The fitness effects of outcrossing distance depend on parental flowering phenology in fragmented populations of a tallgrass prairie forb

The phenomena of isolation-by-distance and isolation-by-time shapecontra mating patterns and population genetic processes, such as inbreeding and outbreeding depression, which influence progeny fitness. However, the effects of parental isolation in time on offspring fitness remain understudied, especially in combination with isolation-by-distance. We planted offspring from a common garden experiment involving 13 populations of the tallgrass prairie forb Echinacea angustifolia into a prairie restoration and tracked their fitness over 16 yr. Parental source populations were up to 9 km apart, and flowering asynchronies spanned up to 13 d. Using Aster life-history analysis, we assessed how interparent distance and asynchrony affected offspring fitness. Interparent asynchrony modified the relationship between interparent distance and offspring fitness. Offspring with the highest fitness had parents from the most distant populations, with maternal plants flowering later than paternal plants. Notably, the order of parental flowering, rather than the absolute difference in timing, better predicted fitness. Nongenetic aspects of reproductive timing, such as morphological constraints, may have contributed to these effects. We suggest management strategies to promote outcrossing over space and time, including leveraging seed production environments.