Contrasting impacts of mountain pine beetle disturbance in two pine-dominated ecosystems in Colorado

Background/Question/Methods

After causing severe and widespread mortality in lodgepole pine (Pinus contorta) forests both west and east of the Continental Divide, mountain pine beetle (Dendroctonus ponderosae; MPB) has increasingly affected an adjacent alternate host species, ponderosa pine (Pinus ponderosa), in Colorado’s Front Range. We investigated the impacts of MPB on forest succession, fuel loading, and carbon storage, hypothesizing that different ecology and management practices in lodgepole and ponderosa ecosystems would lead to detectable differences in both short- and long-term responses. Rocky Mountain lodgepole pine typically experiences severe, infrequent disturbance followed by pulses of regeneration from serotinous cones, whereas ponderosa pine has had a more mixed-severity, frequent disturbance (and management) regime and episodic regeneration in this region. Using data collected in 2009-11 on 119 plots in lodgepole forests and 146 plots in ponderosa pine-dominated stands, we simulated succession, fuel loading, and carbon storage over a 200-year period using the Forest Vegetation Simulator (FVS). We compared several scenarios: stand responses to the actual impacts of MPB, the possible worst-case impacts of MPB (assuming mortality levels in ponderosa pine similar to those actually experienced in lodgepole), and differing levels of regeneration that might be expected under current vs. future, drier precipitation regimes.

Results/Conclusions

In the lodgepole stands, actual MPB-induced tree mortality affected 72% basal area on average by 2010, but our simulations suggested a return to pre-disturbance levels of basal area and total carbon in 60-80 years, albeit with a marked increase in the abundance of subalpine fir and Engelmann spruce. Outcomes were relatively insensitive to regeneration levels. In the ponderosa pine stands, actual mortality ranged from 0-34% by 2011, but preliminary simulation results suggest that under the “worst-case” MPB scenario, ponderosa stands would demonstrate a slower return to pre-disturbance basal area (>100 years), stronger impacts of alternative future regeneration patterns on both basal area recovery and future species composition, and more variable patterns of post-mortality fuel loading. Our results suggest that alternative management strategies intended to mitigate the interactive effects of either MPB disturbance or associated potential fire hazard may have different impacts over time in the 2 ecosystems. Management actions that were appropriate in lodgepole pine may not be transferable to ponderosa pine, and future patterns of succession, regeneration, fuel loading, and predicted climate over longer time frames should be carefully considered when evaluating management strategies before, during, or shortly after MPB-induced mortality in ponderosa pine.