PS 4-49 - Niche shifts, competition and edaphic differences shape a floral mosaic on vertic clay soils in California's San Joaquin Desert

Abstract: Plant communities that occur on edaphically heterogeneous landscapes are ideal systems for exploring questions of niche and community assembly. Species with affinity for harsh substrates often have well-defined edaphic niches and are ideal models for testing ecological and evolutionary theory. While plants on substrates such as serpentine and gypsum have received much attention, those on other harsh substrates, including on vertic clay soils, have gone unstudied. Vertic clay soils are both chemically and physically challenging to plant establishment and productivity. Plant communities associated with vertic clay soils of the San Joaquin Desert appear to have a distinctive mosaic pattern of species distribution that reflect differences in soil properties across the landscape. We conducted fieldwork and a pot study with 12 native annual plant species with an affinity for vertic clay soils to determine whether the vertic clay soils at two sites in the San Joaquin Desert were heterogeneous, whether soil heterogeneity determined the pattern of species distributions observed, and to examine the competition effects of an invasive annual grass (Bromus madritensis) on these species. We sampled soils from species patches and had soils analyzed to examine species’ realized edaphic niche, and found that the vertic clay soils at both of our sites are internally heterogeneous, that soil heterogeneity does appear to shape the patchy distribution of species at both these sites, and that these species have different realized edaphic niches. In a pot study conducted with the same species, we utilized treatment soils spanning a gradient of chemical stress present at both study sites and found that competition from B. madritensis reduces biomass for all species, and that the effect of competition differed between soil types. Further, we found that species’ edaphic niche optima shift when competition is present, and that competitive ability differed across the gradient of edaphic stress in our treatment soils.