Life-cycle components of selection in Erigeron annuus. I. Phenotypic selection

The magnitude and direction of phenotypic selection on emergence date and seedling size in Erigeron annuus was measured to determine the heterogeneity of selection among sites and the proportion of fitness variance explained by seedling size and emergence date. Three disturbance treatments (open, annual vegetation, perennial vegetation) were imposed to test the hypothesis of stronger selection on seedlings in competitive environments. Selection was most heterogeneous early in the life cycle, with significant spatial heterogeneity in the magnitude of selection on a local scale. The disturbance treatments affected only fecundity selection gradients and selection was strongest in open plots. Significant variation in the sign of selection differentials on emergence date was observed for establishment and fall viability selection episodes; at later stages selection varied in magnitude but not direction. Seedlings in the earliest cohort experienced high mortality during establishment, but increased size and fecundity later in the life cycle. Both stabilizing and disruptive selection on emergence date were observed during establishment, but in general selection was purely directional. At Stony Brook most selection on emergence date operated indirectly through seedling size, whereas at the Weld Preserve direct selection was stronger. There were persistent effects of both seedling emergence date and rosette diameter on adult fitness components, and October rosette diameter explained 18% of the total phenotypic variance in fecundity. Overall, viability fitness components were much more important than fecundity selection. Winter survivorship was the single most important episode of selection.