Ecology and biogeography of red brome (Bromus madritensis subspecies rubens) in western North America

Abstract

Red brome [Bromus madritensis subsp. rubens (L.) Husn.] arrived in western North America before 1880 and has since invaded even relatively undisturbed areas. Populations of this annual grass, native to the Mediterranean Basin, fluctuate widely in response to variations in winter precipitation and can produce large amounts of persistent fine fuels. The biogeography portion of this research describes the arrival, spread, and current extent of red brome in western North America and estimates the potential climatic range of this grass in the USA. The ecology portion of this research investigates interactions between native Sonoran Desert winter annuals and four densities of red brome, from the equivalent of 300 to 1200 plants m⁻², at low (3) and high (540 μg NO₃ - g soil⁻¹) levels of soil nitrogen (N), to evaluate the relative intensity of interactions at low and high fertility. This study found no evidence of red brome occurrence in North America prior to 1879. This grass may have arrived with immigrants or agricultural products after the start of the California Gold Rush in 1848, perhaps in crop seed or in the fleece of sheep. Red brome appears best adapted to areas with minimum January temperatures between -5.4 and 3.9°C and total winter precipitation between 9.0 and 76.4 cm. However, the myriad factors mediating interactions among red brome, other species, and local environments in these areas will determine if this grass becomes established in any of these regions. This study found no evidence of reduced richness or diversity of Sonoran Desert annuals occurring with red brome. Red brome did not exclude species from this study, as neither emergence nor survival of native species were affected by this grass. However, red brome significantly interfered with growth of native species. Declines in total biomass of each of 11 analyzed species growing with red brome averaged 58.4% of biomass produced when this grass was not present. The intensity of interactions between red brome and Sonoran Desert annuals rarely varied with soil fertility and differences in growth of red brome and of native annuals between low and high N were similar.