Physiological ecology of the invasive annual grass, Bromus madritensis ssp. rubens, and its interaction with native Mojave Desert species (California)

Abundance of invasive plants is often attributed to greater competitiveness of these species compared with natives. In a combination of field and glasshouse studies, I examined whether the invasive annual grass Bromus madritensis ssp. rubens is a better competitor than two native winter annuals, Vulpia octoflora and Descurainia pinnata. I examined how timing of Bromus establishment, amount of precipitation, and elevated atmospheric CO2 concentrations contribute to its competitiveness in an intact Mojave Desert shrubland and evaluated Bromus' impact on growth and reproduction for a variety of perennial life forms. In comparison with ecologically similar native winter annuals, Bromus establishes in diverse habitats of the Mojave Desert because it is less sensitive to changes in nitrogen (N) availability or plant density than native species, and it can rapidly utilize soil water and N for greater biomass and N content. As Bromus continues to integrate into the landscape in the Mojave Desert, increased Bromus biomass reduces the growth of perennial grasses such as Achnatherum hymenoides (an early season grass) and Pleuraphis rigida (a late season grass) because the phenology of Bromus overlaps completely or closely follows that of these native species. The competitive effect of native Mojave Desert annuals on perennials is lower than that of Bromus, even in years of high rainfall that are favorable for growth, because seed dormancy of native annuals results in densities and biomass lower than those achieved by Bromus, which does not produce dormant seed The evergreen shrub Larrea tridentata is impacted the least by Bromus because it maintains relatively low rates of growth during all months of the year, even after Bromus has completed its life cycle. Rising atmospheric CO2 concentrations exacerbate the competitive effects of winter annuals and other short-lived forbs and grasses on perennial grasses, but only in years with ample precipitation such as that associated with the El Nino Southern Oscillation, which occurs every 4 to 7 yrs in southwestern North America.