Precipitation and invasive winter annual grass data for the Great Plains

Aim: Climate change is predicted to increase spatial extent and equilibrium abundance of many invasive species, and there is evidence this may already be happening. In North American grasslands, the most concerning invaders are winter annuals. Understanding winter annual responses to climate change is challenging because these species are regulated by weather during multiple seasons, unlike perennials that are overwhelmingly regulated by growing season precipitation. We quantified downy brome (Bromus tectorum L. (ITIS)) and Japanese brome (Bromus japonicus Thunb.) responses to precipitation and temperature. These functionally similar invasive winter annual grasses are destroying wildlife habitat, reducing livestock production, and increasing wildfire risks across vast portions of the western U.S. Location: Great Plains, U.S.A. Methods: Using Bayesian methods to integrate experimental and long-term (30 yr) monitoring data, we estimated the effects of precipitation and temperature on biomass production of bromes and native species. Results: Unsurprisingly, brome biomass increased with the current growing season (spring) precipitation. Alternatively, brome biomass declined with previous growing season precipitation, perhaps because previously wet conditions strengthened perennial competitors of bromes. These positive and negative effects of growing season precipitation largely cancelled out and left mean brome biomass unchanged. This suggests bromes will be insensitive to changes in growing season precipitation. Conversely, bromes proved highly sensitive to fall precipitation. Fall precipitation is necessary for germination, and brome biomass in the current growing season increased with precipitation the previous fall (p Conclusions: Fall precipitation is projected to increase in much of the western U.S., and a mere 5% (3.0 mm) increase would cause an 11% (7%, 14%) [mean(95% confidence interval)] increase in brome biomass. These results should urge increased focus on fall weather to understand winter annual plant responses to climate change.