Forest insects and pathogens (FIPs) are increasingly recognized as major drivers of mortality. The combination of mountain pine beetle (Dendroctonus ponderosae) and the invasive pathogen, white pine blister rust (Cronartium ribicola), has caused precipitous declines in populations of white pines. While many studies investigate outbreaks of FIPs, few consider how the interacting effects of FIPs and abiotic drivers of mortality may vary across stress gradients. Here we ask how the interacting impacts of beetles, blister rust, fire and climate shift with decreasing minimum temperatures. Using a combination of downscaled climate data, fire history maps, and tree-level observations of blister rust and beetles, we test how the magnitude of the drivers of mortality and the importance of possible interactions vary across a climate-stress gradient.
Results/Conclusions
While we expected that abiotic controls (e.g., temperature) on biotic drivers of mortality (FIPs) increase across an altitudinal gradient, we found instead that abiotic controls greatly vary in strength and direction. For example, though all abiotic drivers of mortality decline with decreasing temperature, the invasive pathogen, blister rust, is less constrained than beetles. In addition, at low altitudes, the interactions between abiotic and biotic drivers of mortality lead to differential outcomes. For instance, hotter temperatures at low elevations can lead to differential impacts on FIPs. Blister rust may be negatively impacted, while mountain pine beetle benefits when abiotic stress is high at low altitudes. Finally, the presence of fire over the past twenty years negatively impacts blister rust but not beetles. Our results suggest that the temperature stress gradient across altitude has a strong control on the interactions among drivers of mortality, with the greatest control exerted at high elevations. The interactions at low elevations, however, can lead to surprise outcomes. These results highlight that interactions among drivers of mortality complicate efforts to predict temperate forest health under climate change.