Impacts of Herbicide Brush Management on Hydrology & Sedimentation

Introduction

Image Details

Walnut Gulch is an experimental watershed in southeastern Arizona that is managed by the Southwest Watershed Research Center. Walnut Gulch has experienced an increase of whitethorn acacia (Vachellia constricta), a common native woody shrub, into areas that were historically dominated by native grasses. Shrub encroachment often leads to larger patches of bare ground, increased erosion, and reduced infiltration. This can start a cycle of degradation leading to the loss of grasslands and the ecosystem services they provide. Between 2008 and 2018, Walnut Gulch was treated with the herbicide tebuthiuron (‘spike’) over 22% of its area to reduce whitethorn acacia and other woody plant cover. 

The increase of woody plants like whitethorn acacia into historical grasslands can result from the interactions of less frequent fires, livestock overgrazing, increases in atmospheric carbon dioxide, and climate change climate change
Climate change includes both global warming driven by human-induced emissions of greenhouse gases and the resulting large-scale shifts in weather patterns. Though there have been previous periods of climatic change, since the mid-20th century humans have had an unprecedented impact on Earth's climate system and caused change on a global scale.

Learn more about climate change -driven drought and heat waves, among other factors. The conversion of grassland ecosystems has important ecological and economic consequences, including the loss of fertile topsoil, vegetation cover, and associated habitat and forage values.

Shrub management strategies like herbicide treatments seek to reduce woody plant cover to allow for the recovery of grasslands. However, field studies of the impacts of herbicide treatment on runoff and sedimentation of shrub-encroached grasslands are lacking. An understanding of the impacts of treatment on the runoff and sedimentation processes which reinforce the transition to shrubland ecosystem states may allow managers to assess the potential for recovery of grasslands using herbicides. 

Researchers at the University of Arizona and at the Agricultural Resource Service used field rain simulations, modeling approaches, and assessments of physical landscape attributes to compare runoff, sediment loss, and vegetation cover on Limy Slopes ecological sites at Walnut Gulch. 

Key Issues Addressed

In shrublands, patches of bare ground are often larger and more connected than in grasslands, allowing greater movement of water and sediments. Increased soil erosion and reduced infiltration capacity causes the spaces between shrubs to become depleted of these resources. As fertile soil accumulates around the bases of shrubs, ‘resource islands’ form. 

The formation of resource islands advantages shrubs and reinforces the transition to shrubland ecosystem states. Grasses are unable to establish in the resource-depleted spaces between soil-accumulating shrubs, allowing spatially disparate resource accumulation and depletion to continue. Shrub management actions like herbicide treatment seek to remove woody plants and halt this accumulation of resources. Although herbicides have been employed widely as a brush management technique, field studies on the impacts of herbicide treatment on runoff and sedimentation processes of shrub-encroached grasslands are lacking. 

Common practices to reduce woody plant cover include prescribed fire and mechanical removal techniques like grubbing and chaining. While these may be effective in reducing woody species abundance, they may increase the amount of bare ground by removing the shrub skeleton. As a result, already heightened runoff and sediment loss may be exacerbated initially. In contrast, herbicide treatments that leave the skeleton of the shrub intact may minimize runoff and erosion. While herbicides are an effective tool used widely to reduce shrub cover, uncertainties remain as to the efficacy of herbicide treatments in preventing amplification of runoff and erosion following treatment. 

Project Goals

• Compare vegetation cover, runoff, and soil loss between herbicide-treated and untreated sites
• Determine if changes in runoff and soil loss following herbicide treatment disrupt the accumulation of resources in resource islands, allowing for the redistribution of soil nutrients and increased infiltration capacity in interspaces
• Assess whether impacts to vegetation, runoff, and soil loss vary across spatial scales
• Assess which structural characteristics of soil and vegetation are reliable indicators of runoff and erosion vulnerability

Project Highlights

Long-Term Hydrologic Data: The Southwest Watershed Research Center houses datasets of precipitation, sedimentation, and runoff for WGEW dating back to the 1950s. 

• Grasses Rebound: Grass cover was greater on treated sites compared to untreated sites. At the fine scale, grass cover in interspaces increased from 0% in untreated sites to 25% in treated sites, and from 12% in the canopy of untreated sites to 75% in treated sites. Grass cover similarly increased at the coarse scale from 13% on untreated sites to 61% on treated sites. Increases in grass cover led to an overall increase in vegetation cover from 56% to 78% at the coarse scale, and shrub cover was reduced from from 40% on untreated sites to less than 1% on treated sites. This marked a transition back to grass dominance on treated sites. 
• Reduced Runoff and Erosion in Resource Islands:At high rainfall intensities (100 and 120mm/hr) runoff in resource islands on treated sites was reduced by 70-80% when compared with untreated sites at the fine scale. Reductions in runoff led to a similar reduction in erosion. Soil loss in resource islands was 86-88% less on treated sites than untreated sites. Infiltration rates, runoff, and sediment loss were similar in interspaces between treated and untreated sites. 
• Hillslope Scale Responses: Researchers paired rainfall simulations which provide insights at fine (0.5m² plots) and coarse (2m x 4.5m plots) scales with the hillslope-scale Rangeland Hydrology and Erosion Model to compare treatment impacts across scales. At the hillslope scale, erosion was reduced across rainfall intensities in treated sites. Runoff, however, showed no difference between treated and untreated sites. 
• Basal Gaps Predict Cumulative Runoff and Sediment Yield: The length of basal gaps (the distance between plant bases) predicted the amount of runoff and erosion across study sites. Short basal gap lengths limited cumulative runoff and sediment yield, while larger basal gap lengths were correlated with increased cumulative runoff and sediment yield. This indicates that the size of continuous bare ground patches is a determinant of runoff and sediment loss and supports the use of basal gap length as a method for assessing impacts of treatments. 

