Goldilocks forbs: Survival is highest outside—but not too far outside—of Wyoming big sagebrush canopies

In arid and semiarid systems, positive effects of nurse shrubs generally occur immediately underneath and around shrub canopies, creating microsites that can be targeted to promote plant establishment in restoration settings. Alternatively, the best microsites may occur in the interspace zone immediately surrounding nurse shrubs if positive abiotic effects extend beyond nurse shrub canopy boundaries and if competition with nurse shrubs is reduced in that zone. In the Intermountain West, U.S.A., we investigated survival of transplanted herbaceous seedlings at different distances from Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis) canopies. We planted two native perennial forb species, Munro's globemallow (Sphaeralcea munroana) and common yarrow (Achillea millefolium), and two native perennial grass species, bluebunch wheatgrass (Pseudoroegneria spicata) and bottlebrush squirreltail (Elymus elymoides), at four distances from sagebrush canopies at six sites across the Intermountain West, repeated across 2 years. Under above-normal precipitation, proximity to sagebrush influenced first-year survival of the forb, but not grass, species. Globemallow and yarrow survival were highest mid-way between the canopy dripline and maximum interspace distance between neighboring sagebrush plants. Ground cover characteristics and globemallow survival covaried with respect to distance from shrub, suggesting ground cover characteristics as indicators of suitable planting microsites. Under drier conditions, survival of all species was low and unaffected by distance from canopies. Our results demonstrate the value of fine-tuning the canopy-interspace paradigm to more carefully consider how plant performance may differ across zones within the interspace region between plants, especially when the goal is to maximize plant establishment in nondrought years.