COMMUNITY AND ECOSYSTEM ECOLOGY Within-Stand Spatial Distribution of Tree Mortality Caused by the Douglas-Fir Beetle (Coleoptera: Scolytidae)

The Douglas-Þr beetle, Dendroctonus pseudotsugae Hopkins, causes considerable mortality in Douglas-Þr, Pseudotsuga menziesii (Mirb.) Franco, forests. Within-stand distribution of mortality was examined in affected stands using geostatistical techniques. A 10 3 10 m grid was established in two 4-ha study sites. Live and beetle-killed host basal area was measured at each node. In a 16-ha stand, a variable-resolution grid was established and the same information collected. The relationship between Douglas-Þr basal area and Douglas-Þr basal area killed was examined using non-spatially explicit and spatially explicit linear regressionmodels.Apositive linear relationshipwas observed between the variables. SigniÞcant spatially explicit models suggest that the relationship is also true at Þne scales. Relative variograms were constructed for Douglas-Þr basal area before and after the Douglas-Þr beetle outbreaks. For the 4-ha sites, increased spatial dependency in the distribution of Douglas-Þr basal area was observed as a result of the Douglas-Þr beetle outbreak. For the 16-ha site, kriging was used to estimate live Douglas-Þr basal area before and after the outbreak to a 10-m resolution and the stand rated for potential mortality illustrating the potential applicability of geostatistical techniques to rating a stand for potential mortality. Cross-validation analysis indicated that although the potential exists for large estimation errors, themajority of the estimateswere within acceptable ranges. The study suggests that geostatistical approachesmaybe suitable to extend our understanding bark beetle ecology and improving the application of extent of mortality models.