Relationships between forest structure , composition , site , and spruce beetle occurrence in the Intermountain West DeRose ,

Engelmann spruce forests are structurally and compositionally diverse, occur across a wide range of physiographic conditions, and are the result of varying disturbance histories such as fire, wind and spruce beetle. The spruce beetle is a natural disturbance agent of spruce forests and has population levels that fluctuate from endemic to epidemic. Conceptually, structural and compositional diversity of Engelmann spruce forests are thought to provide resistance to spruce beetle activity, and have traditionally been used to assess stand risk to the beetle. We used FIA data from nine states in the Intermountain West to test the effect of stand structure, composition, and site attributes on the occurrence of spruce beetle. A suite of independent variables from FIA data were used to predict the presence or absence of spruce beetle. Results suggested structural variables such as trees per acre and stand density index ratio were influential in the model. Similarly, compositional variables such as percent stand density index of aspen, lodgepole pine, Douglas-fir, and subalpine fir were also influential. Co-occurrence of multiple species with Engelmann spruce helped explain some of the patterns of spruce beetle occurrence. Ubiquitously occurring species such as subalpine fir and Douglas-fir explained less of the variation in model predictions. Elevation and latitude were significant predictors of spruce beetle occurrence; however, co-variation made direct interpretation of these independent variables difficult. Although statistically weak, the model results were ecologically interpretable and provided a range-wide context in explaining spruce beetle activity at some sites in the Intermountain West. 1 Corresponding author (R. Justin DeRose) 2 Department of Wildland Resources and Ecology Center, Utah State University, 5230 Old Main Hill, Logan, UT 84322-5230, Phone: 435-797-3219, Facsimile: 435-797-3796, Email: [email protected] 3 Department of Wildland Resources and Ecology Center, Utah State University, 5230 Old Main Hill, Logan, UT 84322-5230 4 Forest Inventory and Analysis, Rocky Mountain Research Station, 507 25th Street, Ogden, UT 84401 USDA Forest Service Proceedings – RMRS-P-56 39. In: McWilliams, Will; Moisen, Gretchen; Czaplewski, Ray, comps. 2009. 2008 Forest Inventory and Analysis (FIA) Symposium; October 21-23, 2008: Park City, UT. Proc. RMRS-P-56CD. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. 1 CD. 2