Intra‐site sources of restoration variability in severely invaded rangeland: Strong temporal effects of herbicide–weather interactions; weak spatial effects of plant community patch type and litter

Abstract

Abstract Invasions by exotic annual grasses (EAGs) are replacing native perennials in semiarid areas globally, including the vast sagebrush‐steppe rangelands of western North America. Efforts to eradicate EAGs and restore perennials have had mixed success, especially in relatively warm and dry areas where EAGs had high dominance prior to intervention. Greater consideration of the ecological sources of variability in EAG treatment outcomes may improve success. We hypothesized that herbicide and restoration outcomes would be influenced by restoration strategy (type of herbicide, seeding or planting, timing of treatment) and underlying spatial variability associated with plant community patch type and litter, all applied in a landscape‐scale experiment in a severely invaded area in Southern Idaho, USA. EAGs, specifically medusahead (Taeniatherum caput‐medusae [L.] Nevski), were strongly reduced for up to 3 years (maximum observation period) by the pre‐emergent herbicide indaziflam, whereas the pre/post‐emergent imazapic reduced EAGs only when applied twice. Indaziflam effects were greater when post‐spray moisture was greater, and also when co‐applied with imazapic, but reapplying indaziflam did not lead to additional reduction of EAGs. Imazapic and indaziflam each stimulated species‐specific, secondary invasion by exotic and/or invasive tall forbs. Application of the broadleaf herbicide aminopyralid provided only a fleeting 1 year of control of a dominant, highly noxious forb skeletonweed (Chondrilla juncea L.). Underlying heterogeneity in plant community patch type (dominant herb species) explained only ∼5% of variation in the herbicide effects, and manipulation of litter prior to spraying had no effect. Several years of seedings and planting resulted in no establishment of native perennials. Herbicides, especially indaziflam, appear to be an effective tool for reducing EAGs for multiple years in the challenging restoration conditions we evaluated, particularly if their application coincides with suitable moisture. However, restoring the perennials required for longer term resistance to reinvasion is a serious challenge that could be avoided with preservation of perennials.