Expansion of an introduced species of cordgrass, Spartina densiflora, in Humboldt Bay, California

The dominant plant in Humboldt Bay salt marshes is 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. I investigated the relative contributions of vegetative tiller production and seed germination to the establishment and expansion of S. densiflora. In salt marsh stands, I compared increases in basal area of plants surrounded by potential competitors and those of areas without competing plant species. Plants growing in bare areas without competitors had significantly higher rates of vegetative expansion (p0.0001). I measured viable seed production, germination rates, seedling survivorship, and growth of adult plants in six salinity treatments. Approximately 1,977 ± 80 viable seeds are produced per plant (0.25 to 0.5 square meters). The number of germinating seeds was inversely related to increases in salinity. Salinity treatments at 19-35 ppt produced significantly lower germination rates than salinities of 0-18 ppt (p0.0001). Seedling survivorship was 50% at 4 ppt and 8-14% at 11 ppt. Lateral expansion of adult, greenhouse grown plants occurred in all salinity treatments, with modest decreases in the highest salinity treatments (p0.05). My findings indicate that S. densiflora readily expands in bare areas without competitors primarily by vegetative expansion. Lateral tillers are produced by S. densiflora throughout the year, indicating that plants do not become seasonally dormant. Plants in undisturbed salt marshes expand more slowly, limited by potential competitors such as Salicornia virginica and Distichlis spicata. My results suggest that seed germination and survivorship increase during periods of significant freshwater influence. Although S. densiflora produces prolific amounts of seed, colonization of mature salt marshes may be limited by competitors and high salinities. Colonization of protected open areas is more likely, especially if sediment salinities are substantially decreased. Once established, S. densiflora spreads rapidly. Planting of native species such as Salicornia virginica, Distichlis spicata, or Jaumea carnosa may prevent monospecific stands of S. densiflora from developing. Phenological differences and physiological plasticity probably account for the expansion of S. densiflora in disturbed marshes and restoration sites.