Source: TEXAS A&M UNIVERSITY submitted to NRP
THICKETIZATION OF OAK SAVANNAS: CAN RESTORATION LEAD TO GREATER REGIONAL GROUNDWATER RECHARGE?
Sponsoring Institution
National Institute of Food and Agriculture
Project Status
ACTIVE
Funding Source
AFRI COMPETITIVE GRANT
Reporting Frequency
Annual
Accession No.
1027794
Grant No.
2022-67019-36267
Cumulative Award Amt.
$750,000.00
Proposal No.
2021-09129
Multistate No.
(N/A)
Project Start Date
Jan 1, 2022
Project End Date
Dec 31, 2026
Grant Year
2022
Program Code
[A1411]- Foundational Program: Agricultural Water Science
Recipient Organization
TEXAS A&M UNIVERSITY
750 AGRONOMY RD STE 2701
COLLEGE STATION,TX 77843-0001
Performing Department
Ecology And Conservation Biolo
Non Technical Summary
Oak savannas are an important vegetation type in the Southern USA, making up more than 120,000 km2.Over the past 150 years, these landscapes have been radically transformed by cultivation and subsequent abandonment, altered fire regimes, urbanization, and fragmentation. This altered fire regime has allowed undesirable plants to invade the understory, creating dense thickets of vegetation--a process described by some as thicketization.The overarching goal of our proposed project is to develop a better understanding of the ecohydrological implications of thicketization in oak savannas. We are particularly interested in determining if groundwater recharge could be enhanced via the creation of a more open structure. We will focus our field studies on the Post Oak Savanna ecoregion in eastern Texas. We believe that this ecoregion holds particular promise for increasing groundwater recharge at a regional scale because about one-third of its area is underlain by the deep sandy soils of the Carrizo-Wilcox recharge zone. We will employ a variety of field measurements at two locations in the Post Oak Savanna, all of which are designed to better quantify water fluxes across these landscapes (especially recharge). Using both remote sensing and hydrological modeling, we will be able to scale up our results to the entire region and apply them to other oak savannas. A secondary but very important goal of this project is to advance ecohydrology research opportunities for early- career scientists and undergraduate students.The project will support one full time post-doctoral scientist and heavily involve three Ph.D. students.
Animal Health Component
75%
Research Effort Categories
Basic
25%
Applied
75%
Developmental
(N/A)
Classification

Knowledge Area (KA)Subject of Investigation (SOI)Field of Science (FOS)Percent
11203201070100%
Knowledge Area
112 - Watershed Protection and Management;

Subject Of Investigation
0320 - Watersheds;

Field Of Science
1070 - Ecology;
Goals / Objectives
The overarching goal of our proposed project is to advance ourunderstanding of how thicketization affects ecohydrological functioning in the POS ecoregion of Texas.The specific goals and objectives of the proposed project are as follows:Goal 1: Characterize the components of the water budget for both open areas (savannas and pastures) and closed-canopy woodlands (Section 2.3).We will meet this goal by carrying out tasks designed to achieve the following specific objectives: Estimating soil water fluxes in open and in closed-canopy sites (Objective 1.1, Section 2.3.1); Understanding tree transpiration dynamics in thicketized woodlands (Objective 1.2, Section 2.3.2); Determining interception losses in thicketized woodlands (Objective 1.3, Section 2.3.3); and Estimating long-term recharge rates in open and in closed-canopy areas (Objective 1.4, Section 2.3.4).Goal 2: Identify where trees are accessing water (Section 2.4).We will use natural isotopes to determine the depth and seasonality of water use by trees (Objective 2.1, Section 2.4.1).Goal 3: Determinethe spatial and temporal changes in vegetation andevapotranspirationfrom 1986 to the present (Section 2.5).To meet this goal, we will use remote sensing to determine theextent to which thicketized woodlands have increased in the POS since 1986, with particular emphasis on whether the population of evergreen trees is increasing (Objective 3.1, Section 2.5.1); and we will quantify the influence of climate and vegetation on ET by estimating monthly and annual ET at field and regional scales (Objective 3.2, Section 2.5.2).Goal 4: Parameterize and validate hydrological models for simulating groundwater recharge as influenced by climate, soils, and vegetation cover (Section 2.6).Using HYDRUS-1D modeling, we will elucidate the temporal and spatial variability of recharge within the Carrizo-Wilcox recharge zone (Objective 4.1, Section 2.6).Collectively, the data from our study sites will allow us to verify the overarching hypothesisand determine (a) the extent to which thicketization is altering the ecohydrology of the POS ecoregion, and (b) the extent to which woody plant management may lead to increased recharge of the Carrizo-Wilcox Aquifer.
Project Methods
The overall goal of our work in the Carrizo-Wilcox recharge zone is to advance ourunderstanding of how thicketization affects ecohydrological functioning in the POS ecoregion of Texas, and in particular to determine how and to what extent woody vegetation affects groundwater recharge.We will address this goal through a comprehensive assessment of water fluxes in this landscape and how they are influenced by vegetation and soils--focusing in particular on understanding soil water dynamics and recharge[31]. Our field measurements, summarized in Table 1, will be concentrated at two primary locations (Figure 6): one within the Carrizo-Wilcox recharge zone (CWRS), characterized by deep sandy soils; and the other outside the recharge zone (CPS), where the soils are more clayey.n addition, we will collect soil cores for chloride analysis at four auxiliary locations along a climatic gradient within the Carrizo-Wilcox recharge zone (Figure 1). Data from these measurements will be used to calibrate and validate hydrological models, which can then be used to generalize our results. Finally, we will useremote sensing analyses to better understand changes in vegetation cover across the region and to identify large-scale patterns in evapotranspiration as influenced by vegetation.At each site we will set up meteorological stations in two open-area locations. All four stations will be instrumented for recording basic meteorological parameters (temperature, relative humidity, wind speed, wind direction, and rainfall), and one of the stations at each site will also be instrumented for measuring surface energy balance parameters--including net radiation, ground heat flux, and surface temperature. The net radiation and ground heat flux data will be used to estimate available energy at the surface[34], and the surface temperature data will be used to calculate sensible heat flux[35].

