DISSERTATION RESEARCH:Seeking Evidence for Glacial Refugia of North American White Spruce

该项目旨在调查从末次冰盛期到全新世(过去约20000年)北方森林生物群的发展。关于白云杉的历史有两个长期存在的假说，白云杉是北美北部针叶林的优势树种。第一种假设认为，避难所的人口仅限于美国东南部。第二种假设涉及在最后一次冰川盛期期间，在贝林贾东部(阿拉斯加和邻近的加拿大)的无冰区存在少量种群，在全新世期间，该盛期扩大并合并了南部种群。在过去的几十年里，大量的古生态研究(花粉和大型化石分析)试图验证这些假说。不幸的是，由于传统古生态方法的固有局限性，以前的研究没有产生明确的证据。新兴的DNA技术现在有望解决这些假说。该项目的初步叶绿体DNA数据显示，阿拉斯加有独特的单倍型，整个北美西部没有遗传多样性梯度，支持第二个假设。这项拟议的研究对于理解生物对气候变化的反应很重要。白云杉避难所的存在表明，少量种群能够在极端恶劣的气候条件下生存数千年。它还表明，白云杉对气候变暖的迁徙速度比之前认为的要低得多。分子生物学和古生态学的创新结合为未来研究气候强迫的长期生物响应提供了一个模型。同样重要的是，这个项目为研究生和本科生提供了研究机会。

This project aims to investigate the development of the boreal forest biome from the last glacial maximum through the Holocene (the past ~20,000 years). Two long-standing hypotheses exist about the history of white spruce (Picea glauca), a dominant tree species of North American boreal forests. The first posits that refuge populations were restricted to the southeastern United States. The second hypothesis involves the existence of small populations in the ice-free areas of eastern Beringia (Alaska and adjacent Canada) during the last glacial maximum, which expanded and merged with southern populations during the Holocene. Over the past decades, numerous paleoecological studies (pollen and macrofossil analyses) attempted to test these hypotheses. Unfortunately, previous studies yielded no unambiguous evidence because of inherent limitations of conventional paleoecological approaches. Emerging DNA techniques now promise to resolve these hypotheses. Preliminary chloroplast-DNA data from this project show unique haplotypes in Alaska and no genetic-diversity gradient across western North America, supporting the second hypothesis. The proposed research is important for understanding biotic response to climatic change. The existence of a white spruce refuge in Beringia implies that small populations are capable of surviving extremely severe climatic conditions for thousands of years. It also suggests that the migration rates of white spruce in response to climatic warming are much lower than previously thought. The innovative integration of molecular biology and paleoecology offers a model for future studies of long-term biotic response to climatic forcing. Equally important, this project provides research opportunities for graduate and undergraduate students.