Soil, competition, and niche shifts shape the floral mosaic of an annual plant diversity hotspot

PREMISE: Plant species with affinity for harsh substrates often have well-defined edaphic (soil) niches and are ideal for exploring questions of community assembly. Vertic clay soils are chemically and physically challenging to plant establishment and productivity, and annual plant communities associated with these soils of the San Joaquin Desert (California, USA) form a distinctive mosaic pattern of species that reflects differences in soil properties across the landscape. METHODS: We analyzed soil properties to determine how heterogeneous soils at two field sites in the San Joaquin Desert differed among the realized niches of 12 native annual forb species with an affinity for vertic clay soils. We then conducted a pot study with the same species to test if species differed in their realized and fundamental edaphic niches, and to examine the competition effects of an invasive annual grass (Bromus rubens) on these species' edaphic niches. RESULTS: From our field study, we found some differences in the vertic clay soils between the realized niches of species at both sites. In our pot study, we found species had similar fundamental edaphic niche optima in our treatment soils and that several species' competitive ability varied with the edaphic stress in our treatment soils. For some species, differences in competitive ability led to shifts in edaphic niche optima, likely contributing to more divergent realized niches. CONCLUSIONS: The combination of competitive pressure and abiotic stress drove differences between the realized niche and fundamental niche for species in a novel, heterogeneous study system.