Tree-Ring Evidence of Climate and Environmental Change, Beartooth Mountains, Wyoming, U.S.A.

Long-lived, subalpine tree species like whitebark pine and Engelmann spruce may eventually cease to exist due to the combination of climate change and exacerbated native and invasive biological threats. While this loss would have dire consequences for mountain ecosystems, it would also result in the irreversible loss of valuable climatological and ecological data preserved in the growth rings of these trees. The purpose of this dissertation research was to develop extended whitebark pine and Engelmann spruce tree-ring chronologies for use in regional analyses of climate and disturbance, and more importantly to demonstrate the potential of these tree species and the need for increased tree-ring based work. From a high-elevation site in the Beartooth Mountains of Wyoming, I collected hundreds of samples from both living and remnant whitebark pine and Engelmann spruce, and used dendrochronological methods to develop two millennial-length tree-ring chronologies for these species. Using information gained from these chronologies, I examined evidence of climate and environmental change. First, I reconstructed the millennial-length history of a subalpine “ghost” forest. I determined that massive whitebark pine and Engelmann spruce trees had established and lived in the Beartooth Mountains during the warm and dry Medieval Warm Period but perished during the colder Little Ice Age. Next, I evaluated climate drivers of tree growth and implications for ongoing climate change. I discerned the key climatic factors controlling whitebark pine and Engelmann spruce growth, but also discovered that tree responses to climate may be changing. Finally, I investigated the influence of volcanic cooling events in the Beartooth Mountains. I discovered frost and narrow rings associated with climate-changing volcanic eruptions and contributed important information to our understanding of volcanic cooling. I hope that my work may serve as a guide for future efforts to collect and analyze data from whitebark pine and other species that are, and may be, threatened with extinction. I also call for increased contribution of tree-ring data, from whitebark pine especially, but also from other subalpine species, to accessible archives, such as the International Tree-Ring Data Bank (ITRDB), which will make tree-ring data available for current and future scientists.