Winners and losers: Competition and the invasive grass Bromus inermis

Abstract

Competition can cause the loss of certain species and concurrent dominance of other species, especially with invasive species. Here we explore the mechanisms behind competition between an invasive grass species, smooth brome Bromus inermis, and two dominant native grass species, big bluestem Andropogon gerardi and little bluestem Schizachyrium scoparium. Bromus inermis is now found in all contiguous states in the United States and may outcompete native species in North America, particularly when grazed. Grazing was simulated by cutting the plants once per year and three times per year, done monthly in the summer (plus an uncut control treatment). We grew all three species in the greenhouse as monocultures, as well as in a mix of B. inermis + A. gerardi and B. inermis + S. scoparium. We found that there was considerable yield suppression when smooth brome competed with both big- and little bluestem mixes with B. inermis after three cuts. Non-structural carbohydrates and total nitrogen declined considerably aboveground for the B. inermis + A. gerardi mix after three cuts. The clearest overall impact was that of cutting frequency, with plants receiving three cuts per season having lowest biomass. We conclude that the maintenance of biomass in B. inermis, despite the level of simulated herbivory, was probably the reason that they remained competitively dominant over the two native bluestem species.