Restoration temporarily supports the resilience of sagebrush-steppe ecosystems subjected to repeated fires.
Abstract
Many ecosystems are experiencing increased fire frequencies and species invasions that can erode their resilience and cause a shift to alternative states. In the sagebrush-steppe, a semi-arid shrubland ecosystem in North America, restoration treatments are often implemented following wildfire to enhance their resilience to invasion. However, little is known about the long-term effectiveness of these treatments. We investigated whether repeated restoration efforts provide greater resilience in sagebrush-steppe communities initially dominated by species with different post-fire regeneration traits and subjected to compounding wildfires and invasion by Bromus tectorum over 25 years. We studied 37 permanent transects (Columbia Basin, Washington, USA) in which species abundance was recorded multiple times from 1992 to 2017. We quantified community change and its relationship with fire, restoration, and moisture availability. Resilience was evaluated by quantifying community resistance and stability indices. The greatest change occurred in communities where the obligate seeding shrub Artemisia tridentata was initially common. Repeated fires led to the extirpation of this shrub and eventual dominance of B. tectorum. Herbicide applications temporarily suppressed B. tectorum post-fire. Seeding treatments and above average precipitation initially increased native cover. Although communities where resprouting species were common showed the least change, repeated fires did lead to a gradual but substantial decline (86%) in resprouting shrubs. Synthesis and applications. Our findings show that repeated restoration efforts, together with elevated precipitation, can support native species re-establishment in systems experiencing altered disturbance regimes and species invasions. Our unique long-term dataset demonstrates, however, that many such interventions have short-lived effects due to the strong 'unhelpful resilience' of highly invaded systems. This implicitly suggests that many such systems have experienced fundamental shifts in ecosystem state. The likelihood of this occurring is strongly associated with the dominant species post-fire regeneration traits. We predict that community composition and resilience will continue to degrade in the sagebrush-steppe unless management prioritizes fire suppression and an adaptive restoration approach that considers resource availability.