Seed mass and emergence time effects on performance of Panicum dichotomiflorum Michx. across environments

The effects of seed mass and emergence time on growth and reproduction of the annual, C₄ grass, Panicum dichotomiflorum Michx., were investigated in an experimental field study. Individual seeds of P. dichotomiflorum of known mass were sown into marked plots in four newly-abandoned fields that differed in soil nutrient availability, soil moisture and the time of the year in which they were abandoned. Seedling emergence was monitored daily for four weeks following sowing. In none of these fields was there any relationship between seed mass and emergence time. Analyses of covariance indicated that the effects of seed mass and emergence date on plant performance varied among fields. Seed mass affected performance in one field (SOUTH EARLY) where abiotic stress after emergence was high. In contrast, later emergence significantly reduced performance in all four fields. We used a path analysis to decompose the direct and indirect effects of seed mass and emergence time on plant performance in each field. The path analysis revealed that in all four fields, final weight had a significant positive effect on seed production. The effect of seed mass and emergence time had variable (and often opposite effects) on these traits. Seed mass only had a significant positive, direct effect on seed production in the most abiotically stressful field (SOUTH LATE). In the other fields, the effects of seed mass on fecundity were mediated by the effect on final weight. Thus, the effects of seed mass and emergence time on growth and reproduction in P. dichotomiflorum vary across environments.