Spatio-temporal variation of salt marsh seedling establishment in relationto the abiotic and biotic environment

Contrary to our expectations, soil salinity and moisture explained little o f the spatial variation in plant establishment in the upper intertidal mars h of three southern California wetlands, but did explain the timing of germ ination. Seedlings of 27 species were identified in 1996 and 1997. The seed lings were abundant (maximum densities of 2143/m(2) in 1996 and 1819/m(2) i n 1997) and predominantly annual species. CCAs quantified the spatial varia tion in seedling density that could be explained by three groups of predict or variables: (1) perennial plant cover, elevation and soil texture (16% of variation), (2) wetland identity (14% of variation) and (3) surface soil s alinity and moisture (2% of variation). Increasing the spatial scale of ana lysis changed the variables that best predicted patterns of species densiti es. Timing of germination depended on surface soil salinity and, to a lesse r extent, soil moisture. Germination occurred after salinity had dropped be low a threshold or, in some cases, after moisture had increased above a cri tical level. Between 32% and 92% of the seedlings were exotic and most of t hese occurred at lower soil salinity than native species. However, Paraphol is incurva a and Mesembryanthemum nodiflorum were found in the same environ ments as the native species. In 1997, the year of a strong El Nino/Southern Oscillation event with high rainfall and sea levels, the elevation distrib ution of species narrowed and densities of P. incurva and other exotic spec ies decreased but densities of native and rare species did not change. The 'regeneration niche' of wetland plant communities include the effects of mu ltiple abiotic and biotic factors on both the spatial and temporal variatio ns in plant establishment.