Mechanism of Invasion and Dominance of Coastal Grassland by Mesembryanthemum crystallinum

The invasion of Mesembryanthemum crystallinum into coastal grassland was observed at Surf on the central coast of California, USA. The species became established in areas unoccupied or sparsely occupied by other plants, and then grew into surrounding areas. The following season, few grassland seedlings established beneath dried Mesembryanthemum as compared to adjacent grassland. The reduction in numbers of grassland seedlings did not appear to be the result of limiting levels of moisture, light or macronutrients which were found in lower levels in the grassland, nor to be due to grazing by small mammals, since the pattern persisted in the absence of grazing. The differential pattern of seedling establishment was correlated with high levels of salt found in the soil beneath dried Mesembryanthemum. Mesembryanthemum crystallinum is an annual plant which accumulates salt throughout its lifespan. After the plant dies, this salt is released with leaching by fog and rain. The salt produces a detrimental osmotic environment preventing growth of nontolerant species. The salt does not appear to have a direct toxic influence on grassland species. Osmotic interference resulting from accumulation and release of salt appears to be the means by which Mesembryanthemum crystallinum dominates areas previously occupied by grassland.