IS COEXISTENCE OF SONORAN DESERT ANNUALS MEDIATED BY TEMPORAL VARIABILITY IN REPRODUCTIVE SUCCESS

Models of annual plants with a persistent seed bank have shown that te mporal variation can promote coexistence if the reproductive success o f species is favored in different environments determined by temporall y variable conditions. This study investigates whether this mechanism may explain the coexistence of three Sonoran Desert species (Pectocary a recurvata, Plantago patagonica, Schismus barbatus). In a 2-yr experi ment, factors that vary across years (water and seedling density) were manipulated. In addition, the dominant spatial feature, presence or a bsence of Larrea cover, was also included as a factor. Our aim was to document fitness hierarchies in different types of years. Seedlings we re mapped monthly for survival and reproductive success. To compare sp ecies, we used 10 yr of data to calculate the average value that seeds of different species have for population growth. Shifts in fitness hi erarchies were found for two species pairs (Pectocarya-Schismus and Pe ctocarya-Plantago), depending on density and either the year or the wa ter level. Surprisingly, all species had higher survival at higher den sities 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.