Severity of a Mountain Pine Beetle Outbreak Across a Range of Stand Conditions in Fraser Experimental Forest, Colorado, United States

The recent mountain pine beetle (Dendroctonus ponderosae Hopkins) outbreaks had unprecedented effects on lodgepole pine (Pinus contorta var. latifolia) in western North America. We used data from 165 forest inventory plots to analyze stand conditions that regulate lodgepole pine mortality across a wide range of stand structure and species composition at the Fraser Experimental Forest in Colorado, USA. Forest inventory data were then combined with Landsat 7 Enhanced Thematic Mapper Plus (ETM+) imagery and boosted regression trees modeling to map outbreak severity (proportion of basal area killed). The outbreak severity map was evaluated with training (pseudo-R2 = 0.63, RMSE = 0.13) and independent test plots (pseudo-R2 = 0.42, RMSE = 0.27). This map was used to compare pine mortality in regenerating clearcuts and mature stands, which would have been problematic otherwise since regenerating clearcuts were underrepresented in the forest inventory data. Mortality spanned from 0 to 99% of stand basal area, proportional to the abundance of pine in surveyed stands. During the outbreak, mortality was highest for larger-diameter trees; however, contrary to earlier outbreaks, beetles also attacked younger stands. Pine mortality was lower in stands regenerating from clearcut harvests conducted between 1954 and 1985 than in mature stands and was more closely related to topographic factors than stand age or clearcut size; mortality was highest on southerly aspects and lower elevation sites, favorable to lodgepole pine. The best predictors for mapping outbreak severity were the change in the Normalized Difference Moisture Index between pre- and end-of-outbreak imagery and the end-of-outbreak ETM+ band 5. A better understanding of mortality patterns relative to forest management can inform management planning and assessment of the influence of bark beetle outbreaks on ecosystem processes