Wildfire disturbance affects species interactions of a harmful invasive annual in second year of post-fire vegetative recovery

The increasing frequency of wildfires in Southern California’s Mediterranean-type habitats has been facilitating the displacement of native plants by invasive annuals. Black mustard ( Brassica nigra ) is an abundant, allelopathically harmful, invasive forb, which readily colonizes soil niches following most disturbances. Wildfires, however, are unlike other forms of disturbance because they can fundamentally alter plant–soil interactions through both physical and chemical changes in the soil. Here, a comparative field study of burned and unburned sites suggests that the Woolsey Fire—the largest wildfire ever recorded in California’s Santa Monica Mountains—inhibited dispersal of B. nigra and changed how it interacts with other plant species in the second year of post-fire recovery. More surprisingly, native plants were more likely to replace B. nigra than non-native plants in burned sites. These results indicate the possibility of post-fire seeding with specific “fire follower” native plant species may allow native flora to occupy soil niche space until longer-lived, competitive native shrubs establish.