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

世界各地的树木正在以惊人的速度死亡，面对日益严重的干旱和火灾，人们对森林健康的担忧日益加剧。在美国，东部森林受干旱和火灾的影响比西部森林要小。然而，一些东部树种——尤其是东部铁杉和美洲山毛榉——在过去的几千年里经历了大规模的人口崩溃。因此，东部森林可能面临比通常认为的更大的崩溃风险。该项目将收集新的数据，并应用强大的数学工具来检验关于铁杉和山毛榉森林为何遭受大幅减少的假设。研究人员将关注严重干旱、更大更频繁的火灾以及树种之间更激烈的竞争所起的作用。最终，该项目旨在1)更好地了解触发树木种群突然崩溃的基本过程，2)从而了解和减少当前对森林健康的风险。该项目将根据在四个地点收集的湖泊沉积物岩心，分析森林组成、火灾状况和干旱的长期记录（6000年，以十年为分辨率），其中两个地点现在山毛榉和铁杉丰富，另两个地点只有山毛榉丰富。在每个地点，过去树木数量下降的时间将通过化石花粉和放射性碳测年的记录来确定，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，该项目将通过一种需要精确测量单个花粉粒碳同位素含量的创新方法来识别过去的干旱事件。因为所有的记录都来自同一沉积物岩心，事件的相对顺序将被精确地知道。在两个地点，根据湖泊沉积物的特征重建湖泊水平衡的变化，将为从碳同位素推断的干旱和水胁迫信号提供独立的检查。一个新的社区级统计模型将整合数据，并用于检验关于干旱、火灾事件和竞争对过去树木数量下降的相对重要性的假设。所有收集到的数据都将保存在Neotoma古生态数据库中，这是一个由专家管理的开放的长期生态记录公共存储库。这里开发的统计软件的所有源代码都将公开提供，并将举办一个讲习班，培训学生和早期职业科学家使用即将开发的新方法。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识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.