Collaborative Research: Mechanisms of tree population collapses in eastern North America: Disentangling causes of abrupt ecological change during the Holocene

全世界的树木正在以惊人的速度死亡，在干旱和火灾日益严重的情况下，人们对森林健康感到担忧。在美国，东部森林受干旱和火灾的影响比西部森林小。然而，一些东部树种-特别是东部铁杉和美国山毛榉-在过去的几千年里经历了大规模的人口崩溃。 因此，东部的森林可能比通常认为的更有可能崩溃。 该项目将收集新的数据，并应用强大的数学工具来测试为什么铁杉和山毛榉森林遭受了大幅度下降的假设。研究人员将重点关注严重干旱、更大和更频繁的火灾以及树种之间更大的竞争所起的作用。 最终，该项目旨在1）更好地了解引发树木种群突然崩溃的基本过程，2）从而提供信息并减少当前对森林健康的风险。（6,000年，十年分辨率）的森林组成，火灾状况和干旱，根据在四个地点收集的湖泊沉积物岩心-两个在今天山毛榉和铁杉丰富的地方，两个在只有山毛榉丰富的地方。 在每个地点，过去树木种群下降的时间将通过化石花粉和放射性碳年代记录来确定，而过去的火灾事件将通过分析沉积物中木炭碎片的丰度来确定。 由于植物水分胁迫会影响植物组织中的碳同位素种类，因此该项目将通过一种创新方法来确定过去的干旱事件，该方法需要精确测量单个花粉粒的碳同位素含量。 由于所有记录都来自同一沉积物岩心，因此可以精确地了解事件的相对顺序。 在两个地点，根据湖泊沉积物的特点重建湖泊水平衡的变化，将提供一个独立的检查从碳同位素推断的干旱和缺水信号。 一个新的社区一级的统计模型将整合数据，并用于测试有关干旱，火灾事件和竞争对过去树木种群下降的相对重要性的假设。 收集的所有数据将存档在Neotoma古生态学数据库中，这是一个由专家管理的长期生态记录的开放式公共存储库。 所有在此开发的统计软件的源代码都将公开，并将举办一个研讨会，对学生和早期职业科学家进行新方法的培训。该奖项反映了NSF的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。

Trees are dying at alarming rates worldwide, raising concerns about forest health in the face of increasing droughts and fires. In the US, eastern forests have been less affected by droughts and fires than western forests. However, several eastern tree species -- notably eastern hemlock and American beech -- have experienced big population collapses over the last several thousand years. Hence, eastern forests may be at greater risk of collapse than commonly thought. This project will collect new data and apply powerful mathematical tools to test hypotheses about why hemlock and beech forests have suffered big declines. Researchers will focus on the roles of severe droughts, bigger and more frequent fires, and greater competition among tree species. Ultimately, this project seeks to 1) better understand the fundamental processes that trigger abrupt collapse of tree populations and 2) thereby inform and reduce current risks to forest health.The project will analyze long-term records (6,000 years, at decadal resolution) of forest composition, fire regime, and drought, based on lake sediment cores to be collected at four sites -- two at places where beech and hemlock are abundant today and two where only beech is abundant. At each site, the timing of past tree population declines will be established by records of fossil pollen and radiocarbon dates, while past fire events will be identified through analysis of the abundance of charcoal fragments in sediments. Because plant water stress affects the kinds of carbon isotopes in plant tissue, this project will identify past drought episodes through an innovative approach that requires precise measurements of the carbon isotopic content of individual pollen grains. Because all records will be co-located from the same sediment cores, the relative sequence of events will be precisely known. At two sites, reconstructions of changes in lake water balance, based on characteristics of the lake sediments, will provide an independent check on the drought and water-stress signals inferred from carbon isotopes. A new community-level statistical model will integrate data and be used to test hypotheses about the relative importance of drought, fire events, and competition on past declines of tree populations. All data collected will be archived in the Neotoma Paleoecology Database, an open public repository of long ecological records, curated by experts. All source code for statistical software developed here will be made publicly available, and a workshop will be held to train students and early career scientists in the new methods that will be developed.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.