Vegetation development on different extensive green roof types in a Mediterranean and temperate maritime climate

• Climatic conditions greatly affect extensive green roof vegetation development.
• Deeper substrate and water retention layer result in better vegetation performance.
• Sheltered extensive green roofs can sometimes result in higher cover values.
• Weed and bryophyte establishment is part of the natural vegetation dynamics

Green roofs offer a way to improve ecosystem services in urban areas. In the Mediterranean, specialized companies are still unfamiliar with green roof installation because they consider dry conditions and appropriate plant selection as obstacles. Also in a temperate maritime climate, green roof design and vegetation composition need to be adapted to account for the projected climate changes, including an increase in temperature and more irregular precipitation events. This paper discusses the results of a two-year experiment on unirrigated extensive green roofs which were constructed in both a Mediterranean (Avignon, France) and a temperate maritime climate (Heverlee, Belgium) and on which 18 drought-adapted plants with Mediterranean habitat affinities were tested. Experimental plots differed in solar exposure and substrate type (substrate depth of 5 cm and 5 or 10 cm above a water retention layer). Climatic conditions affected vegetation development in both locations, with fluctuating cover values in the Mediterranean reflecting the impacts of summer droughts and rainy autumn periods. Overall, the deepest substrate depth of 10 cm with a water retention layer resulted in the highest cover and abundance values and the highest species diversity throughout the experiment. Both bryophytes and weeds established spontaneously over the two years, and their importance in terms of biodiversity value is discussed. To improve functional biodiversity of the extensive green roof, we advise to use succulent species together with annuals, perennial herbs and geophytes. The design and species recommendations shed some light on the possibilities for extensive green roof design in regions with summer drought and irregular precipitation and regions facing climate change.