Maintaining genetic diversity is important for population persistence in many insect-pollinated forbs, particularly those that occur in fragmented habitats. In cities, genetic diversity of plant populations established from nursery stock can be low due to the collection and propagation methods. Fragmentation and lack of effective pollinators in cities can inhibit gene flow by impeding the movement of pollen, exacerbating the loss of genetic diversity over time. The combination of low genetic diversity and lack of gene flow could eventually lead to inbreeding depression or local extinction and negating some of the ecological benefits of including native plants in urban landscapes. In this study, we hypothesized that 1. populations of a self-compatible forb established on urban green roofs from nursery stock would have lower genetic diversity than naturally occurring populations and 2. limited gene (pollen) flow between isolated green roof populations would result in high incidence of self-pollination and inbreeding. We used nine microsatellite loci to measure the neutral genetic diversity between three established green roof and three natural populations of the bee-pollinated forb, Penstemon hirsutus. We established additional green roof populations and conducted a paternity analysis to measure gene flow and compare inbreeding between parental and offspring generations.
Results/Conclusions
We found that neutral genetic diversity was lower in green roof populations established from nursery stock than in naturally occurring populations in the same region, supporting our first hypothesis. However, natural populations had higher inbreeding compared to those on green roofs. Our paternity tests supported our second hypothesis; although inbreeding differed between populations, an average of 75% of offspring had pollen donors from within the same roof population. On the other hand, our results also demonstrate that pollen is exchanged across the larger metapopulation, with at least 25% of offspring resulting from pollination between roofs, likely carried out by small bees. These findings demonstrate that gene flow between native plant populations occurs in fragmented urban habitats. Green roofs may therefore help to maintain genetic diversity of native plant populations as parts of larger metapopulations in urban environments.