Native plants provide vital ecosystem services in deserts. However, with increasing fire frequency and changing precipitation patterns, invasive plants often outcompete the native plants thus altering the plant community. Increases in fall precipitation, which are projected in the future, and changing fire regimes may alter the competitive balance between invasive plants and native plants. To better understand these relationships, we collected 240 soil cores from areas along the burn boundaries of five independent 2005 wildfires. Replicated combinations of three treatments (fire, seed mix, precipitation timing) were applied. Fire treatments included burned, unburned, and reburned. Seed mixes included invasive plants, native plants, and a mixture of invasive and native plants. Precipitation timing treatments occurred either in early October or early December. After the growing season, the plants from the soil cores were sorted by species and measured and weighed. Reproductive success was also measured by counting the number of flowers and seeds produced by each plant.
The most significant effect we found was in the success of the invasive species Bromus tectorum, Bromus rubens, and Schismus arabicus when an early precipitation treatment was applied. In the case of all three invasive grasses, each was more prolific in density and biomass when early precipitation was received. This indicates that these grasses respond well to and are more competitive with early fall precipitation. Our data suggest that if desert ecosystems of North America experience more frequent and higher amounts of fall precipitation as projected, invasive grasses will become more competitive. Overall, Bromus rubens and Schismus arabicus were more responsive to early precipitation timing than Bromus tectorum, whereas Bromus tectorum was more responsive to reburn treatments.