Abstract
Standard succulent vegetation mixes developed mostly in temperate climates are being increasingly used on green roofs in different climate zones with uncertain outcome regarding vegetation survival and cover. We investigated vegetation on green roofs at nine temperate, cold, and/or wet locations in Norway and Sweden covering wide ranges of latitude, mean annual temperature, annual precipitation, frequencies of freeze-thaw cycles, and longest annual dry period. The vegetation on the roofs were surveyed in two consecutive years, and weather data were compiled from meteorological databases. At all sites we detected a significant decline in species compared to originally intended (planted/sown) species. Both the survival rate and cover of the intended vegetation were positively related to the mean annual temperature. Contrary to a hypothesis, we found that intended vegetation cover was negatively rather than positively related to mean annual precipitation. Conversely, the unintended (spontaneous) vegetation was favoured by high mean annual precipitation and low mean annual temperature, possibly by enabling it to colonize bare patches and outcompete the intended vegetation. When there is high mortality and variation in cover of the intended vegetation, predicting the strength of ecosystem services the vegetation provides on green roofs is difficult. The results highlight the needs for further investigation on species traits and the local factors driving extinction and colonizations in order to improve survivability and ensure a dense vegetation throughout the successional stages of a green roof.
Highlights
Green roofs are becoming increasingly popular in urban areas, partly because of their architectural values and partly due to their potential multi-functionality (Dusza et al 2017)
Accumulated precipitation, duration of the longest drought, and frequencies of freeze-thaw cycles differed between years, but there was no significant difference in vegetation cover between the years (p > 0.05, in electronic supplementary material)
We examined vegetation dynamics on standard Sedum mixes grown in vegetation mats on roofs in Scandinavian climates and effects of weather-related factors, and detected significant losses of species on all of roofs, in nine locations, in the second survey year
Summary
Green roofs are becoming increasingly popular in urban areas, partly because of their architectural values and partly due to their potential multi-functionality (Dusza et al 2017) They can make important contributions to restoration of some of the ecosystem services lost through construction of buildings in densely populated areas In efforts to ensure the establishment of dense, persistent vegetation cover, to a large extent the industry relies on Sedum species that grow naturally in shallow substrates, tolerate long periods of drought, and form dense ground cover (Dvorak and Volder 2010) Their lack of requirement for deeper substrates helps to keep building loads down (Durhman et al 2007), an especially attractive feature in areas with cold climates and potentially heavy snow loads in winter
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