Urbanization affects plant species diversity of stormwater ponds in a large German city

Abstract

Urban stormwater ponds (SPs) are important for flood protection strategies in areas with heavily sealed soils as they mitigate the negative effects of floods. SPs with temporary waters (TWs) are only flooded when water levels or surface runoffs are high, while SPs with permanent waters (PWs) contain a permanently flooded area. In urban areas, SPs are important secondary habitats for plants and animals as natural aquatic and semi-aquatic habitats are often destroyed during urbanization. In 2017, a large survey on vascular plant diversity of 80 SPs was conducted in the city of Hamburg (Germany). The aim of the study was to analyze plant diversity of TWs and PWs and to identify relationships between plant diversity and different urbanization parameters. In total, 30% of the plant species pool of Hamburg was found in the surveyed SPs representing 0.03% of the total area of Hamburg. We found significant differences in the number and proportion of aquatic, threatened and non-native species between the shoreline and bottom of TWs and PWs. The number and proportion of aquatic species was positively correlated with the proportion of urban green and negatively correlated with the degree of soil sealing in the direct surrounding of SPs. For the proportion of threatened species in the bottom of TWs, a positive relationship was found with the proportion of urban green in the surrounding of SPs. For the number and proportion of non-native species in the shoreline of SPs, negative relationships were found with the proportion of urban green and the number of different habitat types in the surrounding of SPs. Further, a positive relationship with the degree of soil sealing was identified. Our results highlight the nature conservation value of urban SPs for plants and showed the negative effects of urbanization on several plant species groups.