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Summer Research Project 2018

  • Title: Group Project: Structure and function of New England forest trees: Predicting future forest composition by looking back in time
  • Summer Supervisors: Craig Brodersen; Brett Huggett; Jay Wason
  • Researchers: Craig Brodersen; Brett Huggett
  • Project Description:

    How were the forest communities we see today assembled? Are these communities changing and what will New England forests look like in 25, 50, 100 years? These are questions that have traditionally been asked from a whole-forest perspective, but much less is known about how the individual species that make up those communities will respond to a changing environment. In particular, we're interested in understanding how and why some species come to dominate some forests, while in other forests greater diversity is maintained.

    Answering these questions largely depends on our ability to understand the physiology of the individual species at different life history stages from seedlings and saplings to canopy trees. The young trees we see today at forest edges and canopy gaps are, potentially, the mature canopy trees of the future. Assuming that physiology and anatomy are fixed and universal properties grossly underestimates the potential of forest trees to adapt to changing environments. Rather, the degree of plasticity of each species at each life history stage is likely to drive the dynamics of competition between species for resources. Understanding these traits and the interrelationship between the structure and function of trees will allow us to create a robust, predictive model capable of predicting how forests will change over time. This information can then be used to better manage forest resources both from a conservation perspective but also from the perspective of timber and forest product production, as well as help to understand whether forests will become carbon sinks or sources in the future.

    Growth and reproduction in trees are dependent on a steady supply of water to the leaves where that water is used in support of photosynthesis. Any dysfunction that arises in the water transport system of plants has the potential to cause both immediate and long-term losses to carbon gain. The resiliency of a tree species is intimately tied to the structure and function of the water transport system (i.e. the xylem). Our goal is to characterize tree xylem networks and assess the degree of plasticity of those networks. The degree of xylem network flexibility should determine how well plants are able to adjust to climate variability, but to date our understanding of these anatomical and physiological processes work is limited.

    Much of the uncertainty surrounding wood anatomical traits is due to the variability of forest systems, the complexity of xylem networks, and the lack of suitable tools to quickly and accurately assess variability in plant structure. This project aims to use traditional ecophysiological and dendrochronological tools coupled with newly developed 3D imaging techniques to formulate a physiological response model for dominant tree species at different life history stages. Focusing our research on trees growing in our common garden drought experiment at Harvard Forest, we will use traditional anatomical methods to characterize the xylem network at different points along the water pathway from the roots to the leaves. These measurements will tell us about the degree of plasticity and the different anatomical traits that are used by the plants to deliver water at different points along the pathway from the soil to the leaves. Next, fresh wood samples along that pathway will be used to determine the air-seeding threshold, which will tell us the physiological threshold where water conduits become dysfunctional. Additional sensors (thermocouple psychrometers) will be used along the root to shoot pathway to determine the degree of water stress.. Finally, we'll analyze tree rings to determine the degree of plasticity within the xylem network. Growth ring data can then be coupled with environmental data from existing micrometeorological towers at Harvard Forest to show how light, rainfall, and temperature affect tree growth and xylem connectivity. Using high resolution microCT we can then analyze those same tree cores digitally. Here, the advantage lies in advanced modeling software that allows us to understand the connectivity of the xylem network. In this way we can determine the redundancy, connectivity, and resiliency of a xylem network. We will then simulate how those xylem networks will perform over a wide range of environmental conditions from the past, present, and future. In this way we can determine the "tipping point" for each species and each life history stage to understand how far a plant can be pushed outside of its comfort zone before the water transport system fails. Coupled with data collected in 2016 from forest-grown trees at Harvard Forest, we will then start to build a predictive model that will inform us about the trees that are most at risk (i.e. those with the least amount of plasticity) and the species that are most likely to be dominant in the future (i.e. those with the greatest degree of plasticity).

    Day-to-day responsibilities will include a mix of fieldwork, laboratory work, and data analysis. The undergraduate researcher will be involved in the anatomical and physiological measurements in the field and, maintaining the plant water stress instruments, and downloading and interpreting data. The student will also be responsible for collecting plant material that will be scanned at a microCT imaging facility. Once the 3D data is collected the student will help in processing and analyzing the network properties of the different species and coupling each tree ring's anatomical features to the environmental data from meteorological towers.

    These data will then populate the model that predicts the tipping points and the dynamics of forest community change at different time scales. Brett Huggett will have primary responsibility of mentoring in collaboration with Craig Brodersen. Following a full week of training at the start of the program, mentors will be onsite once per week to work with the mentee. While mentors are not onsite at Harvard Forest, both mentors and the mentee will schedule regular Skype conversations as much as needed to discuss research progress. Experience/interest in forest ecology and plant physiology in addition to a willingness to learn new laboratory techniques, attention to detail, self-motivation, and enthusiasm are preferred.

  • Readings:

    Brodersen, Craig R. "Visualizing wood anatomy in three dimensions with high- resolution X-ray micro-tomography (µCT)–a review–." IAWA Journal 34.4 (2013): 408-424.

    Choat, Brendan, et al. "The spatial pattern of air seeding thresholds in mature sugar maple trees." Plant, Cell & Environment 28.9 (2005): 1082-1089.

    McElrone, A. J., et al. "Water uptake and transport in vascular plants." Nat Educ Knowl 4.6 (2013).

    Wason, Jay W., Brett A. Huggett, and Craig R. Brodersen. "MicroCT imaging as a tool to study vessel endings in situ." American Journal of Botany 104.9 (2017): 1424-1430.

  • Research Category: Physiological Ecology, Population Dynamics, and Species Interactions