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Research Project: Understanding Water-Driven Ecohydrologic and Erosion Processes in the Semiarid Southwest to Improve Watershed Management

Location: Southwest Watershed Research Center

Title: Restoration of a shrub-encroached semiarid grassland: implications for structural, hydrologic, and sediment connectivity

item JOHNSON, J.C. - University Of Arizona
item Williams, Christopher - Jason
item GUERTIN, D.P. - University Of Arizona
item ARCHER, S.R. - University Of Arizona
item Heilman, Philip - Phil
item Pierson Jr, Frederick
item WEI, H. - University Of Arizona

Submitted to: Ecohydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/4/2021
Publication Date: 3/19/2021
Citation: Johnson, J., Williams, C.J., Guertin, D., Archer, S., Heilman, P., Pierson Jr, F.B., Wei, H. 2021. Restoration of a shrub-encroached semiarid grassland: implications for structural, hydrologic, and sediment connectivity. Ecohydrology. 14(4). Article e2281.

Interpretive Summary: Land managers have implemented brush management treatments for over a century with the intent of restoring shrub-encroached grasslands and their valued ecosystem services (e.g. forage production for domestic livestock and soil conservation). Despite extensive historic and ongoing investment in these conservation practices, our understanding of the ecohydrologic mechanisms governing the distribution of soil and water resources necessary for the successful restoration of grasslands remains limited. We compared an herbicide (tebuthiuron) brush management treatment at a shrub-encroached semiarid grassland five years post-treatment to a nearby untreated site to determine whether and to what degree the conservation practice reduced shrub cover, increased herbaceous cover, and decreased erosion. Runoff, infiltration, and erosion rates were estimated across multiple spatial scales via artificial rainfall, overland-flow simulations, and hillslope-scale modeling. Our results suggest the treatment was effective in reducing shrub cover, increasing grass cover, and decreasing erosion. Of note, increases in grass cover and infiltration were disproportionately observed within remnant dead shrub skeleton microsites. Although areas surrounding these resource-conserving patches infiltrated rainfall at similar rates to the untreated site, reductions in the gaps between vegetation helped to limits the ability of runoff to transport sediment via concentrated flow. This study illustrates how changes in soil and vegetation structure initiated by conservation practices might shift a landscape from a “resource-shedding” to a “resource-conserving” ecohydrologic state. Further, we provide evidence that an easily monitored measure of vegetation structure (i.e. basal gaps) may be an indicator of whether a rangeland is vulnerable to elevated runoff and erosion. The research findings provide public and private land managers valuable insight and considerations for implementing brush management conservation practices on arid lands.

Technical Abstract: Cross-scale structural and functional connectivity feedbacks can amplify exogenous forces in dryland environments leading to ecosystem state change (e.g. from grassland to shrubland). Attenuation of these connectivity feedbacks would ostensibly be required to restore transitioned ecosystems to their former state. We compared structural, hydrologic, and sediment connectivity on a shrub-encroached semiarid grassland in southeastern Arizona, USA to that of a nearby site where encroaching shrubs had been treated 5 yr earlier with tebuthiuron herbicide. Soil/vegetation attributes were quantified and paired with hydrologic experiments at fine (0.5 m2) to hillslope (50 m2) scales. Fine-scale rainfall simulations showed interspaces between shrubs were hydrologically similar between treatments, while the herbicided shrub patches were more resource conserving than those within the control [terminal infiltration rates of 105 and 71 mm·h-1, respectively (120 mm·h-1 rainfall intensity; 45 min)]. High structural connectivity of bare ground (basal gap lengths > 200 cm) was correlated with increased concentrated flow runoff and accompanied by greater sediment yields within the untreated site at a coarse scale (~ 9 m2). Hillslope-scale modeling suggested a divergence between hydrologic and sediment connectivity: runoff from high intensity rainfall was similar between sites, while predicted sediment yield was 44% less within the tebuthiuron-treated site. Our results indicate (i) hydraulic properties of zones between shrubs may be resistant to change from herbicide treatments, (ii) disruption of structural connectivity of these interspaces may help to limit runoff and energy needed for sediment transport, and (iii) sediment connectivity is reduced by restoration of the grassland ecological state.