Interspecific Competition Between Seeds: Relative Planting Date and Density Affect Seedling Emergence

The interactions between seeds in the soil are poorly understood. We performed greenhouse experiments to investigate the effects of seed density, relative frequency, and relative planting date on the emergence of seedlings in the species Senecio vulgaris, Capsella bursa—pastoris, and Poa annua. We found that for both Poa and Senecio, the probability of emergence significantly decreased with an increase in total seed density. Neither the density of conspecifics nor heterospecifics alone could explain this decline in the probability of emergence. We also found that, for all three species studied, the rate of emergence accelerated in the presence of previously planted seeds. A second experiment indicated that this acceleration involves a response to leachate from previously germinated seeds.