Dr. David Thoma

Dr. David Thoma, Ecologist, Inventory and Monitoring Program, National Park Service

Research Interests: Investigating how natural resources in national parks are impacted by and responding to weather and climate change.

Research Highlights: Land managers working in America’s National Forests and Parks need location-specific information about the vulnerability of plants and animals to climate change. Typically, assessments of climate rely on measurements of air temperature and precipitation. Yet, when it comes to discerning how climate and climate change might affect the habitats of plants and animals, temperature and precipitation alone aren’t enough. For example, plants take their water from the soil, but how long it remains there depends on factors such as soil type, slope, and slope aspect (or the direction a slope faces).

To get a more comprehensive view of how climate affects terrestrial ecosystems, scientists have begun to investigate the ways in which water availability—not just total precipitation—affects plants and animal communities. This approach uses a concept called water balance.

Water balance refers to the movement of water in and out of an ecosystem and it accounts for the different forms that water may take (i.e., liquid, solid, and gas). Assessments of water balance are calculated with models that incorporate data on temperature and precipitation to estimate soil moisture, evapotranspiration, runoff, water deficit (i.e., when plants’ need for water surpasses what’s available), and more. Compared to using only measurements of temperature and precipitation, the consideration of these variables in concert enables scientists to better predict how plant and animal species will respond to climatic conditions because it offers a more accurate portrayal of the conditions plants and animals are actually experiencing.

One scientist incorporating water balance into his work is Dr. David Thoma, an ecologist with the National Park Service’s (NPS) Inventory and Monitoring Program. Based in Bozeman, Montana, the Inventory and Monitoring Program conducts long-term ecological monitoring in National Parks and collaborates with a variety of partners and stakeholders to provide valuable scientific data to park managers and members of the public alike.

Thoma works in several parks in the Greater Yellowstone area, including Yellowstone, Grand Teton, Big Horn Canyon, and 16 other parks on the Colorado Plateau. In these locations, he uses water balance to study the sensitivity of the natural resources in these areas to changes in weather and climate.

“If we understand how sensitive a species or resource like stream flow is to weather and climate, then we can estimate its vulnerability to climate change in the future,” he said. “This helps us determine if we can effectively resist or direct change or if we should let it play out.”

In addition to his work with the NPS, Thoma also serves as a User Working Group member of NASA's Land Processes Distributed Active Archive Center (LP DAAC). Located at the USGS Earth Resources Observation and Science (EROS) Center in Sioux Falls, South Dakota, LP DAAC ingests, processes, archives, and distributes data products related to land processes in NASA’s Earth Observing System Data and Information System (EOSDIS) collection. These data are crucial to the investigation, characterization, and monitoring of biological, geological, hydrological, ecological, and related conditions.

Thoma’s current work involves the development of more efficient ways to make ecological now casts (i.e., assessments of current conditions), short-term forecasts, and long-range projections of ecological conditions using a water balance model.

“We have found that the abundance, timing, and phase of water (solid, liquid, gas) explains most of the responses we observe in our monitoring programs,” he said. “This is because ‘water is life’ and ecological processes are sensitive to water availability. We use a water balance model that translates temperature and precipitation into more proximal measures of climate that more directly influence natural resources in parks.”

NASA Earth science data are an “essential” part of delivering credible science in support of park management, Thoma said.

“I use Moderate Resolution Imaging Spectroradiometer (MODIS) Normalized Difference Vegetation Index (NDVI), and Soil Adjusted Vegetation Index (SAVI) datasets to identify where and how fast vegetation change is occurring, then link signals of change with water balance variables like soil moisture and drought stress calculated from climate data obtained from the Oak Ridge National Laboratory DAAC’s (ORNL DAAC) Daymet ” he said. “We monitor vegetation condition as a proxy for fundamental ecosystem processes associated with energy, nutrient, and water cycles that support life in parks.”

Thoma also uses climate data from Daymet to understand how change in vegetation conditions may be affected by both weather and site conditions, such as soil properties and slope aspect. These complex interactions would be difficult to monitor over large park landscapes via field sampling alone, but because NASA data is available over large areas every day, they greatly facilitate his work.

“Because almost everything we care about in national parks is climate sensitive, from water flow in a desert spring to wildfires in forest parks, it is imperative that we have access to accurate, complete, and high-frequency observations of climate and vegetation condition to help us unravel interactions among climate, species traits, and site characteristics,” Thoma said. “If we find relationships that are sufficiently strong and credible, we can use that information in now-casts and long-range planning to help park managers make near-term tactical decisions that can guide ecosystem transformation caused by climate change.”

For example, as part of a 2020 case study conducted in the Great Sand Dunes National Park and Preserve, Thoma and his colleagues used NDVI satellite imagery to measure vegetation “greenness,” an indicator of plant health, and then identified the climate and water balance variables that most strongly affected plant growth in different park vegetation types. They found that vegetation at different elevations responded to different aspects of climate and water balance. Generally, plant growth at high elevations was more responsive to temperature than soil moisture whereas low elevation plant growth responded more to soil moisture than temperature.

According to an article about the case study on the NPS website, these conclusions are significant, as park managers can use them to identify which climate variables are most important for different vegetation types, as well as the amount of water needed to keep plants healthy and the water deficit that different plant types can tolerate.

“With this new understanding of the climate–plant growth relationship across park vegetation types, managers can track water balance conditions as an indicator of vegetation condition. This can help identify vulnerable vegetation types before it’s too late to act and it can help inform restoration strategies.”

