Maintenance of salt barrens inhibited landward invasion of Spartina species in salt marshes

Abstract

Spartina spp. (cordgrasses) often dominates intertidal mudflats and/or low marshes. The landward invasion of these species was typically thought to be restrained by low tidal inundation frequencies and interspecific competition. We noticed that the reported soil salinity levels in some salt marshes were much higher than those at the mean higher high water level, which might inhibit the landward invasion of cordgrass. To test this possibility, we transplanted Spartina alterniflora across an elevational gradient in an invaded salt marsh in the Yellow River Delta National Nature Reserve, where a salt accumulation zone (i.e., salt barren) was previously observed. We found that S. alterniflora was significantly inhibited by the salt barren in high marsh regions, although it performed better at upland and low marsh regions. A common garden experiment further elucidated that S. alterniflora performed best at low salinity levels and that this species is less sensitive to inundation frequency. Our results indicated that the salt barren inhibited the landward invasion of S. alterniflora in salt marshes and provided a natural barrier to protect the upland from invasion. Though field observations suggest that S. alterniflora could propagate along tidal channels, which provide low-salinity corridors for the dispersal of propagules, natural salt barrens can inhibit the landward invasion of Spartina in salt marshes. However, artificial disturbances that break the salt barren band in salt marshes (e.g., artificial ditches) might accelerate the invasion of Spartina spp. This new finding should alert salt marsh managers to pay attention to artificial ditches and/or other human activities when attempting to control Spartina invasion.