Abiotic Effects on the Annual Plant Assemblage of Southern California Upper Intertidal Marsh

I tested whether two types of experimental complexity, the testing of multiple factors or temporally varying factors, affected predictions of the germination and establishment of the annual plant assemblage in the upper intertidal marsh of southern California. First, I characterized the conditions that stimulate germination of 29 species in the field, of which 11 are exotics. Large rainfalls cause a temporary decrease in soil salinity and increase in soil moisture that is a germination window. Second, I documented the spatial and temporal heterogeneity of germination. Species had asynchronous changes in seedling density during the 1996 and 1997 germination windows. The timing of germination was correlated with soil salinity decreasing below 20-35 ppt or soil moisture increasing above 40%, depending on the habitat. Contrary to expectations, soil salinity and moisture explained a small amount of the spatial variation in germination compared to perennial plant cover, soil texture, elevation, and wetland identity. Third, I determined whether it was necessary to test multiple abiotic factors to explain the spatial and temporal variation of species' germination in the field. In growth chamber experiments, species differentially responded to constant soil salinity, constant soil moisture, temperature, and photoperiod. The effects of salinity, temperature, and photoperiod could explain the restriction of some species to the cool-season germination window. Finally, I compared the germination and establishment of the assemblage at constant levels of soil salinity and moisture compared to temporally varying salinity and moisture of differing amplitudes, durations, frequencies, and seasonal timing in greenhouse experiments. Germination rates, germination speeds, and biomass of all eleven species tested differed in varying, compared to constant, treatments. In these greenhouse experiments, exotic species were more salt and moisture tolerant, faster to germinate, and less sensitive to varying soil salinity and moisture than the native species. In conclusion, this system is characterized by high spatiotemporal variation in soil salinity and moisture and species' germination and establishment. Species' germination responded to multiple abiotic factors and differed in treatments with constant soil salinity and moisture compared to treatments with temporal variations in salinity and moisture; future experimenters should consider testing multiple varying factors.