Progress 01/01/24 to 12/31/24

Outputs
Target Audience:Our target audience consists of (1) local land owners (2) managers in state and national agencies (3) and fellow academics Changes/Problems:Our project has been delayed because of disruptions from COVID and the loss of one of our research sites early in the project. This has meant that field projects were delayed in getting implemented. We have found a substitute research site that meets our needs. We have found that extracting deep cores for chloride analysis presents some challenges because the sandy substrate is very difficult to extract in intact cores. For this reason our coring has been restricted to just a few sites. We are hopeful that we can revise the sample extraction technique such that we can address these challenges. What opportunities for training and professional development has the project provided?Our project has supported research for 3 Ph.D. students, one MS student and one Post Doc. In addition, we have provided meaningful research experiences for 6 undergraduatestudents. How have the results been disseminated to communities of interest?Yes--we have published many papers and presentations outlining our research What do you plan to do during the next reporting period to accomplish the goals?For the remainder of the project we will be focusing on (1) isotope measurements to determine fate of water in these landscapes (2) tree transpiration measurements using sap flow techniques (3) parameterization and validation of the HYDRUS hydrological model and (4) evaluation of regional evapotranspiration using ET remote sensing techniques

Impacts
What was accomplished under these goals? We have made very good progress at Characterizing the components of the water budget for both open areas (savannas and pastures) and closed-canopy woodlands. See publications.In addition, we have provided a detailed characterization of vegetation change across the Post Oak Savanna. Current work is focusing on parameterizing and applying hydrological models using field data that we have collected.

