No abstract

Novel habitats can support biodiversity by amending what has been lost through urban development. However, the effects of fragmentation, disturbance, and altered availability of resources in cities can prevent many local plant species from establishing and persisting. Novel habitats like green roofs could be colonized by native plants if species could overcome particularly harsh environmental conditions. To do so, green roofs could be designed using a habitat analog approach wherein natural habitats with similar abiotic characteristics inform the potential species pool. In this study, we tested the efficacy of using a habitat analog approach to establish native plant communities on green roofs. We surveyed vegetation in 18 dry prairies of three subtypes (gravel hill, dolomite, and sand) and planted replicates of two communities (sand prairie and rock prairie, which combined gravel hill and dolomite) on green roofs to determine which prairie was the most suitable analog. We investigated the effect of three environmental variables on plant establishment, survival, and growth: soil continuity (continuous soil vs. isolated trays), planting method (seeds vs. pre-grown seedlings), and the addition of native arbuscular mycorrhizal (AM) fungi. We determined the most suitable habitat analog and measured the effect of the environmental variables by conducting vegetation surveys in the experimental roof plots for three years. The experimental rock prairie communities more closely resembled the target than did the sand prairies, supporting the hypothesis that shared soil properties are important for establishing analogous plant communities in novel habitats. At the community level, survival and growth were higher in continuous soil, highlighting the importance of belowground components of constructed habitats. We found no effect of planting method or addition of native AM fungi on plant survival or growth. Overall, our results support using a habitat analog approach to select native species that support biodiversity in constructed novel habitats like green roofs.

Gene flow between populations can help maintain genetic diversity and prevent inbreeding, which is especially important for small, fragmented habitats. Many plant species rely on pollinators to move pollen between populations. In urban areas, insufficient pollinator services may result in limited gene flow, which can have negative consequences such as genetic drift and inbreeding depression. Furthermore, restored populations that are established with few founders of low genetic diversity may have limited long-term population persistence. Here, we tested the hypotheses that populations of a self-compatible forb established on urban green roofs from nursery stock are genetically depauperate and that limited gene (pollen) flow between populations will result in increased inbreeding. We compared the neutral genetic diversity of Penstemon hirsutus, using nine microsatellite loci, between three green roof populations established from nursery stock and three natural populations. We also established ten experimental populations on green roofs and measured rates of outcrossing and inbreeding and identified the movement of pollen within and between roofs using a paternity analysis. We found that neutral genetic diversity of populations established from nursery stock was lower than that of natural populations, although the level of inbreeding was also lower on the green roofs. In our experimental populations, we found that the rates of outcrossing and inbreeding varied between the roof populations. Our results suggest that inbreeding may be correlated with cover of co-flowering species but not with any of the other measured site properties. The location of likely pollen donors suggested that on average, 75% of pollen was derived from plants within the population (including self) and 25% came from plants on different roofs. Our results document realized pollen movement within and between green roofs, demonstrating that these habitats provide important connectivity in a fragmented environment.