Effects of water stress and soil microbial communities on phenology and performance in Clarkia xantiana
Barring dispersal limitation, a species' distribution is ultimately constrained by a failure to adapt to novel conditions outside of its range. Research on range limits in plants has historically focused on abiotic drivers, but recent work has shown that interspecific interactions can be equally important in constraining species’ distributions through direct or interactive (biotic x abiotic) effects. Soil microbial communities are known to have important effects on plant performance and local distributions of plant species, but their potential to drive geographic distribution patterns is unclear.
Clarkia xantiana is an annual plant restricted to the southern Sierra Nevada foothills, with two subspecies that inhabit parapatric ranges in the east and west. Historical and population genetic evidence suggests that the subspecies border has remained stable for many generations despite no physical barriers to dispersal. We utilized a fully factorial experiment to assess the effects of water availability and local / foreign soil microbial communities on multiple aspects of performance and phenology in the two subspecies. Plants from one population of each subspecies were grown in the greenhouse with one of five regional soil microbial inocula, and subjected to high or low water stress. Inoculum sources were either from one of the two focal populations, from one other population within each subspecies’ range, or were a sterilized control.
ANOVA was used to test the effects of species, microbial community, water level and the interaction of microbial community x water on flower production, shoot biomass, days to first flowering and days to end of flowering. Species and water level had highly significant (P < 0.001) effects on all four response variables, with C. x. xantiana producing more flowers, accumulating more biomass, and beginning and ending flowering later than C. x. parviflora. Plants under high water stress were smaller and produced fewer flowers, and began and ended flowering earlier than well-watered plants. Microbial community had a significant (P < 0.01) effect on both phenological response variables but not on flower production or biomass. However, there was a marginally significant interaction between microbial community and water level on flower production. Further analyses of the data are underway, but these preliminary results provide evidence that for C. xantiana, regional variation in soil microbial communities can affect important aspects of flowering phenology, and that water stress (which also varies across the subspecies’ border) is important to both phenology and fitness in both subspecies.