Summer 2016 represented another record-breaking year for global temperatures, and the Northeast experienced one of the most severe droughts in memory. We know that the ability of plants to store water in their tissues can serve as a buffer against drought and its detrimental effects, yet how they do so is not well understood. As a Katharine H. Putnam Fellow, my research at the Arnold Arboretum focuses on water storage in trees—particularly how much water can be stored in the woody stems of trees and shrubs and how much of that water can actually be used. In untangling some of the uncertainty surrounding water storage and the strategies employed by plants under water stress, we may be able to improve our predictions of how our trees and our forests will respond to a changing climate.
Anatomical studies can be revealing in understanding how plants function, so my research combines in situ (on site) measurement of water storage and anatomical measurements of plant tissues. The woody stems of trees, shrubs, and vines consist of many cell types, but primarily include fibers for mechanical support, parenchyma for transporting and storing carbohydrates, and vessels for transporting water from roots to leaves. Fibers and parenchyma are the two most abundant tissue types and together they make up the bulk of what we refer to as wood. While recent studies suggest that fibers play the pivotal role in water storage, how they do so and the mechanisms that control water release remain unresolved.
So what are the possibilities? Parenchyma cells lie closer in proximity to vessels than the fiber cells, so potentially the water stored in parenchyma may be released more readily into vessels. On the other hand, the water in fibers is free and not bound to other substances as it is in the parenchyma, so hypothetically it could move more easily between tissues. I hypothesize that trees with a higher water demand (those “thirstier” than others) will exhibit larger water storage depletion to meet high water demand, and that their anatomical structure facilitates this behavior. Also, my study aims to determine whether the structural requirements for water storage are compromised by the structural requirements of two other functions played by fibers and parenchyma. Presumably, the stronger the twig, the less space is available for storing water or food.
My research at the Arboretum comprises a wide-ranging comparative study of anatomical and functional traits and a physiological study of the hydraulic mechanisms in fibers versus parenchyma. The first project seeks broad trends across a diverse species set, and the second will investigate the structural mechanisms driving those trends. The trees I selected for the study cover a varied range of anatomies and include a diverse suite of 30 deciduous, flowering plant species from 23 families. Last summer, I collected small twigs, each about one centimeter (0.4″) in diameter, and measured how much water was stored and released during the day. I also collected data on leaf water potential (indicates how “thirsty” a tree is), wood density, tree height, and mechanical strength. In the Weld Hill labs, I investigate the anatomical structure of these samples using microscopy and image analysis techniques. Taken together, this information will reveal how much water is stored and released and any correlations between anatomical structure, water storage, and the other traits measured in the study.
The Arnold Arboretum provides an excellent study site for this work due to its diverse living plant collections growing in a common environment and their close proximity to state-of-the-art laboratories. Through my Putnam Fellowship, I hope to make significant advances in our understanding of the structural diversity of plants and how these differences affect how they grow and their capacity to survive extreme conditions. Since water and carbohydrate storage create important buffers that plants can tap into during periods of stress, understanding their hydraulic behavior will facilitate more accurate predictions of current and future responses to our changing climate. On a broad scale, forests play a major part in the global carbon cycle, but altered water regimes may modify this role—and hence the trajectory of climate change. The more we know about how plants store and release water the better we can manage and protect our trees, our forests, and our future.