The Complex Dendroclimatology of Pinus aristata

Abstract

In the southwestern United States persistent aridity has led to the development of a system of institutions to manage and distribute limited water resources. The management of such systems was based on an imperfect understanding of potential variability in the region’s climate. With anthropogenic warming already altering the Southwest’s climate, understanding the past potential for extreme drought and the role temperature played during drought will provide a baseline for water managers to anticipate risks to this system. Tree-ring reconstructions of climate provide a useful metric for assessing past shifts in precipitation, streamflow, and temperature. Despite an abundance of multimillennial hydroclimate reconstructions for the Southwest, few contemporaneous, skillful tree-ring reconstructions of temperature are similarly available. In an attempt to address this gap, I evaluate the complex climate sensitivity of Rocky Mountain bristlecone pine (Pinus aristata Engelm.), a long-lived, climatically sensitive proxy with the potential to be utilized in temperature and hydroclimate reconstructions. The first study of the dissertation begins with an extensive evaluation of climate sensitivity across ten new and previously sampled collection sites. I identified a robust spring hydroclimate response throughout the lower elevation collections and a weaker but still significant late summer temperature response intermixed with hydroclimate sensitivity in the upper elevation collections. In the second study, I utilized the Vaganov-Shashkin proxy system model to simulate tree-ring growth in P. aristata, identifying temperature thresholds controlling climate sensitivity. My results provided further confirmation of the late-spring/early-summer hydroclimate response in low elevation P. aristata. The upper elevation P. aristata were found to respond to temperature variability under cooler pre-anthropogenic warming conditions. This provides a direction for future research to investigate past temperature sensitivity at sites that no longer contain a clear temperature response or to aid in the identification of locations where cooler growing season conditions may still produce a temperature sensitivity in P. aristata growth. For the final study, I used the results from the climate sensitivity analysis and proxy system modeling to develop the first April-June drought index reconstruction for the Southern Rockies. I utilized this reconstruction to evaluate the multi-century relationship between winter snowpack and spring drought conditions in the Rio Grande headwaters. Initial analysis of the observed relationship between winter snowpack and spring hydroclimate conditions indicated the potential, albeit rare, for extreme spring conditions to ameliorate or exacerbate winter moisture conditions and alter streamflows. I found no consistent multi-year relationship between the new spring drought index reconstruction and an existing snowpack reconstruction, but did identify periods of simultaneous fluctuations that coincided with extreme drought and pluvial events, and provided evidence that during major drought events dry spring conditions may have exacerbated the severity of winter moisture deficits. Overall, the findings in this body of research confirm that P. aristata is a climatically sensitive species, capable of providing sufficient growth response to construct a hydroclimate reconstruction. Additionally, the identification of a late summer temperature response using both climate sensitivity analysis and proxy system modeling provides a solid foundation to pursue the development of a skillful, multi-millennial reconstruction of temperature using P. aristata.