Publications

  • Type: Other Journal Articles Status: Published Year Published: 2025 Citation: Atalar, F., P. A. M. Leite, and B. P. Wilcox. 2025. A Comparison of Three Methodologies for Determining Soil Infiltration Capacity in Thicketized Oak Woodlands and Adjacent Grasslands. Water 17:518
  • Type: Other Journal Articles Status: Published Year Published: 2025 Citation: Wang, H.-H., W. E. Grant, A. G. Birt, and B. P. Wilcox. 2025. Modeling rangelands as complex adaptive socio-ecological systems: An agent-based model of pyric herbivory. Ecological Modelling 501:111020.
  • Type: Other Journal Articles Status: Published Year Published: 2024 Citation: Leite*, P. A. M., S. Di Prima, L. M. Schmidt, and B. P. Wilcox. 2024. A simple infiltrometer automated with a user-friendly pressure datalogger. Vadose Zone Journal n/a:e20366.
  • Type: Other Journal Articles Status: Published Year Published: 2025 Citation: Olariu*, H. G., B. P. Wilcox, and S. C. Popescu. 2024. Examining changes in woody vegetation cover in a human-modified temperate savanna in Central Texas between 1996 and 2022 using remote sensing. Frontiers in Forests and Global Change 7.
  • Type: Other Journal Articles Status: Published Year Published: 2024 Citation: Schmidt, H. E., J. M. Osorio Leyton, S. C. Popescu, E. Noa Yarasca, S. Sarkar, and B. P. Wilcox. 2024. Connecting the Dots: How Ecohydrological Connectivity Can Support Remote Sensing and Modeling to Inform Management of Woody Plant Encroachment. Rangeland Ecology & Management 95:84-99.
  • Type: Other Journal Articles Status: Published Year Published: 2024 Citation: Wilcox, B. P., M. C. Caldeira, P. A. M. Leite*, R. Lobo-do-Vale, and M. N. Bugalho. 2024. Understory shrubs improve soil infiltrability in overgrazed Mediterranean oak woodlands, but have little impact on ungrazed woodlands. Forest Ecology and Management 569:122186.
  • Type: Conference Papers and Presentations Status: Published Year Published: 2024 Citation: Anderson, J., and B. P. Wilcox. 2024. Ecohydrology of Yaupon Holly (Ilex vomitoria).in American Geophysical Union Annual Meeting, Washington DC.
  • Type: Conference Papers and Presentations Status: Published Year Published: 2024 Citation: Atalar, F., F. G�kbulak, and B. P. Wilcox. 2024a. The effect of forest cover changes on throughfall and stemflow in black pine plantation. Water Science Conference. American Geophysical Union, Saint Paul, MN
  • Type: Conference Papers and Presentations Status: Published Year Published: 2024 Citation: Dominichi, B., S. Basant, J. Anderson, and B. P. Wilcox. 2024a. Sap flux changes in woody encroached Post Oak Savannahs. Water Science Conference. American Geophysical Union, Saint Paul, MN
  • Type: Conference Papers and Presentations Status: Published Year Published: 2024 Citation: Anderson, J., S. Basant, B. P. Wilcox, and P. A. M. Leite. 2024. Canopy interception under encroached woodands of Texas Post Oak Savannas. Water Science Conference. American Geophysical Union, Saint Paul, MN
  • Type: Conference Papers and Presentations Status: Published Year Published: 2024 Citation: Atalar, F., P. A. M. Leite, and B. P. Wilcox. 2024b. Comparative multimethod analysis of woody plant encroachment effects on infiltratin rates in different soil types.in American Geophysical Union Annual Meeting, Washington DC
  • Type: Conference Papers and Presentations Status: Published Year Published: 2024 Citation: Basant, S., F. Atalar, M. Wang, H. Olario, M. E. Everett, and B. P. Wilcox. 2024. Deep drainage responses to vegetation change due to Post Oak Savanna restoration. American Geophysical Union Annual Meeting, Washington DC
  • Type: Conference Papers and Presentations Status: Published Year Published: 2024 Citation: Dominichi, B., P. A. M. Leite, and B. P. Wilcox. 2024b. Effects of brush control on soil health indicators in a Post Oak savanna undergoing woody plant encroachment.in American Geophysical Union Annual Meeting, Washington DC
  • Type: Conference Papers and Presentations Status: Published Year Published: 2024 Citation: Wang, M., B. M. Wyatt, S. Basant, and B. P. Wilcox. 2024. Deep drainage responses to vegetation change due to Post Oak Savanna restoration. American Geophysical Union Annual Meeting. American Geophysical Union, Washington DC.
  • Type: Conference Papers and Presentations Status: Published Year Published: 2023 Citation: Anderson, J.*, B.P. Wilcox, and S. Basant. 2023. Canopy interception under woody encroached woodlands of Post Oak Savannas. Annual meeting of the American Geophysical Union, San Francisco.
  • Type: Conference Papers and Presentations Status: Published Year Published: 2023 Citation: Wang, M., B.M. Wyatt, S. Basant*, B.P. Wilcox. 2023. Revealing the influence of land cover dynamics on temporal and spatial variability of groundwater recharge within the Carrizo-Wilcox recharge zone. Annual meeting of the Soil Science Society of America, St Louis
  • Type: Conference Papers and Presentations Status: Published Year Published: 2023 Citation: Wang, M., B.M. Wyatt, S. Basant*, B.P. Wilcox. 2023. Revealing the influence of land cover on temporal can spatial variability of groundwater recharge. Annual meeting of the American Geophysical Union, San Francisco
  • Type: Conference Papers and Presentations Status: Published Year Published: 2022 Citation: Olariu, H.*, P.C. Popescu, M. Lonesome, B.P. Wilcox. 2022. Evaluating the potential for using ICESat-2 to map forest cover and metrics in arid and semiarid landscapes. Annual meeting of the American Geophysical Union, Chicago.


Progress 01/01/23 to 12/31/23

Outputs
Target Audience:Our target audience is other academics, resource managers, water managers, and private land owners. Changes/Problems:We had hoped to collect soil cores for depths up to 10 meters. We are finding that it is difficult to get samples from this depth using the hand augers. We are evaluating the cost effectivenessof using professional drillers to collect deeper samples . What opportunities for training and professional development has the project provided?One graduate student was sent to the Isotope Camp at the University of New Mexico this summer. In addition several undergraduates have received training on how to collect soil water using the neutron probe. Graduate students continue to develop skill sets as they are tasked with major responsibilities for the data collection and analysis. How have the results been disseminated to communities of interest? Nothing Reported What do you plan to do during the next reporting period to accomplish the goals?Weplan to continue collecting the soil water data, precipitation data and interception data. We are also collecting water and plant samples for isotope analysis. We are currently using remote sensing to determine changes in woody cover in the Post Oak Savanna. We will also use remote sensing to estimate evapotranspiration in the Post Oak Savanna. We are in the early stages of establishing sap flow gages on several species of trees that are in the study area.

