Effects of Seed Size and Seedling Emergence Time on the Fitness Components of Ambrosia trifida and A. artemisiaefolia var. elatior in Competition with Grass Perennials

Germination physiology responsible for seedling emergence timing of Ambrosia trifida and A. artemisia‐efolia var. elatior as well as the effects of seed size and seedling emergence time on the fitness were studied in order to understand successful invasion by these annuals of a perennial grass community. Seed biomasses of these species differ greatly ( A. trifida , 34.6±14.7 mg; A. artemisiaefolia var. elatior , 4.7±1.8 mg). Seed germination /dormancy responses to temperature were rather similar. The primary dormancy of the seeds of both species could be effectively removed by a combination of moist chilling and alternating temperature regimes. The variation of the rate of germination among individual seeds in the laboratory tests well explains the variation of seedling emergence time in the field. Microsite variation of light availability had little effect on the fitness and its components in either species. In A. trifida , both initial seed biomass and emergence time significantly influenced the fitness and its components. No significant correlation between the fitness and seed size was found for A. artemisiaefolia var. elatior.