Green roofs and stormwater planters: hydraulic efficacy, biodiversity benefits and experimental optimization

For over 50 years, scientists have warned us of the consequences we would face if nothing was done to protect the environment and tackle social injustice. These warnings went largely unheard. Biodiversity is being lost at unprecedented rates, while changes in extreme weather patterns lead to ecological displacement, increasing human urban population. When surfaces are sealed off, flood risks increase and natural habitats are fragmented. To address these problems, cities are turning to green infrastructures. These infrastructures absorb and purify rainfall and runoff, while supporting biodiversity and improving human wellbeing among other things. This study, based in Dublin, Ireland, looked at a number of green infrastructures. First, 6 full-scale green roofs were studied for their invertebrate mesofauna and macrofauna over a 1-year period. The difference between the fauna of the various roofs was analysed, as well as the difference with a ground control. The mass loss of teabags buried for 3 months in the green roofs, gave indirect information on decomposition activities. Then, the capacity of retaining, delaying, and purifying rainwater of 2 full-scale green roofs and 2 control roofs was compared over a 6-month period. Finally, 5 simulated rain events were used to test the hydraulic performance of 60 stormwater planters with different designs, vegetation, and earthworm presence. Plant survival and flowering success were assessed a year after installation. Woodlice, slugs, snails and millipedes were dominant on the three intensive and semi-intensive roofs while flies, spiders, true bugs, wasps and snails were more prominent on the three extensive roofs. Mesofauna taxa did not split as clearly but some less studied taxa were found, such as tardigrades, psocopterans, pseudocentipedes, proturans and neelipleona springtails. Difference in soil moisture, soil depth and vegetation structural complexity could explain some of these observations. The ground control was more biodiverse than its associated green roofs, although it had a lower invertebrate abundance. Teabags mass loss was highest on the two intensive roofs and lowest on the three extensive roofs. Semi-intensive roofs had mostly similar mass loss compared to their ground control. Extensive roofs were better than bare roofs at retaining and delaying runoff, though a negative impact of rain intensity was observed. Water quality from the green roofs was variable, with higher loads of zinc, E. coli and sometimes phosphorus compared to bare roofs. Yet, their pollutant load was mostly lower than the control one, and their pollutant concentrations mostly below regulatory thresholds. Design impacted the stormwater planters’ water retention capacity mostly in the first few simulations. Hardy plants absorbed more water, although not significantly, but were adapted to all designs, contrary to ornamental plants. The tested designs were not compatible with the soil moisture requirements of earthworms thus they migrated or perished. This research can inform the design and use of green infrastructures in an urban context. Green roofs can support invertebrates and indirectly their vertebrate predators. Possible negative effects relating to invasive species and ecological trap require further investigation. Alternative designs to standard shallow extensive roofs should be encouraged to improve their biodiversity and hydraulic benefits. A rain harvesting stormwater planter with a mix of ornamental and hardy plants could combine higher hydraulic performances with people’s preferences of flowers over grasses. Finally, scientists such as hydrologists, ecologists and sociologists should collaborate, together with the wider public, to broaden their views on the impacts, both positive and negative, that green infrastructures can have on society and the environment