Mechanisms of expansion for an introduced species of cordgrass, Spartina densiflora, in Humboldt Bay, California

The dominant plant in Humboldt Bay salt marshes in Spartina densiflora, a species of cordgrass apparently introduced from South America. At several salt marshes and restoration sites around Humboldt Bay, distribution of this plant has increased significantly. We investigated the relative contributions of vegetative tiller production and seed germination to the establishment and expansion of S. densiflora. Lateral spread of plants surrounded by competitors were compared to areas without competing plant species. Plants growing in areas without competitors had significantly higher rates of vegetative expansion (p<0.0001). Viable seed production, germination rates, seedling survivorship, and growth of adult plants were measured in six salinity treatments. Approximately 1,977±80 viable seeds are produced per plant (0.25–0.5 m²). The number of germinating seeds was inversely related to increases in salinity. Salinity treatments between 19‰ and 35‰ produced significantly lower germination rates than salinities of 0–18‰ (p<0.0001). Seedling survivorship was 50% at ≤4‰ and 8–14% at ≥11‰. Lateral expansion of adult, greenhouse-grown plants occurred in all salinity treatments, with modest decreases in the highest salinity treatments (p<0.05). Our findings indicate that S. densiflora expands primarily by vegetative expansion, and lateral tillers are produced by throughout the year. Spartina densiflora produces prolific amounts of seed, but recruitment in mature salt marshes may be limited by competitors and higher salinities. At restoration sites, planting of native species such as Salicornia virginica, Distichlis spicata, or Jaumea carnosa may prevent monospecific stands of S. densiflora from developing.