A warmer and drier climate in the northern sagebrush biome does not promote cheatgrass invasion or change its response to fire

Abstract

Dryland shrub communities have been degraded by a range of disturbances and now face additional stress from global climate change. The spring/summer growing season of the North American sagebrush biome is projected to become warmer and drier, which is expected to facilitate the expansion of the invasive annual grass Bromus tectorum (cheatgrass) and alter its response to fire in the northern extent of the biome. We tested these predictions with a factorial experiment with two levels of burning (spring burn and none) and three climate treatments (warming, warming + drying, and control) that was repeated over 3 years in a Montana sagebrush steppe. We expected the climate treatments to make B. tectorum more competitive with the native perennial grass community, especially Pseudoroegneria spicata, and alter its response to fire. Experimental warming and warming + drying reduced B. tectorum cover, biomass, and fecundity, but there was no response to fire except for fecundity, which increased; the native grass community was the most significant factor that affected B. tectorum metrics. The experimental climate treatments also negatively affected P. spicata, total native grass cover, and community biodiversity, while fire negatively affected total native grass cover, particularly when climate conditions were warmer and drier. Our short-term results indicate that without sufficient antecedent moisture and a significant disruption to the native perennial grass community, a change in climate to a warmer and drier spring/summer growing season in the northern sagebrush biome will not facilitate B. tectorum invasion or alter its response to fire.