PDF _ R40203 - Moutain Pine Beetles and Forest Destruction: Effects, Responses, and Relationship to Climate Change
6-Feb-2009; Ross W. Gorte; 16 p.

Abstract: The mountain pine beetle is a native insect of western U.S. pine forests. It survives by killing infested trees, usually individually, but occasionally in epidemics. Mountain pine beetle epidemics are particularly associated with lodgepole pine, a common western tree that typically grows in dense, even-aged stands. The beetle is a seasonally adapted species that thrives in areas where it can complete its life cycle in one year. The beetle has evolved a mass-attack approach to overwhelm tree defenses through large numbers, and adults congregate on large trees under stress. Widespread stress (e.g., a regional drought) sets the stage for an epidemic.

Mountain pine beetle epidemics are recurrent events in western forests. The current epidemic can be separated into three distinct events: the central U.S. Rocky Mountains, interior British Columbia (Canada), and high-elevation pines. Two aspects of the current epidemic are widely believed to have been exacerbated by climate change: (1) increased temperatures farther north and at higher elevations (allowing complete life cycles in areas previously not susceptible to the beetle) and (2) possibly regional drought (making trees more susceptible to beetle attacks).

Controlling a mountain pine beetle epidemic can be problematic. Individual trees can be protected by insecticide sprays, but the cost of preventive spraying at a landscape scale is prohibitive. Once a tree is infested, nothing can be done to save the tree. In the long run, silvicultural treatments to provide less dense, more diverse forests may reduce the extent of future epidemics, but epidemics cannot be prevented.

One concern about the consequences of the current epidemic is the possible increase in wildfire threats. Little research has been done to assess the change in threats and impacts of wildfires. The limited existing information suggests that tree mortality due to mountain pine beetles may have little effect on the threat or impacts of wildfire in the affected areas, because lodgepole pines (live or dead) naturally burn in extensive crown fires that typically kill most of the large trees. Furthermore, because of the natural regeneration cycle of lodgepole pine and because the beetles do not kill small trees, natural regeneration of the pine forests is likely.

However, warming as a result of climate change could have two consequential ecological outcomes. First, the beetle outbreak in the high-elevation pine ecosystems could significantly alter these ecosystems, because these pines are much slower to regenerate and small highelevation pines are highly susceptible to the white pine blister rust (an introduced fungus). The second concern is the potential for the mountain pine beetle to spread across northern Canada through the boreal jack pine forest, and become an invasive pest of eastern pine forests.

There are economic consequences of the mountain pine beetle epidemic. The aesthetic values of the area—such as property values and tourism—will likely be harmed by the extensive tree mortality. Much of the beetle-killed timber could be used, for lumber or for biomass energy, but the substantial volume of timber available far exceeds existing capacity to use the wood, and expanding capacity could be unsustainable—a short-run surplus and long-run shortage. Also, Canadian lumber faces restrictions on shipments to U.S. markets. [read report]

Topics: Climate Change, Forests

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