Water retention, wash-out, substrate and surface temperatures of extensive green roof mesocosms—Results from a two year study in SW-Germany

• The succulent Phedimus floriferum uses the C3-photosynthetic pathway.
• Cooling depends on maintaining high vegetation cover during heat spells.
• Water retention is higher in grass species and is slightly related to plant cover.
• Wash-out of solids decreases rapidly over time and is independent of species.

A mesocosm experiment with monocultures and mixtures of five common extensive green roof plant species was performed to study water retention, wash-out of solids and thermal behavior over 24 months. Plants were grown as equal-sized plug plants at the same density in a typical system configuration consisting of a standard substrate, a system filter and a drainage element. At the study site, both summers proved to be hotter (>2 °C) and drier than the long-term averages, creating severe drought stress in all the species and most probably being representative of future summers under climatic change in Southern Germany. While the mini-shrub Thymus serpyllum died off in the first season in the monocultures but not in the mixtures, most individuals of the herb Dianthus carthusianorum and the legume Lotus corniculatus died during the second season. Later on, the grass Koeleria glauca turned senescent as well and only the succulent Phedimus floriferus cv. “Weihenstephaner Gold” (=Sedum kamtschaticum var. floriferum), was able to stay alive in all of the combinations until the end of the experiment. 13C isotopic analyses confirmed that despite the severe drought the sedum used the C3-instead of the CAM-photosynthetic pathway. As shown by continuous measurements of substrate temperatures and infrared thermometry, the species turned out to have the highest and longest cooling potential out of the tested species. Overall, vegetation cover and shoot mass affected substrate cooling and water retention. Water capture was highest in the legume Lotus corniculatus and the grass Koeleria glauca, while water use efficiency and dry mass accumulation were highest in P. floriferus (312 g m−2 per year). However, the combination of the succulent with the N-fixing legume L. corniculatus yielded 37% more dry shoot mass and a higher vegetation cover. During the whole period, water retention of the bare substrate amounted to 29% of the precipitation volume leaving 366 mm of water for evaporation. Planted mesocoms retained on average 40% (i.e. 506 mm) of the rainfall that could be used for evapotranspiration. Wash-out of solids from the bare substrate amounted to 394 g m−2, but was not significantly higher than the loss of substrate from planted mesocosms. Under the extreme climatic conditions in present study, plant mixtures were not superior to monocultures in terms of cooling potentials, water retention and wash-out, but plant survivorship and maintenance of a high vegetation cover proved to be the main drivers of these services.