Altered disturbance regimes are implicit within climate change, and changes in the extent and frequency of disturbances may have significant effects on ecological processes. The potential for altered disturbance regimes on forested landscapes to affect the global carbon budget is considerable, especially when feedbacks between disturbance-mediated carbon loss from ecosystems and climate change are likely. Insect outbreaks are second only to wildfires as the largest source of tree mortality in western North America. Insect outbreaks affect the distribution of live and dead biomass as well as rates of forest productivity and recovery, likely influencing carbon balance very differently than fires, but little empirical data exist to quantify changes in carbon storage following insect outbreaks. We estimated carbon budgets of subalpine forest stands across the Greater Yellowstone Ecosystem (GYE) to examine short- and long-term changes in carbon storage following mountain pine beetle (MPB) outbreaks. The GYE historically experienced MPB outbreaks such that we were able to employ a chronosequence approach to sample stands initially attacked by MPB < 2 years ago, 3-5 years ago, 15-25 years ago, and 30-40 years ago, replicated across stands that experienced 30-50% mortality and > 50% mortality. Aboveground biomass and coarse roots of trees were estimated using site-specific allometric equations, and carbon in dead wood, forest floor, fine roots, soil, and understory aboveground biomass were measured directly.
Initial results suggest a patchy distribution of tree mortality, such that carbon loss via decomposition will occur mainly at local scales. All stands experiencing MPB-caused mortality exhibited a shift in carbon storage from live to dead biomass, though the magnitude of this shift varied with the severity of the outbreak. Given that most stands in the GYE are currently experiencing relatively low levels of mortality, MPB outbreaks in the area are likely to create only very short sources of carbon to the atmosphere (5 years), the longevity of which is limited by rapid recovery of forest productivity supported by the competitive release of surviving trees (by 15 years). Patterns of beetle-killed trees in stands attacked over 15 years ago suggest similar patchiness and severity of MPB outbreaks as that occurring today. The fact that the GYE outbreak is ongoing and may increase in severity, as well as that many other forest stands in western North America are experiencing > 90% tree mortality, suggest that regional effects of insect outbreaks on forest carbon budgets may be substantial and perhaps unprecedented.