No abstract

Green roofs provide multiple environmental benefits. These include storm-water retention, water purification, insulation of the building, air quality and carbon sequestration and biodiversity enhancement. Green roofs can be classified as sustainable extensive green roofs (no additional fertilizer, no gardening, no irrigation and shallow growing substrate) or as intensive green roofs (additional fertilizer, gardening, irrigation and deeper growing substrate). In this review, we concentrate on multi-disciplinary benefits of green roofs - i.e. biodiversity enhancement (i.e. reconciliation ecology), storm-water retention and air-pollution abatement as these benefits highly support sustainable buildings. We have found that under conditions of very fine-scale heterogeneity, species diversity (both plants and arthropods) is unlikely to increase, particularly if there is no irrigation. Under large-scale heterogeneity (such as different types of green roofs), the regional diversity can increase. Some plants compete with each other and others can create facilitation. Under conditions of irrigation in the hot, arid conditions of the Middle East, photovoltaic panels may likely produce more electricity and the heterogeneity in solar radiation and moisture can result in greater biodiversity. Rainwater can also be deterred from storm drainage by using gray water irrigation for specific plant assemblages. Green roofs can help mitigate the negative effects of air pollution and CO2 emissions - in particular, by absorbing pollutants and sequestering carbon dioxide.

Green roofs are expected to contribute to higher biodiversity in urban surroundings. Typically, green roofs have been designed with low plant diversity. However, plant diversity can be enhanced by controlling resource availability and creating distinct niches. Here we hypothesize that by using different drainage heights during the short plant-growing season in a semi-arid green roof system we can create distinct niches and plant communities. Our experiment took place at the University of Haifa, north Israel. We tested three different heights of drainage outlet: 10 cm under the surface of the substrate (Low), 1 cm under the surface of the substrate (Medium) and 3 cm above the surface of the substrate (High) on plant species-composition in green-roof gardens. Grasses cover was higher in High and Medium drainages while forbs cover was higher in Low drainage. Species richness was the highest in Low drainage while diversity indices showed the opposite trend. We conclude that by changing the height of the drainage we can create different niches and change species composition in a short time period of one growing season. This way we can create more diverse green roof communities and enhance biodiversity in urban areas, particularly in semi-arid regions.