Is Coexistence of Sonoran Desert Annuals Mediated by Temporal Variability Reproductive Success

Models of annual plants with a persistent seed bank have shown that temporal variation can promote coexistence if the reproductive success of species is favored in different environments determined by temporally variable conditions. This study investigates whether this mechanism may explain the coexistence of three Sonoran Desert species (Pectocarya recurvata, Plantago patagonica, Schismus barbatus). In a 2—yr experiment, factors that vary across years (water and seedling density) were manipulated. In addition, the dominant spatial feature, presence or absence of Larrea cover, was also included as a factor. Our aim was to document fitness hierarchies in different types of years. Seedlings were mapped monthly for survival and reproductive success. To compare species, we used 10 yr of data to calculate the average value that seeds of different species have for population growth. Shifts in fitness hierarchies were found for two species pairs (Pectocarya—Schismus and Pectocarya—Plantago), depending on density and either the year or the water level. Surprisingly, all species had higher survival at higher densities in one or another treatment. Habitats were not important to the fitness hierarchies; all species had higher mean survival and fitness in the open than under shrubs.