Abstract:Extensive green roofs are a promising type of urban green that can play an important role in climate proofing and ultimately in the sustainability of our cities. Despite their increasingly widespread application and the growing scientific interest in extensive green roofs, their aesthetics have received limited scientific attention. Furthermore, several functional issues occur, as weedy species can colonize the roof, and extreme roof conditions can lead to gaps in the vegetation. Apart from altering the function of a green roof, we also expect these issues to influence the perception of extensive green roofs, possibly affecting their acceptance and application. We therefore assessed the preferences of a self-selected convenience sample of 155 Flemish respondents for visual aspects using a discrete choice experiment. This approach, combined with current knowledge on the psychological aspects of green roof visuals, allowed us to quantify extensive green roof preferences. Our results indicate that vegetation gaps and weedy species, together with a diverse vegetation have a considerable impact on green roof perception. Gaps were the single most important attribute, indicated by a relative importance of ca. 53%, with cost coming in at a close second at ca. 46%. Overall, this study explores the applicability of a stated preference technique to assess an often overlooked aspect of extensive green roofs. It thereby provides a foundation for further research aimed at generating practical recommendations for green roof construction and maintenance.

Questions: (a) Will seed banks develop on extensive green roofs (EGRs) -prime examples of novel ecosystems -as in (semi-)natural systems? If so, which type of seed bank (transient or persistent) can be expected? (b) Do EGR characteristics, such as roof age and substrate depth, influence seed bank seed density and species richness? (c) Can differences in successional stages be noticed in the seed bank and vegetation on EGRs? Location: Flanders and Brussels regions of Belgium. Methods:We conducted a field sampling campaign in which the vegetation and seed bank of 109 EGRs were sampled. Information on roof characteristics (e.g. age, substrate depth) was also collected. Seedling emergence was monitored under standardized greenhouse conditions. Species traits were collected from several existing databases. Data analysis consisted of (generalized) linear mixed modelling.Results: A total of 18,466 seedlings from 139 different species emerged in the seed bank analysis. Average similarity between vegetation and the seed bank was low, and the majority of species could be considered spontaneous colonizers. Most species were classified as having a transient seed bank. Life forms were shown to be significantly different between vegetation and seed bank. Older EGRs had higher seed bank species richness and seed density, both of which were also positively influenced by sampling date and the vegetation species richness. A discrepancy between seed bank and vegetation successional state, characterized by functional signatures, was present for EGRs. Conclusions:EGRs do develop seed banks and show parallels with seed banks in other novel ecosystems. Seed bank species richness and seed density are influenced by temporal and vegetation related factors. We can also confirm EGR seed bank succession and the discrepancy between seed bank and vegetation successional state, as often observed in natural and novel systems. Furthermore, we argue that seed banks can function as biodiversity reservoirs, which can be considered in EGR vegetation restoration if a set of several conditions is fulfilled. Overall, this study explores seed bank analysis on EGR as a novel ecosystem model. It thereby provides multiple insights in processes impacting EGR plant species diversity, both on and in the soil. 420 |

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Questions Extensive green roofs (EGRs) are novel ecosystems and essential tools in mitigating the negative effects of urbanization. However, the extent to which traditional community assembly insights apply to spontaneous plant and trait diversity and composition on EGRs and novel ecosystems in general is unclear: (a) is a dispersal filter present because of EGRs’ fragmented nature (e.g., roof height, urban environment); (b) can we confirm the strong abiotic filtering due to extreme EGR conditions (e.g., limited substrate depth); and (c) does the presence of the planted vegetation lead to biotic filtering? Location The Flanders and Brussels region of Belgium. Methods The vegetation of 129 EGRs was sampled during a field campaign. Information on local (e.g., substrate, roof height) and regional characteristics (e.g., connectivity indices) were collected and species traits were collected from multiple databases. All data were used to construct (generalized) linear mixed models that tested the effects of the dispersal‐, local abiotic‐ and local biotic filters on plant community formation. Redundancy analysis was used to investigate predictor effects on functional composition and null models were constructed to assess trait divergence/convergence. Results Our results show that the spontaneous plant species diversity is not affected by their landscape context (dispersal filtering), while their functional composition is. Additionally, the importance of abiotic filtering is confirmed, as substrate depth and exposure have a strong impact on species diversity and decreasing substrate depth negatively impacts functional diversity. Finally, we show that biotic filtering by increased planted species cover leads to trait divergence, changes in functional composition and reduced cover of spontaneous species. Conclusions We find that a broad perspective that includes the regional and local biotic and abiotic environment is needed to assess all factors that influence the vegetation on EGRs. As biotic interactions are present, EGRs should not be considered as static systems.