Bark Beetle Effects on a Seven-Century Chronosequence of Engelmann Spruce and Subalpine Fir in Colorado, USA

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Forest Ecology and Management

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Many important trends in forest development across landscapes and centuries are difficult to measure directly, and a space-for-time substitution in a chronosequence may provide useful insight at these scales. The value of chronosequences for forest ecology and management depends on a number of sources of variation, including geographic differences in site productivity, differences in climate over long periods, and the presence or absence of rapid events such as fire, windthrow, and insect outbreaks. Confidence in the value of a chronosequence may be increased if later resampling shows that each site followed the predominant trajectory expected from the chronosequence pattern. We resampled a 700-year chronosequence of Engelmann spruce (Picea engelmannii) and subalpine fir (Abies lasiocarpa) three decades after the initial sampling. The original chronosequence suggested long-term stasis in both biomass and production after about 200 years of stand developments in the absence of major fire, beetle outbreaks, and windstorms. Three decades later, a spruce beetle (Dendroctonus rufipennis) outbreak had reduced spruce biomass by 68% and total stand biomass by 44% across the chronosequence (to an average of 7.8 kg m−2). There remained no trend in total stem biomass with stand age, averaging 13.9 kg m−2 of stemwood across all ages. Stem production averaged 0.15 kg m−2 yr−1 between 1984 and 2013, higher than the 0.09 kg m−2 yr−1 estimated in 1984. Over the three decades, stand biomass shifted from about 2/3 spruce to 2/3 fir. Stands may be selected for chronosequences based on an absence of rapid events that substantially change stand structure, but this may limit the ability of a chronosequence to represent real long-term patterns across landscapes.