In another 2020 study published in the journal Forests, lead author David Laufenberg of the Department of Ecology at Montana State University, Thoma, and others showed that water balance concepts can be used to improve the outcomes of whitebark pine restoration efforts in the Greater Yellowstone Ecosystem (GYE).

Whitebark pine (WBP) is an important and long-lived subalpine species in the northern Rocky Mountain region and federal agencies have been planting it for at least three decades in the GYE with varying success.

To bolster the outcomes of managers’ restoration efforts, Thoma and his colleagues used a combination of field sampling and a water balance model informed by daily temperature and precipitation data from the Daymet dataset to investigate biophysical gradients for WBP seedling performance at twenty-nine GYE planting sites.

They found that the establishment of WBP seedlings at the sites is linked to actual evapotranspiration (AET), or the actual rate at which water vapor is returned to the atmosphere from the ground and by plants.

“We found that the WBP growth rate was positively correlated with AET and was greatest when cumulative growing season AET was above 350 millimeters,” the researchers write. “Growth rate was not strongly affected by competition at the levels found in this study. However, site density change over time was negatively affected by mean growing season temperature and when more than five competitors were present within 3.59-meter radius.”

Identifying planting sites with these conditions is of critical importance to WBP seedling survival, as these long-lived trees are late to mature.

“If they make it to maturity, trees that are planted this season will not begin to produce cones until the latter half of this century,” the authors write. “[To increase the likelihood of this outcome] We recommend planting efforts that optimize AET for growth rate objectives, minimize water deficit (WD) that cause stress and mortality, and removing competitors if they exceed five within a short distance of seedlings.”

More recently, Thoma, lead author Michael T. Tercek, and others collaborated on a 2021 study published in PLos ONE that used a gridded water balance model incorporating Daymet data, to assess the changes in water use and need across the Continental United States from 1980 to 2019.

Although the model revealed divergent trends in water availability in the eastern (wetter) and western (drier) United States, a more detailed examination of one location in the West—Sequoia National Park in California—revealed a high degree of spatial variability across elevation and topographical gradients.

This insight is noteworthy, said Thoma, because it demonstrates that, at finer scales, environmental heterogeneity is driving a range of plant community responses that may not be adequately characterized by a single trend.

“From a plant perspective, parks in the western U.S. are drying while parks in the eastern U.S. are wetting,” he said. “However, within Sequoia National Park, which has high topographic complexity, wetting and drying are both occurring simultaneously depending on elevation. This helps us understand why and where changes in vegetation condition are occurring.”

In addition, the study’s results show that a water balance model like the one Thoma and his colleagues used in this investigation is capable of identifying important trends and patterns at both particular sites and regional scales. Such information can be used to drive natural resource management decision-making in response to climate change impacts.

“Many U.S. parks are experiencing temperatures outside their historic range of variation and we found these temperature increases are often driving directional changes in plant-available water and water use,” the study authors write. “Water budgets, including changes in AET and Climatic Water Deficit (i.e., when plants’ need for water surpasses what’s available), are more ecologically relevant indicators of the consequences of climate variation than changes in temperature and precipitation, and they provide a more accurate estimate of climate as a driver of natural resource response.”

Naturally the impacts of climate change are multi-faceted and many questions about how it will affect natural systems remain. Nevertheless, as Thoma’s research indicates, NASA Earth science data play a crucial part in his efforts to provide natural resources managers with the scientific insights they need to address the impacts of climate change in some of America’s most renowned parks.

“The more carefully we look, the more we realize how climate-sensitive park resources are,” Thoma said. “If parks are climate sensitive and the climate is changing faster than species can adapt, we need to know what, where, when, and why change is likely so we can intentionally manage parks through the inevitable transitions.”

NASA's EOSDIS supports the ongoing efforts to answer these questions, and by archiving and distributing the data that researchers like Thoma use in their work, it endeavors to help those in the ecological research community answer them.

Representative Data Products and Tools Used or Created:

Available through LP DAAC:

• MODIS NDVI doi:10.5067/MODIS/MOD13A1.006
• MODIS SAVI doi:10.5067/MODIS/MOD13Q1.006
• Application for Extracting and Exploring Analysis Ready Samples (AppEEARS) data analysis tool https://lpdaac.usgs.gov/tools/appeears/

Other data products used:

• Daymet Daily Surface Weather Data on a 1-km Grid for North America; Version 3 (ORNL DAAC) https://daymet.ornl.gov/
• Multivariate Adaptive Constructed Analogs TerraClimate https://www.climatologylab.org/terraclimate.html
• R data analysis tool https://www2.r-project.org/

Read about the Research:

Tercek, M.T, Thoma, D.P., Gross, J.E., Sherrill, K., Kagone, S., & Senay, G.B. (2021). Historical Changes in Plant Water Use and Need in the Continental United States. PLoS ONE, 16(9): 1–19. doi:10.1371/journal.pone.0256586

Laufenberg, D., Thoma, D.P., Hansen, A., & Hu, J. (2020). Biophysical Gradients and Performance of Whitebark Pine Plantings in the Greater Yellowstone Ecosystem. Forests, 11(1). doi:10.3390/f11010119

Thoma, D.P., Tercek, M.T., Schweiger, E.W., Munson, S.M., Gross, J.E., & Olliff, S.T. (2020). Water balance as an indicator of natural resource condition: Case studies from Great Sand Dunes National Park and Preserve. Global Ecology and Conservation, 24: e01300. doi:10.1016/j.gecco.2020.e01300

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