Introduction

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Using four instrumented rangeland watersheds on the Santa Rita Experimental Range near Green Valley, Arizona, our work is addressing critical uncertainties related to woody plant encroachment (WPE) and brush management (BM) of velvet mesquite (Prosopis velutina). These include the role of WPE on the global carbon cycle, the effects of BM on carbon storage potential, biodiversity, and rangeland ecosystem stability and resilience. With this information, we can provide land managers with an objective basis for differentiating among the consequences and cost-benefits of both WPE and BM, as well as developing appropriate timing and location of BM interventions for maximum benefit.

Key Issues Addressed

Woody plant encroachment (WPE) into North American grasslands has been ongoing since the mid 1900s due to a combination of domestic livestock grazing and fire suppression. Land managers have traditionally used various brush management (BM) techniques to reverse shrub encroachment, restore lost forage production, and reduce erosion. The limited success of BM in meeting these goals has raised questions about its economic feasibility as a management tool. However, numerous other ecosystem services--including forage production, carbon storage, and soil retention--are potentially impacted by WPE and BM, and accounting for them would provide a more complete evaluation of BM as a conservation practice.

Project Goals

• Compare ecosystem services between treated and control sites on four mesquite-encroached watersheds by monitoring a suite of response variables
• Collect and integrate field-based ecosystem service data with gas exchange data and drone-based aerial photography

Project Highlights

Treatment Impacts: This work will advance our understanding of the impacts of brush management on forage production, carbon storage, and nutrient cycling at watershed scales.

• Quantifying Ecosystem Services: Several variables are being quantified on control and treated watersheds, both one year prior to, and up to four years after herbicide treatment.
  • Herbaceous Diversity and Net Aboveground Primary Production: Used to understand how BM affects forage production and quality.
  • Coarse Woody Debris and Litter: Used to understand how BM affects aboveground carbon storage dynamics.
  • Soil Organic Carbon and Nitrogen: Used to understand carbon storage in soil and macronutrients available for plant growth as a result of BM practices.
  • Land Surface-Atmosphere Carbon and Water Exchange: Used to better understand ecosystem carbon and water storage dynamics.
  • Soil Erosion and Retention: Used to better understand if BM helps minimize soil loss, which would increase carbon storage and forage production.
• Herbicide Treatment: We worked with Crop Production Services, LLC and Tri Rotor Ag, LLC to apply an herbicide mixture to mesquite via helicopter in June 2016. The mixture consisted of clopyralid + aminopyralid + triclopyr (Brand names Transline + Milestone + Garlon4Ultra) at rates of 21.3oz/ac + 7 oz/ac + 16 oz/ac, respectively. Following treatment, 214 trees were monitored for 22 months to measure mortality by recording the development of new foliage and basal shoots.
• Interdisciplinary Collaboration: This project integrates field, geospatial, and modeling techniques to assess the biological and physical effects of brush management. Because the project assesses several ecosystem services at once, it requires collaboration of people with different sets of expertise to properly interpret and communicate results. To date, the project has afforded collaborative opportunities for ten professors and research scientists, five graduate students, and employment and professional development opportunities for 30 undergraduate students.
• Long-Term Data: The USDA-ARS instrumented eight unit-source watersheds in 1975 throughout SRER to record rainfall, runoff, and sediment, as well as to investigate the effects of manipulative treatments on hydrological processes. A subset of these watersheds is being used for this study, allowing incorporation of four decades of historical data.
• Monitoring Ecosystem-Scale Gas Exchange: The treatment area is centered around a tower installed in 2011. Referred to as an eddy covariance tower, it is equipped with instruments that measure physical and chemical properties of the atmosphere, as well as gas exchanged between the atmosphere and land surface. Changes in gas exchange measured by this tower post-treatment are being compared with a nearby "control" tower about 1.5 kilometers to the west. 

Lessons Learned

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• Herbaceous primary production increased during the growing season following herbicide application, with the contribution of non-native perennials (primarily Lehmann lovegrass) far exceeding that of native perennials.
• Early results suggest mesquite herbicide treatment in this desert grassland may shift the community composition in favor of non-native perennials and native annuals over native perennials. This may be triggered by periods of precipitation shortfall, with unintended adverse consequences for herbaceous biodiversity and primary production.
• Mesquite mortality following herbicide treatment was very low (~5%). If herbicide treatment alone is a viable option for velvet mesquite management, there is a clear need to more rigorously determine the herbicide mixture composition, environmental conditions, and timing that maximizes mesquite mortality and cost-effectiveness.
• While the velvet mesquite in the treated area largely recovered from herbicide application, very few individuals have developed flowers or seed pods in the two growing seasons since. As the pods are a vital food source for small mammals and arthropods, herbicide application might also reduce availability of food for a variety of native animals.

Next Steps

• Utilize erosion and biogeochemical models to predict long-term ecosystem responses to woody plant encroachment and removal
• Characterize the arthropod and small mammal communities and their dietary preferences on treated and control watersheds
• Share results with local land managers through in-person workshops and with the scientific community through conferences and technical publications

Funding Partners

• US Department of Agriculture - National Institute of Food and Agriculture
• US Department of Agriculture - Agricultural Research Service

Resources

• Brush Management & Ecosystem Services Project Website
• Healthy Rangelands: University of Arizona Brush Control Research Video
• SRER Brush Management UAV Flight Video
• A Framework for Assessment of Ecosystem Services
• Archer, S.R. et al. (2023) Prosopis velutina Response to Aerial Herbicide Application. Rangeland Ecology & Management 88:129-134
• Southwest Watershed Research Center, US Department of Agriculture – Agricultural Research Service

Contacts

• Adam T. Naito, Postdoctoral Research Associate, University of Arizona, School of Natural Resources and the Environment: anaito@email.arizona.edu
• Steven R. Archer, Professor, University of Arizona, School of Natural Resources and the Environment: sarcher@email.arizona.edu

Case Study Lead Author

• Ashlee Simpson, CART Graduate Research Assistant, University of Arizona

Suggested Citation

Simpson, A. C. (2018). “Assessing the Impacts of Brush Management on Herbaceous Plant Diversity and Primary Production in Southern Arizona Grasslands.” CART. Retrieved from https://www.fws.gov/project/brush-management-plant-diversity-and-primary-production.