William Pockman, professor of Biology at The University of New Mexico, has received a grant from the Department of Energy for $999,990 for a research project titled Empirical measurements and model representation of hydraulic redistribution as a control on function of semiarid woody ecosystems. His collaborators are UNM Biology professor Marcy Litvak, and Yiqi Luo, a professor in the Cornell University College of Agriculture and Life Sciences.

William Pockman
William Pockman

Researchers rely on computer models to understand how forests and other ecosystems contribute to the amount of carbon dioxide in the atmosphere and how their roles may change as climate fluctuates and changes, Pockman explained. In drylands like the southwestern U.S., the predictions of these models are highly dependent on properly predicting the amount of water available to plants in the soil.

This research seeks to improve model predictions of soil moisture at different depths through time and the predictions of ecosystem carbon balance that depend on soil moisture.

“A major focus of the research is a process called hydraulic redistribution, which occurs when water moves from wet areas of the soil to dry areas through tree root systems. By understanding when hydraulic redistribution occurs, and how it affects soil moisture, we can better understand the factors that explain what controls how much carbon trees and ecosystems are able to sequester from the atmosphere,” Pockman said.

The grant supports simultaneous work at field sites and the development of models that incorporate the patterns revealed by the field research. The field activity will measure hydraulic redistribution by the root systems of trees, and the function of those individual trees and the entire ecosystem, as well as small-scale manipulations to estimate what happens to soil moisture without hydraulic redistribution.  The modeling activity, based at Cornell University, will use cutting edge data assimilation techniques to train models with observations from the field sites.

Each of the researchers brings different areas of essential expertise to the project.

Pockman is the Principal Investigator for the project. His expertise is in how individual plants take up and use water. Litvak is an expert in ecosystem function and has developed the sites equipped to measure the exchange of carbon dioxide, water, and energy between the atmosphere and the ecosystem that includes plants and soils. Luo’s expertise is in ecosystem modeling, and he has been a leader in the use of data assimilation methods to train ecosystem models to better capture the responses observed in the field. UNM graduate student Josh Lopez-Binder in the Litvak lab is also involved in the research.

Marcy Litvak
Marcy Litvak

While this specific research begins with this funding, the field research will build upon long-term funding from the DOE to Litvak’s research group.

“These sites provide an extraordinary setting for this research because the data collected tell us a great deal about how these systems respond to year-to-year variation in precipitation and weather. With the addition of the measurements planned in this new project, we will be able to advance our understanding of forest function and improve the models that we depend upon to understand forest function over areas so large that we can’t possibly make measurements everywhere,” Pockman said.

The team will be conducting the research at five of the nine instrumented tower sites that Litvak runs through the New Mexico Elevation Gradient. These sites have been heavily instrumented to measure ecosystem function, tree function, and environmental conditions since 2007. All these measurements are made continuously using small computers that record data 24/7.

“The new measurements will give us additional insight into comparative patterns of water use, carbon storage and energy exchange in biomes that range from desert ecosystems to high elevation forests, and how sensitive these biomes have been to the changes in climate and disturbance we have been experiencing for the past two decades,” Litvak said.  

Her role in the research is to provide the datasets and infrastructure that allow them to do two things:

First, it allows them to put this research in context of understanding the overall impact of hydraulic redistribution on the ability of ecosystems to function and sequester carbon. If trees are able to hydraulically redistribute water this might modify available soil water in ways that might allow them, and thus, ecosystems, to be more resilient to drought. This is crucial because many models are predicting that drought intensity and frequency are likely to increase in the Southwestern US.

Secondly, these datasets will allow Luo to parameterize his model, and to validate whether adding hydraulic redistribution to his modeling framework allows him to represent ecosystem function more accurately in these biomes.

“This is crucial to being able to predict what our landscapes will look like in the coming decades,” Litvak observed.

This research is important because it offers a way to improve the predictions of earth system models in aridlands.  Aridlands cover nearly 40 percent of Earth’s land surface and thus have a big influence on the performance of earth system models that seek to describe and predict the carbon balance of the entire earth.

Pockman noted that this research has a variety of benefits. It will contribute to improving understanding of the controls on the function of ecosystems that are important to the residents of New Mexico, such as juniper savanna, piñon-juniper woodlands, ponderosa pine forest, and high elevation mixed conifer forest.

“These are important ecosystems that provide essential benefits such as water that flows to our streams and rivers, as well as providing the ecosystems that many of us call home,” he said, adding, “Improved understanding of these forests will also have important implications for similar forests worldwide. The development of models that incorporate our field observations will be important for making predictions about the future of these ecosystems under future climate conditions.”

Also, the funding provided by this grant from the Department of Energy, like most such grants, is primarily spent on salaries of the people who conduct the research, so it is spent locally and benefits the economy of the community and the state, he said.

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