Doctoral Dissertation Research: Multi-Scale Influences of Climate on Upper Treeline Dynamics Along a Latitudinal Gradient in the Rocky Mountains, USA

预计未来落基山脉将继续出现大范围的气温上升。古生态和现代研究表明，上树线的海拔高度对温度波动很敏感，但降水与气温变暖的相应影响尚不清楚。这种不确定性在加拿大和瑞典的研究中得到了突显，在这两个国家，树高线的推进仅限于朝南的斜坡，这表明与气候之间存在非线性关系。我们目前对上树线动态的了解主要是基于局部的小规模研究，而这些研究缺乏对区域、大尺度模式和过程的理解。这项博士论文研究的目的是确定气候对落基山脉沿纬度梯度的多个空间尺度上树线的时空动态的影响。这项研究将利用美国西部北纬40度的降水偶极子来演示温度和降水对南北对比坡面上树线动态的相对影响。树木年代学技术将能够重建树高线上的建树和定居日期。将使用PRISM和SNOTEL气候数据以及统计技术，包括相关、多元回归和叠加历元分析，调查树木建立的气候控制。如果树的建立在不同地点之间基本上是同步的，这可能表明温度在调节树上线生态动态方面具有压倒一切的重要性。相反，如果不同山脉的再生脉冲不同，那么温度和降水制度的时空排列似乎是最重要的。这项研究的结果既填补了我们在理解气候变化对上树线影响方面的理论空白，也填补了落基山脉上树线站点在地方和区域尺度上的数据网络的物理空白。查明和了解依赖气候的生态过程中区域变异性的原因，如林线的海拔推进，构成了应纳入未来气候变化情景的关键问题。山区对相对较快的人为气候变化速度特别敏感，可能对生态、水文和社会系统造成严重影响。揭示气候变化对落基山脉上树线森林的影响，将有助于为增进有关全球变暖导致的植被变化的知识提供必要的框架。作为博士论文研究改进奖，该奖项还将提供支持，使有前途的学生建立一个强大的独立研究生涯。

Broad-scale increases in temperature are expected to continue throughout the Rocky Mountains in the future. Paleoecological and contemporary research suggest that the elevational extent of upper treeline is sensitive to temperature fluctuations, yet the corresponding influence of precipitation acting in conjunction with warmer temperatures remains less clear. This uncertainty is highlighted in studies from Canada and Sweden where upper treeline advance has been confined to south-facing slopes, suggesting a non-linear relationship with climate. Our current knowledge of upper treeline dynamics is largely based on local, small-scale studies that lack an understanding of regional, large-scale patterns and processes. The goal of this doctoral dissertation research is to determine the influence of climate on the spatiotemporal dynamics of upper treeline in relationship to multiple spatial scales along a latitudinal gradient in the Rocky Mountains. This study will utilize a precipitation dipole that exists at 40 degrees north latitude in the western United States to demonstrate the relative influences of temperature and precipitation on upper treeline dynamics on contrasting north- and south-facing slopes. Dendrochronological techniques will enable the reconstruction of tree establishment and colonization dates at upper treeline. Climatic controls on tree establishment will be investigated using PRISM and SNOTEL climate data along with statistical techniques, including correlation, multiple regression, and Superposed Epoch Analysis. If tree establishment is broadly synchronous among sites, that may suggest the overriding importance of temperature in regulating upper treeline ecotonal dynamics. Conversely, if pulses of regeneration differ between mountain ranges, then the spatial and temporal alignment of temperature and precipitation regimes would appear to be most important.The results of this research fill both a theoretical gap in our understanding of the influence of climate change on upper treeline and a physical gap in the network of data on upper treeline sites in the Rocky Mountains at local and regional scales. Identifying and understanding the causes of regional variability in climate-dependent ecological processes, such as the elevational advance of treeline, constitute vital questions that should be incorporated into future climate change scenarios. Mountain regions are particularly sensitive to the relatively rapid rates of anthropogenic climate change, with potentially severe impacts on ecological, hydrological, and social systems. Revealing the impacts of climate change on upper treeline forests in the Rocky Mountains will help provide the necessary framework for advancements in knowledge concerning vegetation change due to global warming. As a Doctoral Dissertation Research Improvement award, this award also will provide support to enable a promising student to establish a strong independent research career.