Bighorn Mountains Dendrochronology

Trapper Creek Canyon Douglas fir chronology

 

 Chronology description

These results are preliminary and unpublished. Please do not cite or distribute.

This updated chronology includes 25 ring width series from 19 living Douglas fir trees along the southern rim of Trapper Creek Canyon near Shell, WY. Sample depth in 1600 is 17 series from 13 trees. Twelve series from 10 trees extend back to 1500. The series from 1 tree begins prior to 1300. The shortest series is 267 years. Each ring width series was detrended using a negative exponential curve or straight line of 0 or negative slope. The length of these series and the conservative detrending allow interpretation of trends as long as 200 years. The red line below is a 25 year cubic smoothing spline which highlights these trends.

Twenty-two of the 25 series had a total of 84 missing rings for an average of 3.4 missing rings per series. The most common years of missing rings are 1800, 1934, and 1966, but about 10 other rings were also missing in at least one core. The location of these missing rings was determined by visually cross dating each core using narrow marker rings which are consistently present in most decades in each core. There is strong intercorrelation of ring widths among series (r = .795), and high mean sensitivity (.381) making crossdating straightforward.

Interpretation

The good correlation of the 20th century portion of this chronology with Wyoming division 4 PDSI and PHDI suggests that the entire chronology can be used as a proxy for regional drought stress and precipitation. The chronology therefore indicates that the last decades of the Little Ice Age between 1770 and 1850 were generally wetter than other times in the last 4 centuries. Two periods lasting 2-3 decades each around 1660 and 1710 were apparently much drier than any subsequent time, and were substantially more severe droughts than the well-documented droughts of the 20th century. Although the oscillations between good and poor growth which are evident in the smoothed chronology appear to be damped in the 20th century, there appears to be one period of sustained water stress between 1952 to 1958 corresponding to a severe drought which is well documented throughout the US. The 1950s drought is more conspicuous in this chronology than is the 1930s Dust Bowl drought, but the driest Dust Bowl year is well recorded by the absence of the annual ring for 1934 in 16 of the 25 series. The only ring missing more frequently is the 1800 ring.

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