Precipitation timing and soil heterogeneity regulate the growth and seed production of the invasive grass red brome

Abstract

Invasions of exotic grasses are increasing the spatial extent and temporal frequency of wildfire. Mitigation efforts to prevent wildfire depend on understanding the biophysical agents that drive spatial and temporal distributions of exotic grasses. We examined the growth and reproduction of the exotic grass red brome (Bromus rubens L.) in response to precipitation timing in the fall period, fertile-island topography, and fire history in the Mojave Desert. Tiller density, shoot height, shoot biomass, and seed production were measured. Early precipitation timing (mid-October versus late November) triggered early germination and led to greater red brome density (1.5-fold), shoot height (1.9-fold), shoot biomass (6.8-fold) and seed production (3.9-fold). Soil cores collected from beneath shrubs (fertile-islands) versus shrub inter-spaces also produced red brome with enhanced shoot height (1.5-fold), and seed production (1.9-fold). The legacy effect of wildfires on soils (10 years after the fires) had no significant effect on the growth or reproduction of red brome. Soils beneath shrubs had significantly less gravel and higher mineral nitrogen (N) concentrations than soils from shrub inter-spaces. Soil gravel % was negatively correlated with red brome density, height, biomass and seed production while mineral N tended to have weak positively correlations with red brome density and seed production. These results suggest that the growth and reproduction of red brome is influenced by fall precipitation timing and soil characteristics. Management planning for fire intervention should consider fall precipitation patterns, soil characteristics and shrub topography as important determinants of red brome growth and fire potential.