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Introduction

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In 1944, the National Park Service established Big Bend National Park (BIBE) in the Chihuahuan Desert grasslands of western Texas. The vegetation in lower-elevation (~2,700 ft) tobosa (Pleuraphis mutica) dominated grasslands at BIBE is characterized by a pattern of banded vegetation strips alternating with bare ground. This pattern of vegetation is important for regulating water infiltration and runoff (; see  for more detailed site characteristics). Several thousands of acres in Big Bend National Park and the Rio Grande Basin are covered by similar grasslands with banded vegetation and fine textured, highly erosive soils. 

Before the park was established, it was used for grazing livestock. A combination of drought and high stocking rates resulted in the loss of vegetative cover. This loss of vegetation left the soil exposed to solar radiation and the erosive effect of rain. In the lower elevation grasslands that have highly erosive fine-textured soils, physical soil crusts formed, reducing infiltration and further increasing erosion. In the 80 years since the creation of BIBE and cessation of grazing, recruitment of native grasses and forbs has remained low and rates of erosion high, preventing the recovery of natural banded vegetation patterns.

Managers at BIBE used heavy equipment in the 1950’s and 1960’s to attempt to restore these low-elevation tobosa grasslands. However, these efforts occurred during drought and were unsuccessful in intervening in cycles of degradation and returning patterns of banded vegetation.

Beginning in 2004, NPS staff began experimenting with mulch, branch mulch, rock mulch, and a variety of planting techniques in highly degraded and eroded soils in the north area of the park. Park staff ended up combining hydroseeding of grass with a commercial bonded fiber matrix (BFM), and overlaying the seeds with branch mulch cover. The BFM keeps seeds in place until the rainy season and prevents widespread removal by harvester ants. In addition, the branch mulch acts as a surrogate for vegetative cover, taking on the vital hydrologic role of interception, allowing for throughfall of precipitation along with shade and significantly lower temperatures. Finally, the treatments were arranged to mimic natural cover in arid landscapes. On undissected surfaces, the treatments were arranged in bands to mimic banded vegetation. On surfaces with rills, the treatments were arranged as chevrons within the rills.

Key Issues Addressed

The orientation of , perpendicular to the flow of water, naturally directs rainfall and runoff in a way that aids the infiltration of limited water resources to allow the landscape to support more vegetation. Loss of banded vegetation has increased erosion from rainfall leading to the formation of detrimental physical (not biological) soil crusts that seal the soil surface, reduce infiltration, and further increase rates of runoff and erosion. This in turn further reduces plant establishment, increases surface water speeds, intensifies raindrop impacts and soil erosion, and increases surface temperatures that can reduce seed germination and harm soil microbes. 

Previous restoration efforts were unsuccessful at repairing underlying soil and hydrological processes. Use of heavy machinery for pitting of the soil surface, tillage, and installation of stock ponds can increase infiltration, but they do not cover the soil surface and prevent erosion and formation of physical soil crusts that are a primary driver of soil erosion. Many of these efforts also intended to restore large swaths of continuous grasslands or small pockets instead of restoring patterns of banded vegetation which are important to the hydrological functioning of the landscape.

The scale of soil degradation at BIBE is too large to address by any one department within the National Park Service. Collaboration and coordination among several departments as well as external partners is needed to provide the resources necessary to address this issue at the scale that is needed.

Project Goals

• Restore the underlying soil and hydrological processes that allow banded vegetation to establish and persist on the landscape by using a combination of techniques to increase infiltration, reduce future erosion, prevent formation of physical crusts, and improve soil microclimate.
• Restore the patterns of banded vegetation, especially of native perennial grasses, by introducing seed with hydromulcher in combination with the aforementioned restoration techniques that seek to improve soil characteristics and restore hydrological processes.
• Increase coordination and collaboration among different departments within BIBE as well as with partners outside the National Park Service. 

Project Highlights

Think Like a Raindrop: When using whole branch mulch from many different species, managers “thought like a raindrop” and applied branches thick enough to intercept 9 out of 10 raindrops instead of following strict depth guidelines.

