Germination response to temperature and moisture to predict distributions of the invasive grass red brome and wildfire

Abstract

The annual grasses red brome (Bromus rubens L.) and cheatgrass (Bromus tectorum L.) are associated with increased frequency and size of wildfires in the Great Basin and Mojave deserts of North America. Invasion success of these exotic grasses is driven in part by germination responses to climate fluctuations. Understanding how climate influences germination characteristics of invasive grasses is critical to predicting desert wildfire. While the germination characteristics of cheatgrass have been reported, the germination requirements for red brome, the exotic grass associated with fire in the Mojave Desert, have not. Herein we characterize germination response of red brome through hydrothermal-time modeling. The hydrothermal-time requirements were matched with historic climate data to estimate optimal germination periods for red brome and its potential association with wildfire. The optimal temperature for germination in a population of red brome seeds was 19.3 °C with a hydrothermal-time constant of 38.9 MPa °C days and a mean base water potential of −1.35 MPa. Based on the hydrothermal model, high red brome germination rates are most likely to occur in wet Octobers when optimal fall temperatures overlap with adequate moisture. Abnormally high precipitation totals in the fall of 2004 created ideal conditions for red brome germination that likely contributed to the large-scale Mojave Desert fires of 2005. Predicted intensification of fall and winter precipitation events in response to climate change will likely drive further increases in red brome abundance and subsequently more widespread fire in regions of the Mojave Desert.