Urbanization Alters Phenology, Mating System Allocation, and Life History of (Balsaminaceae) via Trait-Specific Plasticity and Genetic Differentiation.

Urbanization is a major human-mediated driver of environmental change. Plants in urban environments may differ in timing and investment in key life history traits compared to rural plants as a result of genetic differentiation or plastic responses to the urban environment. However, it is unclear for many species whether genetic differentiation or plasticity has shaped urban phenotypes. is an annual plant that produces self-pollinating and outcrossing flowers, varying in timing and amount based on environmental conditions. In this study, we characterized differences in floral phenology, mating system allocation, and key life history events between urban and rural populations of , in situ and in greenhouse common gardens. We asked whether (1) floral investment varies with urbanization in situ, if (2) the differences between urban and rural populations are maintained (genetic differentiation) or lost/exaggerated (plasticity) in a common garden, if (3) differences can be attributed to shifts in life history strategy, and if (4) urban population traits are more variable (higher coefficient of variation). In situ, we found that urban populations advanced flowering time and invested more in outcrossing flowers compared to rural populations. Within greenhouse common gardens, urban plants maintained advanced flowering and were less variable than rural plants (low CV), indicative of genetic differentiation. In contrast, urban plants lost outcrossing bias in mating system allocation observed in situ, indicating plasticity, although both urban and rural plants were highly variable (high CV) for this trait. Early onset of selfing flowers was tied to earlier germination, but outcrossing onset was not affected by germination time. Flowering probability in urban plants was higher than rural ones in common gardens. Our study demonstrates that urbanization influences plant phenotypes through both genetic differentiation and phenotypic plasticity, but the relative importance of the two mechanisms of change vary among floral traits.