Alaska has experienced recent large-scale mortality of several tree species, including the most commercially valuable forest species, white spruce (Matthews et al. 1997). The spruce bark beetle (Dendroctonus rufipennis) outbreak in southcentral Alaska has affected 2 to 3 million acres in the past 10 years. In heavily affected stands, often all spruce trees greater than 10 cm are killed. The area affected by tree death is one of the largest ever documented from an insect outbreak in North America (Werner 1996). |
Beetles are the proximate cause of tree death in these large outbreaks, but the insects have always been present at low background population levels, and some environmental factor has triggered a rapid population buildup by releasing them from a major control of their numbers (Hard 1985). In the far north, the spruce bark beetle is a heat-limited organism. It normally requires 2 years to complete its life cycle. In exceptionally warm summers, it can complete reproduction in one year (Holsten et al. 1985). Two successive cold winters will reduce survival of the overwintering population to the point that it has little outbreak potential (Holsten and Burnside 1997). Recent (since 1977) climate warming in Alaska has removed those limitations; winters especially have been milder and a few summers (e.g. 1989, 1991, 1993) have been exceptionally warm.
Spruce bark beetles bore into trunks and feed on the live cambium layer between the bark and wood. A major spruce bark beetle outbreak is also facilitated by weak resistance in host trees (Werner and Holsten 1983). Some of the typical factors in weakening the resistance of spruce stands include defoliation, flooding, prolonged drought, and old age in crowded stands. Ken Raffa at the University of Wisconsin has found that trees experiencing fewer than 25 beetle attacks (emergence holes) per square meter of lower bark surface generally survive. Trees that experience 25 to 40 attacks can survive if they are healthy. Trees with more than 40 attacks generally die no matter what their health status. As a result, a major area of mechanical injury such as snow breakage of tree crowns and branches can set the stage for a large bark beetle outbreak by simultaneously weakening trees over large areas. Heavy, wet winter snow during warm winters often serves as such a trigger.
The western strain of the spruce budworm (Choristo-neura fumeriina) is a defoliating insect that feeds on new buds in May or June and then shifts to spruce needles (Furniss and Carolin 1977). Trees under attack often have a reddish-brown scorched appearance (the basis of the scientific name). This insect is also heat limited and near the extreme northern limit of its distribution near Fairbanks (Holsten et al. 1985). A tree-ring study of a spruce budworm outbreak near Fairbanks found that no period for the last 160 years experienced a growth reduction best explained by budworm defoliation until 1993?95 (Juday and Marler 1997). Before the origin of the studied forest (~180 years ago) interior Alaska was in the colder climate of the Little Ice Age, and spruce budworm defoliation almost certainly was not a factor in forest health north of the Alaska Range. So recent warming in Alaska appears to have removed the environmental limitation that prevented outbreaks of spruce budworm in the far north.
What factors can lead to warmer weather?
Warmer summers in interior Alaska are associated with strong high-pressure centers that dominate the weather near the summer solstice when day length is often more than 20 hours. The long hours of sunshine build up heat near the earth?s surface. Such conditions also block major frontal weather systems that introduce moisture into the boreal forest region. Parts of interior Alaska are experiencing a long-term decrease in warm season precipitation (Juday et al. 1997), which is leading to very high moisture stress levels in white spruce trees at lower elevations (Barber et al. 1997, Barber and Juday 1997). Often such weather leads to large forest (or less frequently, tundra) wildfires, usually sparked by dry lightning strikes. While these meteorological conditions have always happened in the past, it is their increasing frequency and severity (e.g., as revealed in tree rings) that appear to be new (Jacoby et al. 1997, Juday et al. 1997).
In general terms, the Aleutian Low represents a system for discharge of heat from the north half of the Pacific to even out the otherwise steep gradient between the Arctic and the low latitudes. Intensification of the Aleutian Low then simply represents discharge of more stored heat. Indicators of a warmer climate in Alaska and the Arctic/boreal sector worldwide are consistent with global warming, but by themselves don?t establish increased greenhouse gasses, increased solar flux, or other possible explanations as the ultimate cause.
What is the impact of the insect outbreak?
A critical issue for the future involves the process of forest regeneration in the years immediately following the death of the spruce forest killed by the beetles. Spruce can become a significant component of the new forest if a few mature spruce trees survive and shed seeds, or if a substantial number of small understory spruce survive the beetle outbreak. In spruce stands with the highest levels of mortality, spruce may not be a part of the new stand. New vegetation on beetle-killed sites may range from grassy shrublands to well-stocked birch or aspen forests.
What can be done?
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