Effects of variable precipitation on the structure and diversity of a California salt marsh community

Abstract. We measured the effects of annual variation in climate and experimentally augmented rainfall on patterns of distribution and above‐ground productivity in annual plant communities at Carpinteria Salt Marsh in central California. In the driest year, Hutchinsia procumbens was codominant throughout much of the upper marsh; however, Hutchinsia was very rare or not present in the wetter years. Conversely, Juncus bufonius was common in the wettest year and absent in the driest year. Elevational distributions of other annual species also differed among years with different total precipitation. In 1989–1990, an exceptionally dry season, supplemental water decreased soil salinity, increased above‐ground productivity of annuals, and caused significant changes in spatial patterns and relative density. In the lowest zone, Hutchinsia occurred only in watered plots and supplemental water increased the density of Spergularia marina. At intermediate elevations Lasthenia glabrata occurred only in watered plots and supplemental water increased the density of Spergularia, Hutchinsia, and Parapholis incurva. At upper elevations, Juncus occurred only in watered plots, and Lasthenia was the only species that increased significantly in density with watering. Unlike natural shifts in species abundance, no species declined significantly in cover in any zone in the watered treatment. Although climatic variation has complex affects on annual plant communities, our experiments isolated important affects of total annual rainfall on the structure of annual plant communities that were similar to those that occurred with natural variation in rainfall. We conclude that variation in total annual precipitation promotes dynamic community composition and spatial distributions among years, and thus increases overall species diversity in the salt marsh. 1994 IAVS ‐ the International Association of Vegetation Science