Restoration of a shrub‐encroached semi‐arid grassland: Implications for structural, hydrologic, and sediment connectivity

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 semi‐arid grassland in south‐eastern Arizona, USA, to that of a nearby site experiencing an increase in non‐native perennial grass (Lehmann lovegrass) abundance 5‐year following treating shrubs 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 (120 mm·h−1 rainfall intensity; 45 min) showed interspaces between shrubs were hydrologically similar on the treated and control sites, whereas herbicided shrub patches were more resource conserving than those within the control (terminal infiltration rates of 105 and 71 mm·h−1, respectively). High structural connectivity of bare ground (basal gap lengths >200 cm) was correlated with increased concentrated flow run‐off and accompanied by greater sediment yields within the untreated site at a coarse scale (~9 m2). Hillslope‐scale modelling suggested a divergence between hydrologic and sediment connectivity: run‐off 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 soils between shrubs are unresponsive to herbicide treatment, (ii) disruption of structural connectivity of these interspaces associated with grass cover increases subsequent to herbicide application attenuated run‐off and the energy needed for sediment transport, and (iii) sediment connectivity is reduced by conversion to a novel grassland ecological state.

Vegetation patterns affect soil aggregate loss during water erosion

“…Vegetation coverage affects the power and energy of runoff by changing the runoff and sediment connectivity in the slope–gully system. Johnson et al (2021) and Urgeghe et al (2021) pointed out that the stronger the connectivity of runoff is, the greater are the energy and sand‐carrying capacity of the runoff, resulting in more soil erosion. In our study, compared with pattern A (bare slope), the vegetation pattern can reduce the runoff and sediment by 7.0%–42.6% and 5.0%–25.7%, respectively.…”

The Importance of Long-term Observations for Understanding Dryland Soil-Water-Vegetation-Climate Dynamics

“…Results showed that 5-yrs post-treatment, the chemically-treated shrubland had shorter basal gap lengths (distances between the bases of vegetation), which were associated with less cumulative runoff and soil loss during overland flow experiments. The study highlighted the utility of widely implemented rangeland monitoring methods (i.e., foliar canopy cover and basal gaps) to assess changes in vulnerability to runoff and erosion (Johnson et al, 2021). This increased the fraction of rainfall events that produced runoff to 23.4%, more than triple the long-term average (7.2%).…”

Ecohydrological connectivity: A unifying framework for understanding how woody plant encroachment alters the water cycle in drylands

“…The connectivity concept is increasingly being employed by hydrologists and geomorphologists to better understand and describe water and sediment fluxes (Bracken et al, 2013;Bracken et al, 2015;Keesstra et al, 2018). It has been usefully applied by hydrologists in many environments, including wetlands (Cohen et al, 2016;Lane et al, 2018;Leibowitz et al, 2018), river systems (Castello et al, 2013;Jaeger et al, 2014;Goodrich et al, 2018), and arctic regions (Bring et al, 2016;Walvoord and Kurylyk, 2016)-at continental and even global scales (Peters et al, 2008;Good et al, 2015)-but it has been especially useful in clarifying ecosystem function and interaction in drylands (Turnbull and Wainwright, 2019;Saco et al, 2020;Calvo-Cases et al, 2021;Johnson et al, 2021). Okin et al (2015) argue that connectivity serves as an "organizing principle to understand dryland structure and function at scales from individual plants to entire landscapes."…”