Stability of exotic annual grasses following restoration efforts in southern California coastal sage scrub.
Abstract
Restoration of semi-arid shrub ecosystems often requires control of invasive grasses but the effects of these grass-control treatments on native and exotic forbs have not been investigated adequately to assess long-term stability. In southern California, coastal sage scrub (CSS) vegetation is one semi-arid shrub community that has been invaded extensively by both exotic grasses and exotic forbs and is a target for restoration. We studied the effects of grass-specific herbicide, thatch removal plus herbicide and mowing on native and exotic species in a heavily invaded CSS community. We followed this grass-control experiment for 6 years to assess the stability of such treatments. We also added a soil disturbance experiment to investigate the potential influence of thatch removal and soil disturbance on exotic and native grasses and forbs. In the grass-control experiment, treatments reduced exotic grass cover to differing degrees. Three years of mowing resulted in lower exotic grass cover, but only for 2 years. Both herbicide and herbicide plus thatch removal reduced exotic grasses more than mowing, and effects persisted for longer. However, reducing exotic grass cover increased seeded species only during the year of seeding. In addition, plots where exotic grasses were controlled by herbicides also experienced increases in exotic forb cover. In the soil-disturbance experiment, treatments did not increase cover of native species, although plots in which soil was disturbed did have less exotic grass cover. In both experiments, plots observed in years with different rainfall experienced widely varied plant cover, emphasizing the influence that precipitation exerts in these systems. Synthesis and applications. In restoration of semi-arid shrub ecosystems, grass control can reduce exotic grasses over the short-term. However, recovery of grasses in the longer term indicates that restoration does not form a new stable state. Restoration and management of semi-arid shrublands may therefore require continual grass control. Exotic forbs should also be considered for control, as they may increase when exotic grasses are removed. Yearly variations in precipitation confound determination of successful restoration efforts, and require long-term observations to detect the response of native species to treatments.