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

在全球范围内，树木正以惊人的速度死亡，在日益严重的干旱和火灾面前，这引发了人们对森林健康的担忧。在美国，东部森林受干旱和火灾的影响小于西部森林。然而，几种东部树种--特别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种群崩溃。因此，东部森林面临的坍塌风险可能比人们通常认为的更大。该项目将收集新的数据，并应用强大的数学工具来测试有关铁杉和山毛榉森林为何大幅下降的假设。研究人员将重点研究严重干旱、更大和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，这个项目寻求1)更好地了解引发树木种群突然崩溃的基本过程，2)从而告知并减少当前对森林健康的风险。该项目将分析森林组成、火灾状况和干旱的长期记录(以十年为分辨率)，基于将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，过去树木数量下降的时间将通过化石花粉和放射性碳年代的记录来确定，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碳同位素的种类，该项目将通过一种创新的方法来识别过去的干旱时期，这种方法需要精确测量单个花粉粒的碳同位素含量。因为所有记录都将来自相同的沉积岩芯，所以事件的相对顺序将被准确地知道。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，将提供对从碳同位素推断的干旱和水分胁迫信号的独立检查。一个新的社区级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。所有收集到的数据都将被存档在新托玛古生态数据库中，这是一个开放的长期生态记录的公共储存库，由专家管理。这里开发的统计软件的所有源代码都将公开，并将举办一个研讨会，培训学生和早期职业科学家使用将开发的新方法。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.