Late-Holocene paleoclimate and treeline fluctuation in Wyoming’s Wind River Range, USA

Abstract

This study presents evidence for treeline advance in the central Rocky Mountains, United States, between 1800 and 800 cal. yr BP. This evidence was generated by systematic survey, sampling, and accelerator-mass spectrometer (AMS) dating of remnant Pinus albicaulis above the modern treeline ecotone of Union Peak, in the Wind River Range of western Wyoming. The survey identified 257 tree remnants, of which AMS dates were generated on 15 samples. Based on AMS results, it appears that treeline was approximately 100 m higher in elevation and covered an additional 1 km of horizontal distance for approximately a millennium, 1800–800 cal. yr BP. These findings are surprising given the tendency of remnant forests across the region to date to the middle Holocene but are consistent with regional evidence for increases in effective moisture that are contemporaneous with the Union Peak treeline advance. The age of death for most of the sampled trees occurred, however, near 800 cal. yr BP, 200 or more years prior to the onset of the type of regional and hemispheric temperature decreases that might be expected to result in treeline decline. Given the contemporaneity of the Union Peak treeline decline with a substantial regional megadrought 820–780 cal. yr BP, it is hypothesized that declining moisture availability played a fundamental role in this phenomenon. If true, the late-Holocene advance and subsequent decline of treeline on Union Peak would emphasize the role effective moisture plays in the complex interactions between temperature, precipitation, and local-scale factors conditioning treeline elevation across the central Rocky Mountain region.