Abstract: Woody encroachment into grasslands and shrublands disrupts ecosystem processes and reduces biodiversity. Tree removal is a widespread strategy to restore ecosystem services and biodiversity in impacted landscapes. However, tree removal can also increase the risk of invasion by exotic annual grasses. In western North America, juniper (Juniperus spp.) encroachment threatens the ecological integrity of intact sagebrush (Artemisia tridentata) shrublands. We used remote sensing to track vegetation changes following juniper removals on 288 parcels totaling 106,333 ha in southern Idaho, USA. We also analyzed vegetation changes following 64 wildfires that burned 152,611 ha of nearby rangeland during the same period. We matched areas within removals and wildfires to similar undisturbed areas, and then used causal impact analysis to estimate the effects of the disturbances. Juniper removals resulted in sustained reduction of tree cover and increased perennial forb and grass cover across nearly all sites, achieving key management goals. Based on the metrics evaluated, juniper removal was more effective than wildfire in delivering long-term restoration in this sagebrush system. However, juniper treatments also stimulated temporary undesirable increases in annual grasses and forbs, indicating the need for additional management to achieve durable conservation outcomes. Intensive mechanical methods initially reduced shrub cover in some treatments, but shrubs recovered to near pre-treatment levels within 7 years. Using a recently-developed metric of ecological integrity for sagebrush ecosystems, we show that these large, long-term projects halted or reversed degradation attributed to juniper expansion, demonstrating that restoration can improve the trajectory of ecosystems when implemented at scale.

Keywords: Bromus tectorum; Juniperus spp.; Restoration; Sagebrush; Tree removal; Woody encroachment