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

全球气候变化模型预测，未来一个世纪气温更高，降水量变化更大，这可能会影响干旱的频率和强度。北美中部落基山区持续了较长时间的干旱，对环境和当地社区的生计造成了不利影响。要提高未来干旱预测的准确性，就需要回顾过去，研究降水和其他与干旱有关的因素的长期变化。从1895年到现在，干旱在仪器气候记录中得到了很好的记录，而年轮宽度已经被成功地用于将这些气候记录追溯到数百年前。这项研究项目将描述近期干旱的气候学特征，并将重建落基山脉中部干旱的时间和空间模式。已经收集了树芯，初步结果表明，从1955年左右开始，气候和树木生长的关系在中高海拔地区变得更加自满。随着时间的推移，这种日益增长的自满情绪使气候-树木生长关系和随后的干旱重建复杂化。以前的研究表明，将稳定的碳同位素与树轮宽度结合起来，可能会增强重建所需的气候信号，而且已经发现碳同位素与半干旱地区的干旱周期密切相关。因此，研究目标将是利用仪器气候记录(温度、降水和Palmer干旱严重程度指数)、多指标方法(树轮宽度和稳定碳同位素)、海拔梯度采样计划和树枝气候时间序列分析来建立落基山脉中部的干旱重建。该项目的多代理设计将解决关于碳同位素是否真的会加强树木年轮宽度随时间和空间的预测能力的知识差距。该项目的结果将为研究树木年轮宽度和碳同位素组成对气候的复杂生态响应提供新的见解。从这些跨越海拔梯度的年度和季节性气候的替代记录中进行推断，将提高识别落基山脉中部地区干旱时空模式的能力。该项目还将通过查明干旱发生的历史背景使当地社区受益，这些背景可纳入目前正在地方和区域两级审查的土地利用规划和节水战略。

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.