Landscape analyses of Douglas-fir beetle populations in northern Idaho

Spatial relationships of Douglas-fir beetle (DFB), Dendroctonus pseudotsugae, infestations were examined over a 13-year period on the Priest Lake Ranger District, a USDA Forest Service ranger district in Idaho, USA. In addition, population trends were examined on three other USDA Forest Service ranger districts. Using aerial detection survey maps and GIS to derive nearest neighbor distances, trends in the proximity of DFB infestations within a single year (t), and between-years (t + 1) were determined and spatial patterns of infestations were investigated. DFB infestations (i.e., trees killed, area affected) were described with patch metrics. When DFB populations were low, average within-year nearest neighbor distances were typically above 1400 m. As DFB populations increased, average nearest neighbor distances dropped below 500 m. Simulation-based analyses of the spatial pattern of DFB infestations suggested a clustered distribution of infestations when populations were high. Between-year average nearest neighbor distances followed a similar pattern, with half of the infestations close to one another especially during epidemic population phases. During periods with endemic populations, infestations were found at various distances (0–5000 + m) from other infestations. Average patch size of infestations on Priest Lake Ranger District varied from 0.8 to 19.7 ha and was positive exponentially related to number of infestations. Average number of trees killed per infestation ranged from 4.7 to 124.3 and was also positive exponentially related to number of infestations. As numbers of infestations increased, infestations formed larger patches with an increasing number of trees killed.