SEED BANKS IN DESERT ANNUALS - IMPLICATIONS FOR PERSISTENCE AND COEXISTENCE IN VARIABLE ENVIRONMENTS

It is widely believed that desert annual plants maintain between-year seed banks, yet few field studies actually have measured the proportio n of the viable seed bank that remains dormant through a season. Dorma ncy and germination fractions were quantified for a guild of winter an nuals on a creosote flat in the Sonoran Desert for three years. Predic tions from two types of theoretical models applicable to temporally va riable environments were examined: (1) the evolution of life history t raits promoting persistence in the face of temporal variation and (2) the role of temporal variation in mediating species coexistence. The d ensity of ungerminated seeds was estimated by collecting soil samples after germination, but prior to new seed set. Seedlings were followed in nearby plots to estimate the density of germinated seedlings and th eir reproductive success. Long-term data collected from permanent plot s over a 10-yr period were used to calculate temporal variation in rep roductive success for each species. Species with higher temporal varia tion in reproductive success had lower germination fractions and small er seeds, consistent with the theory that seed dormancy and large seed size are partially substitutable bet-hedging strategies. The data als o suggested that this system possesses traits that are necessary for t emporal variation to promote coexistence. First, between-year seed ban ks, necessary to buffer populations in unfavorable years, were documen ted for 17 species. Second, there was a strong tendency for year-to-ye ar variation in germination fractions to vary among species. Finally, plants germinated more in years of higher reproductive success. We dis cuss how a correlation between germination and reproductive success en hances the role of temporal variance in success hierarchies in promoti ng species coexistence.