Doctoral Dissertation Research: Drought Sensitivity of Limber Pine in the Central Rocky Mountains Determined From C Isotopes and Ring Widths

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Global climate change models predict higher temperatures and increased variability in precipitation in the coming century, which could affect drought frequency and intensity. A relatively long-term drought has persisted throughout the central Rocky Mountain region of North America with negative impacts on the environment and the livelihood of local communities. Increasing the accuracy of future drought projections requires a glimpse into the past to look at long-term variability in precipitation and other factors associated with drought. Droughts are well documented in instrumental climate records from 1895 to present, while tree-ring widths have been used successfully to extend these climate records back in time for hundreds of years. This research project will characterize the climatology underlying recent droughts, and will reconstruct temporal and spatial patterns of drought in the central Rocky Mountains. Tree cores have already been collected and preliminary results suggest that climate and tree growth relationships are becoming more complacent at the mid to high elevation sites starting around 1955. This increasing complacency over time complicates the climate-tree growth relationship and subsequent drought reconstruction. Previous research suggests that combining stable carbon isotopes with tree-ring widths may strengthen the climatic signal necessary for reconstructions, and carbon isotopes have been found to correlate strongly with drought cycles in semi-arid regions. Thus, research objectives will be to use instrumental climate records (temperature, precipitation, and the Palmer Drought Severity Index), a multi-proxy approach (tree-ring widths and stable carbon isotopes), an elevation gradient sampling scheme, and dendroclimatic time series analysis to develop drought reconstructions for the central Rocky Mountains. It is the multi-proxy design of this project which will address a knowledge gap as to whether carbon isotopes will actually strengthen the predictive ability of tree-ring widths over time and space. Results of this project will provide new insights into the complex ecological responses of tree-ring widths and carbon isotope composition to climate. Inferences from these proxy records of annual and seasonal climate across an elevation gradient will improve the ability to identify spatiotemporal patterns of drought in the central Rocky Mountain region. This project will also benefit local communities by identifying the historical context of drought occurrence which can be incorporated into land use planning and water conservation strategies currently being reviewed at both the local and regional level.