Climate variability and ecological response in the Uinta Mountains, Utah, inferred from diatoms and tree-rings

This research reconstructs paleoclimate and examines ecosystem response to environmental variability for the past 13,000 years in the Uinta Mountains, Utah, an important water resource, ecological conservation, and recreation area. Analysis of present-day diatom species distributions and environmental conditions in 61 subalpine and alpine lakes indicates that dissolved inorganic carbon (DIC), surface-water temperature, depth, water clarity, and alkalinity are important determinants of diatom community composition. Quantitative models for estimating alkalinity and surface-water temperature, based on ecological relationships between diatom species distributions and environmental variables, are also developed and discussed. This analysis helps inform an evaluation of diatom species assemblage changes in a subalpine lake-sediment core spanning the terminal Pleistocene (∼13,000 years BP) through the present at approximately 300-year resolution. The diatom stratigraphy suggests that climate was cold until around 10,900 years ago, then temperatures were higher for approximately 7,500 years before cooling to near modern conditions starting ∼3,500 years ago, coincident with the development of modern Pacific Ocean cycles as well as changes in insolation. Ecological changes within the early and middle Holocene could be related to variability in summer season precipitation and/or changes in lake-water alkalinity, which may indicate enhanced monsoonal circulation.;Tree-ring records thought to reflect past streamflow and snowpack conditions are used to reconstruct climate at annual resolution over the last five to nine centuries. These records suggest that the Uinta Mountains experienced droughts of higher magnitude and longer duration prior to the 20th century. Hydrological conditions appear to have been periodically influenced by regular variability in ocean sea-surface temperatures. However, these relationships are not stable over time. The impact of climate variability on terrestrial ecosystems is demonstrated in an investigation of tree establishment, mortality, and radial growth at the alpine-tundra ecotone. There is spatial divergence in establishment response to climate, and temperature-induced summer moisture deficit may result in future ecological changes at tree-line. This research has implications for local to regional water resource planning and ecosystem management.