Lessons Learned

Image Details

Five years after tebuthiuron herbicide, it is unclear whether the processes driving woody plant encroachment were adequately disrupted to enable long-term grassland recovery. Sites treated with tebuthiuron were characterized by reduced runoff, reduced erosion, and increased vegetative cover of grasses. Contrary to expectations, changes in runoff and erosion were concentrated in the resource islands under shrub skeletons rather than in the interspaces between them. The limited response to treatment observed in interspaces between shrubs may indicate that the underlying feedbacks that maintain shrub-dominance were not interrupted by herbicide treatment. It is unknown whether the increasing herbaceous cover and subsequent reduction of bare ground will lead to homogenization of resources over the long term, allowing for grassland recovery. 

Linking costly field rainfall simulations (which require a rainfall simulator) with the Rangeland Hydrology and Erosion Model (RHEM) allowed researchers to compare changes in runoff and erosion across spatial scales. RHEM is a low-requirement tool available to managers to assess the impacts of shrub removal on runoff and sedimentation without the use of complex and expensive field simulations. By integrating user inputs of soil attributes, ground and foliar cover, and slope dimensions, RHEM allows users to assess impacts of treatment on runoff and sedimentation of a potential treatment area. 

Treated sites were vegetated by Lehmann lovegrass (Eragrostis lehmanniana) and Bush Muhly (Muhlenbergia porteri). Nonnative Lehmann lovegrass was only observed on treated sites, leading researchers to speculate that its expansion is limited in part by shrubs. Researchers additionally note that the shift back to grass dominance may not have occurred in the absence of Lehmann lovegrass. Although this bunchgrass expanded into interspaces, these areas retained vulnerabilities to runoff and erosion. Native grasses may not have the same capacity to grow into these resource-poor interspaces. At Walnut Gulch, herbicide treatments initiated a shift to a grassland dominated by Lehmann lovegrass five years following treatment.

Next Steps

• Future research should aim to determine the long-term impact of herbicide treatments at Walnut Gulch to assess if observed increases in herbaceous cover will lead to resource homogenization over time.
• Future research is needed to determine how immediate outcomes of herbicide treatment, particularly the expansion of Lehmann Lovegrass, interact with ecosystem disturbances like fire to influence site conditions in the long term.
• Researchers note that macropores (large pores in soil allowing infiltration) likely played an important role in mediating the different response observed between interspaces and resource islands. Future research should aim to examine the mechanistic role of macropores in infiltration around the skeleton of herbicide-treated shrubs. 
• While this research traversed fine to hillslope spatial scales, researchers note that the results of herbicide treatment on water and sediment movement at the watershed scale may vary and should be researched further.
• Water is not the only vector of sediment transport and may have influenced the trends observed in this study. Future research should aim to characterize more directly the influence of wind on sediment movement following herbicide treatment.

Funding Partners

• U.S. Department of Agriculture, Agricultural Research Service, Southwest Watershed Research Center (USDA - ARS - SWRC)
• Arizona Agricultural Experiment Station
• This research was a contribution from the Long-Term Agroecosystem Research (LTAR) network. LTAR is supported by the U.S. Department of Agriculture.

Resources

• The Rangeland Hydrology and Erosion Model (RHEM)
  • RHEM documentation
• Walnut Gulch Experimental Watershed (WGEW)
• SWRC Data Access Project
• Johnson, J.C., et al. (2022) "Restoration of a shrub-encroached semi-arid grassland: Implications for structural, hydrologic, and sediment connectivity." Ecohydrology 14(4):1-20

Contacts

• Justin Johnson, USDA ARS, justin.johnson3@usda.gov
• Jason Williams, USDA ARS, jason.williams@usda.gov

CART Case Study Lead Author

Nicolas Katz, University of Arizona

Suggested Citation

Katz, N.A. (2022). “Impacts of Herbicide Brush Management on Hydrology & Sedimentation at Walnut Gulch Experimental Watershed”. CCAST. Retrieved from https://www.fws.gov/project/impacts-herbicide-brush-management-hydrology-sedimentation.