Seed bank dynamics of French broom in coastal California grasslands: Effects of stand age and prescribed burning on control and restoration

Successful control of invasive exotic plants depends to a large degree on the regeneration potential of the target exotic species and other species that might also be influenced by the removal effort. In coastal grasslands of California, exotic French broom (Genista monspessulana) forms both dense stands aboveground and abundant seed banks belowground. Land managers attempt to reduce broom cover and the seed bank through prescribed burning, but before this study, no investigators had examined the effect of repeated burning on French broom and associated grassland species. We found that the soil seed bank of stands that were burned had fewer broom seeds than unburned areas but that repeated burning did not reduce the seed bank beyond what was observed after one fire. Fire also did not have any consistent effect on the seed banks of other grassland species. We also examined the relationship between broom stand age and seed bank size but did not find a strong relationship between them. Our data suggested that the broom seed bank stays constant or declines slightly with stand age. We did, however, find that nonbroom seed numbers decreased as broom stands aged. Our results suggest that fire does reduce the size of the broom seed bank and that control of broom need not be limited to only the youngest stands.