Doctoral Dissertation Research: Treeline Responses to Climate Change in High-Elevation Landscapes of Western Montana

树木线过去和现在的位置受气候条件的控制，是过去气候变化的记录者。中世纪温暖期（MWP）和随后的小冰期（LIA）被认为是在整个冰川期和间冰期反复出现的千年尺度气候波动的温暖期和寒冷期的典型例子。目前在蒙大拿州西部高海拔地区气候波动的时空记录方面存在空白。本次博士论文研究项目的目标是：(1)利用蒙大拿西部选定地点树木生长的年轮记录，对树木生长对气候变化的响应进行对比分析；(2)评估夏季温度对树线火山气溶胶强迫的响应；(3)评估树线霜轮形成和温度的机制、解剖结构和后果；(4)综合这些分析，评估与气候变率有关的过去和现在的树线动态。研究气候变化的范围是理解过去气候变化的时空动态和未来气候系统的潜在行为的重要组成部分。为了分析全球气候变化对林木线动态的影响，研究人员将收集生长在当前林木线下方、上方和上方的白皮松和亚高山冷杉的增量核心和横截面。此外，将从树线以上的残余木材中收集样本，以帮助确定过去的树线波动，并增加对过去森林结构的了解。提出的树木气候学和树木生态学研究的结果将导致以下方面的发展：(1)增加对温度变化对白皮松和亚高山冷杉生长季节影响的认识；(2)提高对物种生态极限下火山强迫温度影响的认识；(3)发展亚高山生态对气候变率的百年（可能还有千年）时间尺度的响应。(4)增加对MWP和随后的LIA期间经历的温度对生态系统过程的影响的理解。这项研究的结果将有助于填补我们对气候在落基山脉北部高海拔地区生态系统动态的主要影响作用的理解空白。因此，未来应该针对美国西部的上层树线进行更系统的气候敏感标本采样，以增加我们对过去气候变化的认识，进一步量化气候变化对高海拔生态系统的影响，并帮助预测未来的气候波动及其如何影响树线进退的波动。这项博士论文研究改进补助金将为有才华的学生提供现场和实验室支持，以继续他的研究和学术生涯。

The past and present positions of treeline are controlled by climatic conditions and act a recorder of past climate change. The Medieval Warm Period (MWP) and subsequent Little Ice Age (LIA) are considered characteristic examples of the warm and cold phases of millennial-scale climatic fluctuations that have reoccurred throughout glacial and interglacial periods. A gap currently exists regarding the spatial and temporal record of climatic fluctuations for high-elevation sites in western Montana. The goal of this doctoral dissertation research project is to: (1) conduct comparative analyses of tree growth responses to climate changes using the tree-ring record from trees growing in selected sites in western Montana, (2) evaluate the summer temperature response to volcanic aerosol forcing at treeline, (3) assess the mechanism, anatomy, and consequences of frost-ring formation and temperature at treeline, and (4) synthesize these analyses to assess past and present treeline dynamics in relation to climate variability. Examination of the range of variation in climate is a significant component for understanding the spatial and temporal dynamics of past climate shifts and the potential behavior of the climate system in the future. To analyze the effects of global climate shifts on treeline dynamics, the researchers will collect increment cores and cross sections from selected whitebark pine and subalpine fir trees growing below, at, and above the present treeline. In addition, samples will be collected from remnant wood above treeline to aid in locating past treeline fluctuations and increase understanding of past forest structure. The results of the proposed dendroclimatic and dendroecological research will result in the development of (1) an increased knowledge of the effects of temperature change on the growing season of whitebark pine and subalpine fir, (2) improved understanding of the effects of volcanic forcing of temperatures at a species ecological limit, (3) the development of the subalpine ecological response to climate variability over century (and possibly millennial) time scales, and (4) increase comprehension of the effect of temperatures experienced during the MWP and subsequent LIA on ecosystem processes.The results of this study will serve to fill gaps in our understanding of the role of climate as a primary influence of ecosystem dynamics at high-elevation sites in the northern Rocky Mountains. Therefore, the upper treeline in the western United States should be targeted for a more systematic sampling of climate sensitive specimens in the future to increase our knowledge of past climatic shifts, further quantify the effects of climate shifts on high elevation ecosystems, and to help predict future climate fluctuations and how they may affect the fluctuation in treeline advance or retreat. This Doctoral Dissertation Research Improvement grant will provide field and laboratory support for a talented student to continue his research and academic careers.