Do tree and stand-level attributes determine susceptibility of spruce-fir forests to spruce beetle outbreaks in the early 21st century?

Stand-level hazard ratings are widely used to determine the susceptibility of forests to insect outbreak. Many of these risk systems were developed from observations and empirical studies of one to a few insect outbreaks and consequently may not reflect variability expected under different climatic conditions. Given the strong and nonlinear linkages between bark beetle outbreaks and climate, there is increased need for understanding the applicability of existing hazard and risk classification systems under current and future climate conditions. Here we examine the susceptibility of 100+ year-old Engelmann spruce to spruce beetle (Dendroctonus rufipennis) infestation during the 2002-2012 period of stand-mesoscale outbreak (i.e. patchy distribution of affected stands rather than a continuous outbreak across a large outbreak). In addition, we examine the effects of pre-1900 disturbance history on stand structural traits related to spruce beetle outbreak. We document the effect of a 1840s spruce beetle outbreak on stand structure 170. years after the outbreak. In contrast, stand structure was not drastically different amongst stands with different last stand-replacing fire dates, which ranged from CE 1740 to 1879. Spruce beetle infestation in the early 2000s was influenced by tree-level characteristics (e.g. DBH), but not by topographic variables or stand structure; thus stand-level effects of pre-1800 disturbance did not discernibly affect the pattern of recent spruce beetle occurrence. Stand-level hazard ratings systems could be improved to consider warm and dry climate conditions, which may remove or relax stand constrains such as in the current outbreak. © 2014 Elsevier B.V