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

Publication Type

Journal contribution (peer reviewed)

Authors

Franzaring, J., Steffan, L., Ansel, W., Walker, R., Fangmeier, A.

Year of publication

2016

Published in

Ecological Engineering

Band/Volume

94/

DOI

10.1016/j.ecoleng.2016.06.021

Page (from - to)

503-515

A mesocosm experiment with monocultures and mixtures of five common extensive green roof plantspecies 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 configurationconsisting of a standard substrate, a system filter and a drainage element. At the study site, both summersproved to be hotter (>2◦C) and drier than the long-term averages, creating severe drought stress in all thespecies 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 notin the mixtures, most individuals of the herb Dianthus carthusianorum and the legume Lotus corniculatusdied during the second season. Later on, the grass Koeleria glauca turned senescent as well and onlythe 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 analysesconfirmed that despite the severe drought the sedum used the C3-instead of the CAM-photosyntheticpathway. 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 washighest in the legume Lotus corniculatus and the grass Koeleria glauca, while water use efficiency anddry mass accumulation were highest in P. floriferus (312 g m−2per year). However, the combination ofthe 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 theprecipitation volume leaving 366 mm of water for evaporation. Planted mesocoms retained on average40% (i.e. 506 mm) of the rainfall that could be used for evapotranspiration. Wash-out of solids fromthe bare substrate amounted to 394 g m−2, but was not significantly higher than the loss of substratefrom planted mesocosms. Under the extreme climatic conditions in present study, plant mixtures werenot superior to monocultures in terms of cooling potentials, water retention and wash-out, but plantsurvivorship and maintenance of a high vegetation cover proved to be the main drivers of these services.