Short-interval disturbance in lodgepole pine forests, British Columbia, Canada: Understory and overstory response to mountain pine beetle and fire

• We compared plant communities where fire burned through red-phase MPB forests.
• Treatments: MPB-only, MPB+low, +moderate and +moderate-high severity fire.
• Communities converged in structure five years post fire but diverged in membership.
• MPB-only plots were dominated by regenerating fir and fire plots by fire-cued pine.
• This community was resilient to the combined effects of red-phase MPB and fire.

The recent mountain pine beetle (MPB) outbreak across western North America’s interior lodgepole pine forests has altered the landscape such that the majority of wildfires in the region will now burn through MPB-affected stands. Study of plant community response to these combined disturbances is critical for our understanding and management of these pine forests. Based on data collected from an experimental burn project in central interior British Columbia, Canada, we investigate variation in understory plant communities along a gradient of disturbance that includes MPB-only and red-phase MPB + fire across three levels of fire severity. We compare communities pre-fire and five years post-fire using three hypotheses that predict community response to disturbance: the light hypothesis, the divergence-convergence hypothesis, and the fire severity begets pine hypothesis.Comparisons of pine and fir tree seedling tallies suggest a divergence in composition along the disturbance severity gradient. Pine seedlings were mostly absent from the unburned treatment but dominated the burned treatments with densities increasing at higher severities. For the non-tree understory community, canopy-opening associated with both unburned and with low through moderate-high severity burning results in a convergence of general plant community structure demonstrated by rank-abundance curves, but divergent community membership associated with fire severity and fire-adaptive traits. Communities in the unburned treatment saw a release due to canopy opening that supported increases in the dominant shrubs and understory fir seedlings. In the burned treatments, shrubs also increased in dominance and community membership was determined by fire-adaptive traits with fire-adapted species being most abundant in the highest severity fire treatments. Overall, the communities appear to be resilient to the combined effects of MPB (red-phase) and low to moderate-high severity burning, but further research is required in the grey-phase of beetle-kill to broaden our understanding of the landscape legacy of this recent insect outbreak.