Genetic Diversity, Ecological Niches, and Climate Change Vulnerability of Aspens in the Upper Midwest:Leaf osmotic potential and stomatal pore index

Quaking aspen (Populus tremuloides) is the most cosmopolitan tree species in North America and an important native at Cedar Creek and across the Midwest. Aspen stands are quite common through eastern, central, and northern Minnesota, and occur sporadically in cool, wet microclimates across the Great Plains. Currently, these stands are in decline, are poorly reproducing in the wild, and are suffering from a range of stresses. Climate change associated phenomena, drought and altered freeze-thaw cycles, have contributed to massive aspen dieback, especially in the American West. We have received funding from the National Park Service to assess the genetic diversity and hybrid status, age structure and health, ecological niche and historical rate of range contraction, and drought and freezing tolerance physiology of an aspen stand of interest at the Niobrara National Scenic River (NNSR) in northern Nebraska. As part of this project, we are also studying genetic diversity and physiological vulnerability to climate change in quaking and bigtooth (P. grandidentata) aspen populations in Minnesota, Wisconsin, Iowa, South Dakota, and Nebraska. We will use genetic markers to identify genetically unique stands and compare growth and survival of these to populations of the parent species under different drought and freeze-thaw conditions. This study will allow us to better pinpoint the causes of decline in the NNSR aspen stands and aspen stands across the upper Midwest, and potentially provide guidance to managers on the prioritization of particular stands for conservation or in identifying genetic sources for any ex situ conservation or assisted migration.

颤杨（Quaking aspen，Populus tremuloides）是北美分布最广的乔木树种，同时也是锡达溪（Cedar Creek）及整个中西部地区的重要本土树种。颤杨林分在明尼苏达州东部、中部与北部地区极为常见，且在大平原区域的凉爽湿润微生境中呈零星分布。当前，这类林分正处于衰退状态，野外自然更新不良，且正遭受多种胁迫的影响。与气候变化相关的干旱、冻融循环改变等现象，已导致大规模的颤杨枯梢，尤其在美国西部地区。我们已获得美国国家公园管理局（National Park Service）的资助，旨在评估内布拉斯加州北部尼奥布拉拉国家风景河（Niobrara National Scenic River，NNSR）内目标颤杨林分的遗传多样性、杂交状态、年龄结构与健康状况、生态位及历史分布收缩速率，以及其干旱与耐寒生理特性。作为本项目的一部分，我们还将研究明尼苏达州、威斯康星州、艾奥瓦州、南达科他州及内布拉斯加州境内的颤杨与大齿杨（bigtooth aspen，P. grandidentata）种群的遗传多样性及其对气候变化的生理脆弱性。我们将利用遗传标记识别遗传上独特的林分，并对比这些林分与亲本种群在不同干旱及冻融条件下的生长与存活情况。本研究将帮助我们更精准地明确尼奥布拉拉国家风景河林分及中西部北部地区颤杨林分衰退的成因，并有望为管理者提供两方面的指导：一是确定优先保护的林分，二是为迁地保护或辅助迁移筛选合适的遗传种质资源。