Landscape restoration in southern California forblands: Response of abandoned farmland to invasive annual grass control

A large-scale experiment using 1-ha plots was done to control invasive Mediterranean annual grasses, primarily Bromus spp., in farmlands abandoned for 20 years in southern California. Treatments were a grass-specific herbicide, and herbicide plus dethatching. Dethatching was done to improve the contact of herbicide with newly growing grass seedlings, and to remove litter that may potentially affect germination of native annuals. Native annuals had increased richness in response to grass control, but had low cover in all but two of six years when precipitation was adequate. Dethatching did not improve forb response. Two exotic species of Erodium were the greatest beneficiaries of exotic grass control, but the positive response of native forbs in spite of Erodium dominance suggests a competitive hierarchy of less intense interaction between the native forbs and Erodium than between native forbs and exotic grasses. After five years, grasses began to recover from the herbicide treatments, indicating the need for periodic management of exotic grasses to maintain richness and cover of native forbs. Shrubs that occur on adjacent hills did not recolonize during these six years, suggesting that the valley bottoms may have always been forblands rather than shrublands. The study shows that restoration can be used to rediscover the former vegetation of long-disturbed landscapes.