Submitted to: Ecological Applications
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/21/2024
Publication Date: N/A
Citation: N/A

Interpretive Summary: Climate change is predicted to benefit many invasive species. In North American grasslands, the most concerning invaders are winter annuals. Understanding winter annual responses to climate change is complicated because these species are regulated by temperature and precipitation during multiple seasons, in contrast with perennials that are predominately regulated by growing season precipitation. Downy brome and Japanese brome are widespread, functionally similar invasive winter annual grasses that are destroying wildlife habitat, reducing livestock production, and increasing wildfire risks in the Western U.S. We combined experimental and long-term (30 yr) monitoring data to estimate effects of weather on biomass production of these bromes. Fall precipitation is necessary for germination, and brome biomass the current growing season increased with precipitation last fall, two falls ago, and likely three falls ago. Fall precipitation is projected to increase in our Great Plains study region, and a mere 5% (3.0 mm) increase would cause an 11% increase in brome biomass according to our model. Brome biomass increased with current growing season precipitation. Conversely, brome biomass declined with previous growing season precipitation, perhaps because wet conditions strengthened perennial competitors of bromes. These positive and negative brome responses to growing season precipitation were of similar magnitude, so they largely cancelled out and left mean brome biomass unchanged. This suggests bromes will be somewhat insensitive to changing growing season precipitation levels. These results should urge an increased focus on fall weather to understand invasive winter annual plant responses to climate change. Fall temperature was not an important predictor in this study, but temperature and precipitation have interacting effects on germination and establishment, so both variables deserve further study.

Technical Abstract: Climate change is predicted to increase spatial extent and equilibrium abundance of many invasive species, and recent research indicates this may already be happening. In North American grasslands, the most concerning invaders are winter annuals. Understanding winter annual responses to climate change is complicated because these species are regulated by temperature and precipitation during multiple seasons, in contrast with perennials that are predominately regulated by growing season precipitation. Downy brome (Bromus tectorum L. (ITIS)) and Japanese brome (Bromus japonicus Thunb.) are widespread, functionally similar invasive winter annual grasses that are destroying wildlife habitat, reducing livestock production, and increasing wildfire risks in the Western U.S. We combined experimental and long-term (30 yr) monitoring data to estimate effects of weather on biomass production of these bromes. Fall precipitation is necessary for germination, and brome biomass the current growing season increased with precipitation last fall (p<0.0001), two falls ago (p<0.001), and likely three falls ago (p=0.09). Fall precipitation is projected to increase in our Great Plains study region, and a mere 5% (3.0 mm) increase would cause an 11%(7%, 14%) [mean(95% confidence interval)] increase in brome biomass according to our model. Brome biomass increased with current growing season precipitation. Conversely, brome biomass declined with previous growing season precipitation, perhaps because wet conditions strengthened perennial competitors of bromes. These positive and negative brome responses to growing season precipitation were of similar magnitude, so they largely cancelled out and left mean brome biomass unchanged. This suggests bromes will be somewhat insensitive to changing growing season precipitation levels. These results should urge an increased focus on fall weather to understand invasive winter annual plant responses to climate change. Fall temperature was not an important predictor in this study, but temperature and precipitation have interacting effects on germination and establishment, so both variables deserve further study.