Collaborative Research: Structure and Function of Whole-tree 3D Xylem Networks in Response to Past, Present, and Future Drought

森林生产力与植物维管系统的生长和维持有关，维管系统将水分从土壤输送到树叶。这些维管系统是由数千个比人类头发直径还小的相互连接的导管组成的网络，统称为木质部。当植物暴露于干旱时，这种运输系统会变得功能失调，导致生长减少，最终导致植物死亡。目前对木质部网络整体连通性的了解是有限的，这阻碍了对水和营养物质如何在植物中分布的完整理解，也限制了预测不同物种如何适应有限的水分供应的能力。该项目的总体目标是表征东北阔叶树木质部网络的三维（3D）结构及其在干旱期间的功能之间的关系。这项研究将确定哪些树种在不断变化的环境条件下最具弹性，建立超过临界点的物种无法从缺水中恢复过来，并开发一个模型来预测在不同长度和强度的干旱下广泛存在的树木死亡率。这些数据将通过预测环境变化情景下树木死亡率的变化，为保护和木材生产管理提供信息。将创建一个在线数据库，其中3D木质部模型可以下载，然后3D打印用于生物学和植物科学课程，提供一个独特的，动手的方法来学习植物功能解剖。该项目涉及一所主要研究型大学和一所主要本科院校之间的密切合作，从而增加本科生接触STEM领域的研究环境和教育。木质部网络连通性是植物解剖学中了解最少的领域之一，主要是因为缺乏合适的可视化工具来研究构成木质部的微观组织的复杂三维（3D）组织。三维木质部网络解剖结构的可塑性了解得更少，但它可能对水、营养物质、病原体或干旱和冻融引起的栓塞的运动产生重大影响。此外，木质部网络组织应该影响通常测量的木质部易损曲线，但描述这些曲线如何产生的机制模型并不存在。在这里，目的是利用生理和解剖测量现有的成年树和幼树，以及幼树在一个普通的花园干旱实验中，明确地测试一系列关于木质部网络连通性影响的假设，在四种主要的东北硬木树种。利用x射线微断层扫描，从根、树干和茎中提取的木材样本将在3D中进行分析，以探索过去15年来树木对环境变化的反应，这些树木靠近哈佛森林的长期生态研究基地塔。然后将开发一个机制模型来预测每个物种在两个生活史阶段的木质部脆弱性和生理临界点，以帮助理解在变化的环境条件下群落动态将如何变化。该项目将支持博士后、初级研究员的职业发展，并为本科生提供研究机会，包括哈佛大学本科生森林研究经验项目的职位。

Forest productivity is linked to the growth and maintenance of plant vascular systems that transport water from the soil to the leaves. These vascular systems are made up of a network of thousands of interconnected conduits smaller than the diameter of a human hair, collectively known as xylem. As plants are exposed to drought, this transport system can become dysfunctional, leading to reduced growth, and ultimately plant death. Current knowledge of the overall connectivity of the xylem network is limited, and this prevents a complete understanding of how water and nutrients are distributed through plants, and also limits the ability to predict how different species will adapt to limited water availability. The overarching goal of this project is to characterize the relationship between the three-dimensional (3D) structure of the xylem network and its function during drought in northeastern hardwood trees. The research will determine which tree species are most resilient under changing environmental conditions, establish tipping points beyond which species cannot recover from water deficits, and develop a model to predict widespread tree mortality under droughts of varying length and intensity. These data will inform conservation and timber production management by predicting shifts in tree mortality given environmental change scenarios. An online database will be created where 3D xylem models can be downloaded and then 3D-printed for use in biology and plant science classes, providing a unique, hands-on approach to learning plant functional anatomy. The project involves close collaboration between a major research university and a primarily undergraduate institution, thereby increasing undergraduate exposure to a research environment and education in STEM fields. Xylem network connectivity is one of the least understood areas of plant anatomy, primarily due to a lack of suitable visualization tools to study the complex, three-dimensional (3D) organization of the microscopic tissues that make up xylem. Plasticity in 3D xylem network anatomy is understood even less, yet it could have significant impacts on the movement of water, nutrients, pathogens, or drought and freeze-thaw induced embolisms. Furthermore, xylem network organization should influence commonly measured xylem vulnerability curves, but a mechanistic model that describes how these curves arise does not exist. Here, the aim is to use physiological and anatomical measurements of existing adult and juvenile trees, as well as juvenile trees in a common garden drought experiment, to explicitly test a range of hypotheses regarding the influence of xylem network connectivity in four dominant northeastern hardwood tree species. Using X-ray micro-tomography, wood samples from roots, trunks, and stems will be analyzed in 3D to explore the responses of trees to environmental changes over the past 15 years within close proximity to the Long Term Ecological Research site tower at Harvard Forest. A mechanistic model will then be developed to predict xylem vulnerability and physiological tipping points for each species at two life history stages to help understand how community dynamics will shift given changed environmental conditions. This project will support the career development of a postdoctoral associate, a beginning investigator, and provide opportunities for undergraduate research, including positions in the Harvard Forest Research Experiences for Undergraduates program.