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

预计未来整个落基山脉的气温将继续大幅上升。古生态学和当代研究表明，树木线海拔高度对温度波动较为敏感，而降水对气温升高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出体现，在这些研究中，树木线上游仅限于朝南的斜坡，这表明与气候存在非线性关系。我们目前对上层树线动态的认识主要基于局部的小规模研究，缺乏对区域大尺度模式和过程的理解。本博士论文的研究目标是确定气候对落基山脉沿纬度梯度的多空间尺度上的上树线时空动态的影响。本研究将利用存在于美国西部北纬40度的降水偶极子来证明温度和降水对北向和南向斜坡上树线动态的相对影响。树木年代学技术可以重建树线上树木的建立和定殖日期。本文将利用PRISM和SNOTEL气候数据以及相关、多元回归和叠加历元分析等统计技术研究气候对树木生长的影响。如果不同地点之间树木的形成大致同步，这可能表明温度在调节上层树线生态动态方面的重要性高于一切。相反，如果不同山脉之间的更新脉冲不同，那么温度和降水制度的时空一致性似乎是最重要的。本研究的结果既填补了气候变化对落基山脉上树线影响的理论空白，又填补了落基山脉局地和区域尺度上上树线站点数据网络的物理空白。确定和了解气候依赖的生态过程中区域变异的原因，如树线的海拔上升，是应纳入未来气候变化设想的重要问题。山区对相对快速的人为气候变化特别敏感，可能对生态、水文和社会系统产生严重影响。揭示气候变化对落基山脉上游林木线森林的影响将有助于提供必要的框架，以提高有关全球变暖导致的植被变化的知识。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.