Impacts
What was accomplished under these goals? We have made good progress in establishing our field measurements and experimental setup at each of our study sites. We have selected two study sites. One being the Gus Engeling Wildlife Management Area and the other is the Range Research Area on the Texas A&M Campus. At each of these sites we have established instrumentation for monitoring soil water, precipitation, and interception. At the Gus Engeling site we have collected soil cores from 10 locations. The cores have been tested for soil chloride and we are now analyzingthese results. In addition we have conducted geophysical surveys at the Gus Engeling site to determine differences in soil water with vegetation cover.

Publications


    Progress 01/01/22 to 12/31/22

    Outputs
    Target Audience:The target audiences are other academics and research scientists who have been reached by publication in the peer reviewed literature. Other target audiences include management agencies including Texas Parks and Wildlife, the Texas Water Development Board, and private land managers. We reach these audiences thru regular press releases. Changes/Problems:Our initial plan called for soil coring to a depth of 10 M using a drilling rig. Unfortunately, the drilling rig that we have available is not suitable for such sandy soils and parent material. We are hand augoring and getting core samples to a depth of 4 M. Pending results from out 4 M cores, we will use professional drillers to get a limited number of 10 M cores. We anticipate that the 4 M cores will provide insightful information with respect to deep drainage. What opportunities for training and professional development has the project provided?We are sending one of our students to the IsoCamp summer course for training on the use of isotopes for accomplishing goal 2. In addition, there are a number of undergraduate students who are working part time on this project and are becoming trained in field and laboratory measurements. How have the results been disseminated to communities of interest?In addition to the peer reviewed publications there have been a number of press releases describing the project. These press releases have been picked up by several regional news outlets. What do you plan to do during the next reporting period to accomplish the goals?We will continue to advance each of the major 4 goals. Goal 1: We are engaging in continuous monitoringof soil water, soil tension, plant interception. We will conduce more chloride coring sampling at the Gus Engleling Study area as well as additional locations across the Post Oak Savanna. We will install the last continuous soil monitoring?station. Goal 2: Sampling for the isotope analysis will start including collection and analysis of plant, soil, and water samples Goal 3: We will complete the land cover analysis and begin the ET remote sensing analysis Goal 4:Following the completion of soil core collection and site installation, HYDRUS-1D simulations will be used to estimate deep drainage across the multiple vegetation conditions considered in the study.

    Impacts
    What was accomplished under these goals? We have made good progress in achieving?Goal 1 stated above. We have established a number of measurement stations at two locations in the Post Oak Savanna including neutron probe access tubes, continuous monitoring stations for soil water and soil water potential, plant interception plots. We will initiate steps to accomplish Goal 2 in the upcoming year. A graduate student has been identified for leading this aspect of the project and training on the use of isotope methodology has proceeded. In addition, we are making good progress in Goal 3. Remote sensing studies are ongoing to determine the extent of tree cover change across the Post Oak Savanna. We plan to advance on estimating ET using remote sensing next year. The graduate student (Wang) assigned to complete objective 4 has familiarized herself with the HYDRUS-1D model, including ongoing coursework providing formal training in HYDRUS-1D model use and applications. Intact soil cores necessary to properly parameterize the model will be collected in Spring 2023.Equipment for measuring soil moisture and meteorological data needed to run HYDRUS-1D simulations have been installed in a grassland and an encroached post oak savannah location since summer 2022, with two additional sites in encroached post oak savannah to be installed in Spring 2023.

    Publications

    • Type: Journal Articles Status: Published Year Published: 2023 Citation: Basant, S., B.P. Wilcox, C. Parada, B.M. Wyatt, and B.D. Newman. 2023. Thicketized oak woodlands reduce groundwater recharge. Science of the Total Environment 862, 160811.
    • Type: Journal Articles Status: Published Year Published: 2022 Citation: Wilcox, B.P., S. Basant, H. Olariu, and P.A.M. Leite. 2022. Ecohydrological connectivity: a unifying framework for understanding how woody plant encroachment alters the water cycle in drylands. Frontiers in Environmental Science 10
    • Type: Journal Articles Status: Published Year Published: 2022 Citation: Olariu, H.G., L. Malambo, S.C. Popescu, C. Virgil, and B.P. Wilcox. 2022. Woody plant encroachment: evaluating methodologies for seminar woody species classification from drone images. Remote Sensing 14 (7), 1665.