<p>Because a system’s invasibility is governed strongly by biotic interactions of competition and facilitation, the stress-gradient hypothesis (SGH) can be used as a framework to understand invasion dynamics. Here, we examine buffelgrass (<i>Pennisetum ciliare</i>), a perennial bunchgrass and notorious invasive species of the Sonoran Desert. To better understand how interactions between buffelgrass and native species might change across the stress gradients of varying precipitation and slope angle, we conducted a field study at three locations near Tucson, Arizona, USA. We measured culm counts of tagged buffelgrass individuals; buffelgrass density, cover, and seedling survival within plots; and native vegetation cover over three years. Consistent with the SGH, under more stressful conditions (either from low precipitation or steep slopes) native shrubs increased in cover while buffelgrass individuals increased in total culm count, signaling possible facilitation. However, the opposite relationship was observed between shrub cover and buffelgrass density: in drier conditions, shrub cover tended to increase while buffelgrass density decreased; and wetter conditions, in which buffelgrass density saw its largest increases, resulted in relatively unchanged shrub cover. Individual- versus population-level responses to precipitation differed: mature buffelgrass individuals could withstand drought and continue growing, while wetter years enabled increases in density and cover, and higher seedling survival. Continual intervention is likely needed to control buffelgrass, including targeting mature individuals during drought to reduce propagule pressure and fuel loads, and intensifying seedling monitoring and follow-up treatments in wetter years.</p>