| In Chapters 1 and 2 of this dissertation, I present new tree-ring based precipitation reconstructions for portions of southern Montana, Wyoming, and northeastern Utah. These reconstructions reveal significant precipitation variability at interannual to decadal scales during the 1200–2000 AD interval. Single-year dry events before the instrumental period (<1900 AD) tended to be more severe than those in the 20th Century. In general, decadal-scale dry events were longer and more severe prior to 1900. In particular, dry events in the late 13th and 16 th Centuries surpass both the magnitude and duration of any droughts seen after 1900. Overall, evidence from these proxy records demonstrates that the climate of the instrumental period (∼1900–Present) is not representative of the full range of precipitation variability the central Rocky Mountain region has experienced in the past 700+ years.; In Chapter 3, I combine high-resolution precipitation reconstructions with packrat midden and tree-ring data to examine how climate variability may have influenced the late-Holocene migration of Pinus edulis in northeastern Utah. Midden and tree-ring records show that pinyon pine arrived at the Dutch John Mountain study site in the mid-13th Century AD, but further expansion on the landscape was delayed by >70 years. Independent evidence from tree-rings of pinyon pine and Douglas-fir (Pseudotsuga menziesii) suggest that the delay in pinyon pine expansion after initial colonization coincided with a severe, multi-decadal drought. Subsequent periods of rapid pinyon spread occurred during a series of unusually wet periods (1300–1500 AD), each lasting >10 years. |
