Abstract
Multiple interacting disturbances including white pine blister rust (Cronartium ribicola), mountain pine beetle (Dendroctonus ponderosae), stand replacement wildfire, and climate change have significantly impacted whitebark pine at a landscape, stand and tree level scale. In Central Idaho, limited research exists to understand the impact stand replacement fire is having on whitebark pine potential habitat. Broad assumptions about whitebark pine across its range assert that fire suppression has greatly limited successful whitebark pine regeneration, however, for Central Idaho this is not the case.A whitebark pine species distribution model (SDM) was developed using random forest to predict potential habitat in Central Idaho and to determine disturbance from stand replacement fire, as estimated by a binary modeled presence-absence threshold of 0.63 (63-100% probability of occurrence. Whitebark pine potential habitat is 23% of Central Idaho and is most strongly predicted by the following environmental variables: biophysical settings, existing vegetation type, elevation, and snow depth. Whitebark pine potential habitat disturbed by stand replacement fire from 1984 to 2021 is 20% in under 40 years. The Sawtooth National Forest has 7% of its potential habitat disturbed, the Salmon-Challis National Forest 18%, the Payette National Forest 31%, and the Boise National Forest 33%.
For Central Idaho, the disturbance needed for whitebark pine regeneration is occurring. This disturbance creates a mosaic of patch sizes and ages required to maintain seed sources for regenerating whitebark pine, but additional research is needed to understand natural regeneration post-fire. Recovery is incredibly slow in these high elevation areas with short growing seasons and approximately 60 years to reach cone bearing maturity, further challenging the recovery of the species. Time will determine if stand replacement fire is a threat rather than a benefit for Central Idaho, with a future projection of disturbance and recovery indicating there could be an issue if disturbance increases through time with climate change, limiting whitebark pine seed sources in addition to mountain pine beetle and white pine blister rust. As a larger proportion of potential habitat is disturbed, management implications can also change if natural regeneration is not successful due to lack of remaining seed sources.
Whitebark pine has been disturbed by stand replacement fire, mountain pine beetle and white pine blister rust in the Payette National Forest. In unburned stands, 51% of mature whitebark pine have been killed with mountain pine beetle a greater impact to past mortality and white pine blister rust expected to cause future mortality. Regeneration of whitebark pine post-fire is at an acceptable level with 242 trees per acre and 36% of the species composition. The rate of mortality and the severity of these stressors will test the adaptive capacity of whitebark pine. Management intervention is necessary to increase frequencies of white pine blister rust resistant genotypes and assist in the recovery of whitebark pine to ensure long-term population viability.