Responses to salinity of Spartina hybrids formed in San Francisco Bay, California ( S. alterniflora × foliosa and S. densiflora × foliosa )

San Francisco Bay (SFB), which supports large populations of the California native cordgrass Spartina foliosa , has been the recipient of introductions of S. alterniflora and S. densiflora. Hybrids have arisen between the native and these exotic species. Sterile F1 S. densiflora × foliosa hybrids have formed numerous times in a number of marshes, while introgressing S. alterniflora × foliosa hybrids are fully fertile and invaded widely in SFB, especially onto naturally-open low tidal flats by inundation-tolerant hybrids. Sarcocornia pacifica , pickleweed, dominates the mid-to-upper marsh zones where the hypersaline conditions that occur during the summer drought, characteristic of this climate, exclude S. foliosa . Here we report on two glasshouse experiments investigating the salinity tolerance of hybrid Spartina . Some hybrids of both origins grew well and flowered at high salinity levels while the parental species grew little and did not flower. Our results imply that mid-zone marshes are also vulnerable to invasion by salinity-tolerant Spartina hybrids. Herbicide control implemented over the last 10 years targeting both the exotic species and their hybrids have reduced their extent. However, efforts in monitoring and management of exotic Spartina and its hybrids must continue as vast areas of tidal marsh restoration are underway and planned in SFB; colonization by Spartina hybrids tolerant to inundation and/or salinity will greatly alter restoration trajectories. These concerns are all the more vital given projections of climate change and its effects on salinity and sea level rise in SFB salt marshes.