbacks depending on the magnitude of these two effects. One thing that appears certain, however, is that the negative feedbacks to climate will predominate locally within the boreal forest. This is important because it is one of the few negative feedbacks that have been identified and that will limit the high-latitude amplification of warming caused changes in snow and ice cover.

Changes in fire regime also have important effects on society locally through changes in other ecosystem services. Rural Alaskan residents, the majority ofwhom are Alaska Natives, depend on subsistence hunting for both a large proportion of their meat supply and maintenance of cultural connections to the land (Magdanz et al, 2002). Changes in fire regime will probably be the major way in which climate warming will change the availability of these subsistence resources (Chapin et al, 2003). For example, caribou, which are among the major subsistence resources for many Athabascan communities, depend on lichens as their primary winter forage. Lichens recover slowly after fire, so caribou tend to avoid burned areas for 80—100 years before using them as winter habitat. Recent and projected increases in area burned are reducing habitat available to caribou and therefore the availability of caribou as a subsistence resource to local communities (Rupp et al, 2006; Nelson et al, in review). Moose, in contrast, are most abundant 15—30 years after fire (Maier et al, 2005), so the recent reduction in fire return interval from 160 to 90 years (see Figure 13.6) should increase moose abundance in interior Alaska. Availability of these and other subsistence resources to local communities depends not only on their abundance but also on accessibility. Wildfires burn remote trapping cabins and topple trees along traplines, making it more difficult and dangerous to hunt and trap (Huntington et al, 2006).

Although fire also affects availability of timber resources, there is currently only a local market for timber, primarily for rough-cut lumber and firewood, and these are harvested mostly where there is good road or over-ice river access.

White spruce, which is the main timber species, burns less frequently than black spruce, so fire has been a relatively modest concern, except where insect outbreaks have killed extensive stands of trees, as in the Kenai Peninsula of south-central Alaska.

Another fire-dependent ecosystem service that is becoming increasingly important and controversial in interior Alaska is the regulation of disturbance spread — specifically fire risk to communities. Early successional post-fire vegetation is less flammable than late successional black spruce, so recent fires act as fuel breaks that reduce the likelihood of future fires (Rupp et al, 2002). Climate warming has two counteracting effects on this fire-vegetation feedback. First, increases in fire extent reduce landscape flammability by increasing the proportion of non-flammable vegetation on the landscape (Rupp et al, 2002; Chapin et al, 2003). Second, these vegetation differences in flammability are least pronounced under hot dry conditions (Kasischke et al, 2002), which reduces the effectiveness of recently burned stands as fuel breaks. The net effect of climate warming thus appears to be an increase in fire risk (Chapin et al, 2003).

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