Following Walker and Lundholm’s recent Journal of Applied Ecology paper, Designed habitat heterogeneity on green roofs increases seedling survival but not plant species diversity, Associate Editor Cate Macinnis-Ng discusses enhancing the ecological benefits of the green roof.

Famed for their spectacular and lush green terraces of trees, shrubs and vines, the Hanging Gardens of Babylon were perhaps amongst the first green roofs.

Modern-day green roofs are rarely as grand but they have become a popular way of adding some greenery to urban landscapes. In addition to adding colour, green roofs have the potential to provide a range of ecosystem services including improved storm-water management, reduced urban heat island effects, better insulated buildings and increased urban biodiversity.

One of the biggest limitations on green roofs is the need to provide sufficient soil and other substrate. Green roofs often have only a thin layer of substrate to keep the weight of the roof down and avoid unnecessary fortification of the structure below. This means that resources needed by plants can be limited. This is particularly true of water so succulents are often used to plant green roofs because of their minimal water needs. Succulents are also well adapted to cope with the hot conditions that can develop across the day on a green roof.

In new research, Walker and Lundholm modified substrate and added logs and pebble piles to see if they could alter the temperature and moisture of the substrate. They found deepening the substrate (to 10-12 cm) reduced the soil temperature by almost 15°C and pebble piles reduced moisture loss in a shade house drought experiment. Furthermore, while there was high mortality after drought, seedling density and species richness of 26 native species (added as seeds) were higher where pebbles were added or on deeper substrate compared to controls. The authors concluded these small modifications improved seedling survival through improvement of microsite conditions. This is a positive step towards using green roofs as conservation areas for local native species. Another intriguing aspect of microsite modification that wasn’t explored was the potential for invertebrates to use logs and pebbles as habitat.

We might not be able to recreate the Hanging Gardens of Babylon in our modern cities just yet but with small modifications, the conservation value of green roofs may be enhanced significantly. As to whether the Hanging Gardens of Babylon actually existed and if so where, we’ll leave that to the archaeologists.

The article, Designed habitat heterogeneity on green roofs increases seedling survival but not plant species diversity is available in Journal of Applied Ecology.