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。这些维管系统由数千个相互连接的导管组成的网络组成，这些导管的直径小于人类头发的直径，统称为木质部。当植物暴露在干旱中时，这种运输系统可能会功能失调，导致生长减慢，最终导致植物死亡。目前对木质部网络整体连通性的了解有限，这阻碍了对水分和营养物质如何通过植物分布的完全了解，也限制了预测不同物种如何适应有限的水分供应的能力。本项目的主要目标是描述东北阔叶树木质部网络的三维结构与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，建立物种无法从缺水中恢复的临界点，并开发一个模型来预测在不同时间和强度的干旱下广泛的树木死亡。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，在那里可以下载3D木质部模型，然后通过3D打印用于生物学和植物科学课程，为学习植物功能解剖学提供一种独特的动手方法。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，从而增加本科生在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最不被了解的领域之一，主要是由于缺乏合适的可视化工具来研究组成木质部的微观组织的复杂的三维(3D)组织。三维木质部网络解剖中的可塑性更是知之甚少，但它可能对水分、营养物质、病原体的运动或干旱和冻融诱导的栓塞物产生重大影响。此外，木质部网络组织应该影响通常测量的木质部脆弱性曲线，但描述这些曲线如何出现的机制模型并不存在。在这里，目的是使用现有的成年树和幼树的生理和解剖学测量，以及常见的花园干旱实验中的幼树，明确地测试一系列关于木质部网络连通性对东北四种优势阔叶树树种的影响的假说。利用X射线显微断层成像技术，将对来自树根、树干和树干的木材样本进行3D分析，以探索哈佛森林长期生态研究遗址塔楼附近树木对过去15年环境变化的反应。然后将开发一个机械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转折点，以帮助理解在环境条件变化的情况下群落动态将如何变化。该项目将支持一名博士后助理、一名初级研究员的职业发展，并为本科生提供研究机会，包括哈佛本科生森林研究经历计划的职位。

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.