Traits for stress-tolerance are associated with long-term plant survival on green roofs

Abstract

Green roofs may contribute to biodiversity conservation. However, these habitats are not typically planted to support biodiversity and are not monitored to see how communities change over time. Plants on green roofs must be adapted to high stress caused by drought and severe temperatures. Over several generations, plant functional diversity may decrease, as only species able to colonize and survive in harsh habitats persist. If particular traits aid in survival, careful species selection could direct the trajectory of green roof succession and ensure future biodiversity support. In this study, we used long-term vegetation surveys from six green roofs in northeast Germany to discern patterns of plant traits and functional diversity. To determine if site managers could predict future green roof diversity by extrapolating from a common pattern of change, we compared our results with analyses of a chronosequence (1-93 years since establishment) of 13 sites in the same region. We also measured site-level properties of the chronosequence sites to explore factors other than time that may contribute to functional diversity. We found that plant functional diversity generally increased during an initial establishment period but subsequently decreased. Mature communities were primarily made up of species highly tolerant of stress and repeated disturbance. Our results differed slightly between the long-term and chronosequence site types, highlighting the uniqueness of green roof communities. Site-specific factors, including building height, vegetated area, soil depth and soil water-holding capacity, likely contribute to plant functional diversity on green roofs.