Historically, coniferous forests have been shaped by insect pests, fire, and climatic controls. In recent years, impacts of fire suppression and global climate change have led to conditions that are conducive to large-scale insect outbreaks. Although bark beetles impact forests from low-elevation pinyon-juniper woodlands to subalpine spruce-fir forests, lodgepole pine forests have been and will continue to be altered to the greatest extent. Due to warming temperatures, the two-year life cycle of bark beetles has been reduced to one year and bark beetle populations have exploded. Older, even-aged lodgepole stands (the majority are > 100 years old) and hot, dry conditions have combined to create the "perfect storm" for bark beetle outbreaks. In addition to adverse impacts on scenic value of the state's forests, insect infestations lead to dramatic stand structure alterations with both positive and negative impacts on local wildlife and forage animals and mature forest species. These impacts, along with increasing attention to the state's carbon sequestration potential cause us to examine greenhouse gas, carbon, and water balance issues associated with rapidly expanding (up to 15-fold in one county in a year) insect infestations in Colorado forests.

Using the DayCent ecosystem model which has been extensively tested for lodgegole pine in the Fraser Experimental Forest, we conducted simulations to illustrate near-term (20 years following needle-drop) consequences of insect attack of three intensity levels (30, 50, and 80 percent mortality).
Results/Conclusions

We found that stand-level evapotranspiration was not impacted at low levels of mortality (30-50%) but was reduced by an average of 4% in stands with 80% mortality. Fifteen to Twenty years following insect attack, however, evapotranspiration recovered to pre-infestation levels. Carbon balance was slightly positive in simulated control stands. Insect-impacted stands showed short-term release of carbon ranging from 900 gC/m² over a twenty year period (30% mortality) to 1600 gC/m² (50% mortality), and 3000 gC/m² with 80% mortality. Most of these carbon losses occur in the first few years following mortality. Within 20 years of initial attack, NEE of insect-impacted forests nearly converge with the control stand. These values do not take into account the possible impacts of salvage logging operations and short-term increased fire danger, both of which could have further negative impacts on terrestrial carbon sequestration. Beyond the widespread statewide concerns about aesthetics, habitat, and fire danger, we must take into consideration the large-scale near-term losses of stored carbon although the potential for long-term recovery is encouraging.