Dr. Rachel Keen has been awarded an EAR Postdoctoral Fellowship to conduct research and professional development activities under the mentorship of Dr. Pamela Sullivan at Oregon State University. Woody plant encroachment occurs when trees spread in grasslands and savannas. Left unchecked, it can cause these areas to transform into forests. Trees and grasses look and behave very differently. When woody plant encroachment occurs, many changes occur aboveground that are obvious and easy to see. For example, there are more trees but less grass and fewer wildflowers. However, there are many changes occurring underground that are more difficult to see. First, trees take up more water from the soil than grass does. As a result, woody plant encroachment can cause the soil to dry out over time in some places. It can even reduce the amount of water flowing in streams. Trees also have bigger, deeper roots than grass. These roots can change how water moves down into the soil after it rains. Larger roots create channels that allow water to move deep into the soil more quickly. The goal of this project is to better understand how woody plant encroachment changes the amount of water in the soil. To achieve this goal, researchers will study four grassland and savanna sites in South Africa. At these sites, researchers will compare the amount of soil water in places covered by grass and nearby places covered by trees. They will then measure how dry the soil gets over time in these places. Soil water impacts the amount of water that ends up in streams and groundwater, and many animals and people rely on these water sources to survive. This makes it important to understand how woody plant encroachment could change the amount of water in the soil in grasslands and savannas. Data from this study will be used to make hands-on science activities for students in high school and college. Researchers in this project will also provide a short course designed to help graduate students succeed.

Vegetation forms a critical link between the atmosphere and the subsurface in terrestrial ecosystems, providing primary controls on water fluxes within the critical zone. Woody plant encroachment (WPE), the spread of woody vegetation (trees and shrubs) in historically grass-dominated ecosystems, represents a major shift in land-cover that is impacting grassland and savanna ecosystems worldwide and may result in substantial alterations to ecosystem-scale water cycling. A primary gap in our understanding of the ecohydrological impacts of WPE is the belowground change in rooting dynamics, and subsequent impacts on soil structure (i.e., creation of macropores) and hydrological processes (i.e., preferential flow) in grasslands and savannas. This study will quantify the impacts of WPE on (1) rooting distributions and soil structure, (2) seasonal wetting/drying dynamics and deep percolation of soil water, and (3) groundwater recharge. This study will take advantage of a natural rainfall gradient in northeastern South Africa to compare soil structural and hydrological responses to WPE at two mesic (mean annual rainfall > 1000 mm) and two semi-arid (mean annual rainfall < 550 mm) savanna sites. Plots will be established in encroached areas with high tree cover and adjacent non-encroached areas where tree cover remains low. Soil moisture time series (0-1 m depth) paired with time-lapse electrical resistivity imaging will provide detailed information on changes in infiltration dynamics and seasonal deep soil moisture in response to WPE. HYDRUS-1D will be parameterized with these data and used to predict changes in deep percolation of soil water and groundwater recharge in different locations and climate conditions. This project will advance the field of ecohydrology by improving our ability to understand and predict hydrological changes in response to WPE and uncovering the mechanisms behind accompanying changes in soil moisture and water yield. Beyond these advances, the data collected in this study will be used to create Data Nuggets, which are data-driven science lessons designed to give highschoolers and undergraduates hands-on experience working with data and asking scientific questions. A journal club-style course will also be developed for graduate students and postdocs aimed at ‘demystifying’ the hidden curriculum in academia, with the goal of making graduate school more accessible, enjoyable, and successful for a wider range of students.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.