Projected future forest conditions and live carbon pools following a mountain pine beetle epidemic in southeastern Wyoming

Background/Question/Methods

Forested ecosystems provide important ecosystem services, including playing an important role in the global carbon cycle.  The capacity of forested landscapes to sustain themselves and the production of ecosystem services, including the capture and storage of carbon, is strongly affected by natural disturbance regimes such as fire and insect outbreaks. Many Rocky Mountain forests are currently experiencing a bark beetle epidemic of unprecedented severity, duration and extent. Forest regeneration and carbon dynamics following bark beetle outbreaks is driven, in part, by the survival and density of understory trees (advance regeneration).  We characterized and compared the density and species composition of advance regeneration, residual overstory, and pre-outbreak overstory in stands with varying lodgepole pine density and mortality following a massive mountain pine beetle outbreak in Southeastern Wyoming and modeled future forest structure and carbon dynamics under several different potential mortality and regeneration scenarios using the Forest Vegetation Simulator .  Models included combinations of two morality scenarios (observed and extreme mortality where all trees over 10 cm dbh were killed) and three regeneration scenarios (zero, observed, and high regeneration where a pulse of lodgepole pine seedling were added to the model in year 1).

Results/Conclusions

While lodgepole pine experienced considerable mortality, it was still the dominant live species in the overstory.  Subalpine fir was the dominant species in the advance regeneration. Three different lodgepole pine forest types (pure lodgepole pine, aspen-influenced, and mixed conifer with significant elements of subalpine fir and Engelmann spruce) were found prior to the outbreak. Our modeling indicated that while the majority of stands recovered their original basal area within 100 years under all scenarios, there was a significant shift in species composition in all forest types except the pure lodgepole pine.  For example, even in the high seedling establishment scenario, lodgepole pine was almost entirely absent from aspen-influenced forest type and subalpine fir was the most abundant species in the mixed conifer forest type after 100 years of recovery. Aboveground carbon recovered more slowly but 100 percent of stands modeled using the observed mortality recovered their aboveground carbon biomass within 100 years.   Recovery time was longer under extreme mortality scenarios. Interestingly, the pure lodgepole pine forest type was the slowest to recover basal area and carbon.  Lodgepole pine forests in the Rocky Mountains will look significantly different following a mountain pine beetle epidemic of this magnitude especially with potential changes in climate.