Methods of detecting past spruce beetle outbreaks in Rocky Mountain subalpine forests

We examined the use of dendrochronological techniques and stand age structure for detecting past outbreaks of spruce beetle (Dendroctonusrufipennis Kirby) in subalpine forests in the Colorado Rocky Mountains. The methods were based on the observation that following beetle-caused mortality of canopy trees, growth rates of subcanopy Engelmann spruce (Piceaengelmannii (Parry) Engelm.) and subalpine fir (Abieslasiocarpa (Hook.) Nutt.) increase severalfold and remain high for several decades or more. Although inspection of unstandardized individual tree ring-width chronologies is useful for detecting past disturbance, standardized mean chronologies based on ≥20 cores assure adequate sample sizes and improve interpretability. Standardization of ring-width series by fitting the observed data to a horizontal line passing through the mean ring width facilitates detection of past outbreaks better than other standardization models such as polynomial or cubic spline functions. The incorporation of samples from dead-standing trees increases sample sizes and reveals patterns in the early history of a stand that may not be detectable if samples only from live trees are used. Patterns of release frequencies based on nearly all the trees in a stand were helpful in confirming interpretations not always clear from the chronologies alone. Variations in tree population age structures were related to the history of disturbance by beetle outbreak, but age structure alone was not a reliable means of detecting past outbreaks.