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

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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.