Here we report shrub-ring isotope records for big sagebrush (Artemisia tridentata) from below the “reverse treeline” in central Utah, USA. Despite sectorial growth, ring-widths of mature plants (age range 9 to 54 years) cross-dated well. Plants were sampled in upland locations (controls) as well as adjacent to mounds (circular patches of denuded soil approximately five m diameter) made by harvester ants (Pogonomyrmex occidentalis) and in swale locations where water drains during monsoon rainfall. For each of the three location types ring-width index (RWI) and carbon isotope discrimination (Δ13C) chronologies were constructed for the years 1989-2018. For each of the three habitats ring-width indices were significantly correlated with summed Spring and monsoon period precipitation (r ≥ 0.42). Δ13C chronologies were strongly influenced by precipitation summed across the previous November to June (r ≥ 0.80) as well as May to July average vapor pressure deficit (r ≤ -0.75). Unlike ring-widths, Δ13C was not related to summer monsoon precipitation (0.09 ≥ r ≥-0.04), indicating water used to support leaf gas exchange was from deep soil layers. Basal area increment of plants that grew adjacent to ant mounds or in swales grew 35% and 61% faster than controls, respectively. Likewise, the 30-year trend in intrinsic water use efficiency (iWUE) was 0.98 µmol CO2 mol H2O−1 yr−1 for controls, but only 0.65 and .60 for and mound or swale sagebrush, respectively. Differences in iWUE between controls and mound or swale sagebrush were greatest during the driest years, indicative of ant mounds and swales providing summer moisture subsidies to sagebrush when the ongoing Megadrought across the Southwestern US has been most severe.