Competitive effects of an invasive grass on native annuals are species-specific and independent of water availability

Dryland native plant communities are threatened by increasing aridity and the progressive expansion of invasive plant species regionally. Red brome (Bromus madritensis ssp. rubens; “B. rubens” hereafter) is one of the most rapidly spreading invasive plants in California. Here, we experimentally tested the independent and joint effects of B. rubens competition and water availability on the establishment, survival, and per capita biomass of three native forb species common to California drylands. In a controlled greenhouse setting, five watering regimes ranging from extreme dry to extreme wet were introduced through controlled watering every 10 days, and B. rubens competition was introduced by growing native plants either in the presence or absence of B. rubens. Bromus rubens did not impact the establishment or survival of any of the three native forbs but did negatively influence the per capita biomass of two of the three native species. Competitive effects did not vary with water availability, suggesting that the negative effects of B. rubens on native competitors may depend more upon species identity than soil moisture conditions in California drylands. We suggest that the native forb Phacelia tancetifolia may be useful in restoration plantings [in areas affected by B. rubens] because this native species maintained consistent biomass across the range of watering levels tested and regardless of B. rubens presence.