Bighorn Mountains Dendrochronology

Preliminary Synthesis

These results are preliminary and unpublished. Please do not cite or distribute.

Based on correlations with 20th century instrumental records and derivitive indices, ring-width chronologies from all three of our study sites have signals for high-frequency variation in growing-season drought stress. Although the low-frequency variation which is conspicuous in all chronologies is at a wavelength which approaches the length of the instrumental record, it appears that those low-frequency oscillations in the chronologies also contains information about climatic variability. Although the three chronologies are from sites spanning 150 km along the Bighorn Basin, and each is from a different species of tree, they have a substantial amount of low-frequency variation in common. This similarity is evident in the graph below of the 25 year cubic-smoothing splines of the chronologies. The similarity of these geographically and taxonomically disparate chronologies suggests that their low-frequency signal is one of regional climate. These chronologies appear to have a reliable record of decadal-scale periods of climatic fluctuations, and are expected to be good proxies for droughts which pre-date the instrumental record.

The major periods of drought in the Great Plains and western US have been summarized by Woodhouse and Overpeck (1998). Most of these past droughts are associated with periods of slow growth in all three of our ring-width chronologies (graph below) confirming that the chronologies are good proxies for drought. Some severe and extended periods of slow growth in the chronologies are not associated with droughts in Woodhouse and Overpeck, suggesting that these periods of drought stress were localized or mostly outside the geographic network of proxies reviewed in that study.

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