Introduced and native plant species composition of vacant unmanaged green roofs in New York City

Vacant spaces, such as unplanted, unmanaged, or infrequently accessed green roofs, may play a critical role in sustaining biodiversity in cities. Initial plant composition of these spaces likely is contingent on seed dispersal and growing medium, yet no studies have directly characterized the effects of location or growing medium depth on plant diversity of unplanted green roofs. To address this gap, we studied first-year community composition of five unplanted green roofs in New York City (NY, USA). On each roof, 12 plots were established with media depths ranging from 5 to 13 cm. Four months after installation, all plants were harvested. We observed 29 species across locations; five species were found on all five roofs, while 13 species each were observed on a single roof. Introduced species accounted for 14 of the 29 species found across roofs. Species richness and composition differed among roofs, while functional richness varied within, but not among, roofs. About 80% of the variation in species richness was accounted for by total green space in the neighborhoods within 100 to 200 m of each roof. Functional richness and plant cover increased with increased growing medium depth, as did the relative abundance of annual and forb species. Our results suggest both location and growing medium depth will affect initial community composition of unmanaged green roofs, but with differential effects on community assembly. These results also highlight the potential role of unmanaged vacant roofs as reservoirs of plant diversity, both native and invasive, in urban spaces.