UCLA

Invasion by non-native annual grasses poses a serious threat to native vegetation in California, and interference with native ecosystems may be facilitated through interaction with wildfires. Our work is the first attempt to use the coupled fire-atmosphere model, WRF-Fire, to investigate how shifts from native, shrub-dominated vegetation to invasive grasses could have affected a known wildfire event in southern California. We simulate the Mountain Fire, which burned >11,000 ha in July 2013, under idealized fuel conditions that represent varying extents of grass invasion. Expanding grass to double its observed coverage causes fire to intensify and spread faster due to increased wind speed. Beyond this, further grass expansion reduces the simulated spread rate and intensity because the relatively low fuel loads of grasses partially offset the positive wind-speed effect. Overall, our simulations suggest that grass expansion would generally promote larger, faster spreading wildfires in southern California, motivating continued efforts to reduce the spread of invasive annual grasses in this region.