Douglas-fir ring width data for reconstructing past periods of western spruce budworm outbreak

The western spruce budworm (Choristoneura occidentalis; "WSB") is one of the most widely distributed native defoliators of coniferous forests in North America, where it plays an important role in shaping ecosystem function. Here we provide tree-ring data used to reconstruct periods of past WSB outbreak at 12 sites across central to northern Colorado over the past 300 years.

Methods
Ring-width data for reconstructing periods of WSB outbreak were collected from Douglas fir in the 1990s, but unpublished. Sample sites were selected based on the availability of large, old Douglas fir and the absence of evidence of recent (i.e., since c. 1940) fire or logging to minimize the potential effects of other disturbances on radial growth. At each site, tree-ring data were collected by preferentially sampling at least 20 large Douglas fir using an increment borer. Cores were transferred back to the lab, where they were dried, mounted, and sanded to a fine polish, following standard dendrochronological methods. Ring widths were then measured to a 0.01 mm precision using a Velmex UniSlide digital encoded traversing table paired with a standard light microscope. To ensure accurate dating, ring-width series were visually cross-dated using the maker year approach and statistically verified using the dplR package in R.