Operationalizing resilience in ecological restoration: A case study in a shrub-encroached semi-arid rangeland ecosystem
Restoration success can be defined as overcoming the resilience of an undesirable state to promote an alternative state that yields greater ecosystem services. Since greater resilience of undesirable states translates into reduced restoration potential, quantifying differences in resilience can enhance restoration planning. Nevertheless, the practical utility of the resilience concept has been limited by the difficulty associated with quantifying thresholds associated with management actions and ecosystem transformability. In the context of shrub-encroached rangeland restoration, shrubland resilience is the capacity of the woody vegetated state to absorb management interventions designed to produce a more desirable grass-dominated state and remain within its current regime. Therefore, differences in the resilience of a state can be quantified in a relative sense by measuring whether a state switches to an alternate state following perturbation or remains in its current stability domain. Here we designed an experimental manipulation to assess the contribution of soils to differences in the relative resilience of a shrub-invaded state to restoration. In this large-scale experiment, we repeated perturbations across a gradient of soil textures to inform restoration practitioners of differences in the relative resilience of shrubland occurring on different soil types to common rangeland restoration practices. On each soil type, we compared the relative ability of the shrubland state to withstand chemical, mechanical, and pyric brush control treatments commonly employed in this study region to untreated controls.
Although shrubland community composition did not differ at the onset of the study, the ecosystems capacity to absorb and recover from brush removal treatments depended on soil type. Shrubland resilience to chemical and mechanical brush removal was highest on coarse, sandy soils. On these soils, brush removal temporarily restored grassland dominance, but woody plants quickly regained pretreatment levels of dominance. However, shrublands on clay soils did not recover following treatments and continued to be grass-dominated for the duration of the study. Fire did not cause a reduction in shrub cover or increase in herbaceous cover in mechanical-treated, chemical-treated, or control plots This study highlights a simple approach for prioritizing restoration actions by mapping the locations and extent of different soil attributes that support shrub-dominated states with differing levels of resilience to mechanical and chemical brush control. This experimental approach provides a basis for operationalizing resilience in restoration and prioritizing management actions across a range of environmental conditions, which is critical given the economic constraints associated with broad-scale mechanical and chemical interventions for rangeland restoration.