• Process-Based Restoration: Managers restored hydrological processes by improving infiltration and reducing erosional processes that lead to the formation of a physical crust on the soil surface. Tilling the soil lightly with a disc plow dragged behind an ATV to a depth of 15cm broke the physical soil crust that lowered infiltration. Then managers used a hydromulcher to spread native grass seed. A tackifier was added to the hydromulcher to prevent seed loss from harvester ants and other seed predators. After hydromulching, managers placed a mulch of full branches from a variety of woody plants up to 1m thick on top of the seeds to mimic the protective cover of natural vegetation that intercepts raindrops and shades the soil.
• Learning from Scale: Initial restoration trials started in the early 2000’s on small 1x1m plots. The small scale of these plots did not significantly reduce soil temperatures or increase plant recruitment. After consultation with the NRCS plant material centers and other partners, managers at BIBE decided to use a hydroseeder to scale treatments and mimic natural patterns of banded vegetation on this arid landscape. Repeated trials revealed that increased levels of tackifier were needed to effectively keep seeds in place and reduce seed predation.
• Improved Soil Conditions: Areas with branch mulch had higher soil moisture and lower soil temperatures, both of which help with plant germination and establishment (). Branch mulch also increased soil organic matter and levels of soil microbes. Mulched areas had more microbes than reference natural grasslands and a similar microbial community four years after application. Within the first year after application, whole branch mulch reduced the high NO3 concentrations that were high enough in some untreated areas to be toxic to plants ().
• Vegetation Response: Early vegetation responses in 2004, after monsoon rains, were observed to be significant compared to untreated areas. Between 2008 and 2017 grass cover decreased but cover of forbs increased, as did cover of plant litter and dead or dormant plant bases.

Lessons Learned

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Focusing restoration efforts on restoring the natural processes that influence soil health and hydrological function, can mitigate the impacts of extreme climatic events like drought and may reduce the impact of these events on the landscape. For example, BIBE experienced an extreme drought in 2011 after the grassland restoration followed by a severe freeze, resulting in very little plant growth (). Because the application of branch mulch improved soil conditions to levels similar to those in the existing native banded vegetation (), the underlying soil hydrological processes (infiltration and soil moisture) were improved which increased the potential for successful plant establishment when drought conditions subsided. 

Several thousand yards of brush were needed to treat the 400+ acres that were restored. Managers obtained woody materials from a variety of local vegetation management projects. The National Park Service’s Fire Management Office provided brush that was removed during fuels reduction work. Park maintenance crews provided woody material that they cleared from the National Park. Invasive species removal projects, including Tamarisk (Tamarix spp.), along the Rio Grande also provided woody material. These efforts benefited from being able to deliver removed biomass to restoration sites, when they were closer than other drop-off locations, and when weather conditions or permitting issues prevented burning. There was a shift in perspectives as people started seeing this “waste” brush as a valuable resource for restoration. 

Using whole branch mulch for restoration requires a lot of labor and coordination to move and spread the branch mulch. Partnerships with volunteer organizations like the Sierra Club allowed managers to overcome high labor costs to apply the whole branch mulch at the restoration treatment areas. Coordinating among several National Park departments was critical to be able to source and transport brush to restoration sites.

Next Steps

• Conduct additional photo-point monitoring during monsoon season and drought to assess continued impacts of restoration plots.
• Collaborate with NPS and partners in the potential continuation of the program and use of the restored area as a field classroom. 

Funding Partner

Resources

• Poster: 
• Presentation: 
• McDonald, A.K., Kinucan, R.J., and Loomis, L.E. (2009) “”&�Բ�����;Ecoyhydrology 2:66-71
• Ortiz, A., and Zak J.C. (2011) “”&�Բ�����;Master’s Thesis at Texas Tech University
• Poulos, H. (2017) “” Final Report to Big Bend National Park.

Contacts

• David Larson, National Park Service: david_larson@nps.gov
• Jeff Bennett, American Bird Conservancy: jbennett@abcbirds.org

Case Study Authors

• Maddison Elliott, CART Student Author
• Jeff Bennett, American Bird Conservancy

Suggested Citation

Elliott, M, E. and Bennett, J. (2022). “Using Soil Science to Restore Desert Grasslands in Big Bend National Park.”&�Բ�����;CART. Retrieved from /project/soil-science-restore-grasslands.