Living at the edge: Varying substrate depth on green roofs affects the microclimate and plant establishment

Green roofs are mainly created as extensive green roofs with a few centimetres of substrate. This limits their water-holding capacity and the number of suitable plant species. Roofs incorporating areas with thicker substrate depths and structural heterogeneity may offer a promising balance between the low weight and cost of extensive greening and improved growing conditions. However, the effects of this design on microclimate and vegetation establishment have been little studied. On a large extensive green roof in Tyrol, Austria, we found that roof areas with increased substrate depth had significantly reduced maximum soil and surface temperatures (from 47.7 and 57.1 °C with 7 cm substrate to 38.7 and 49.6 °C with 14 cm substrate, respectively). Deeper substrate increased water resources and allowed higher cover of vascular species and non-succulent herbs but had no effect on mean plant ecological indicator values. Sedums showed more stable plant water potentials (maximum −0.6 MPa) compared to non-succulent herbs (maximum −2.2 MPa), but the former suffered from leaf temperatures above 52 °C and associated photosynthetic impairment. Deadwood piles on gravel allowed the growth of plant species not otherwise found on the roof. Our results demonstrate that even small variations in substrate depth can create distinct microclimates on an otherwise homogeneous green roof. More favourable temperature and moisture conditions in deeper substrate areas can promote the growth and diversity of plants. Green roofs with varying substrate depths could thus help to avoid uniform sedum-dominated vegetation and contribute to reducing the environmental impact of large industrial buildings. • Interplay between microclimate and plant physiology and composition on a green roof • Increasing substrate depth from 7 to 14 cm reduced soil temperature fluctuations • Non-succulent herbs reached lower water potentials than sedum species • Plant physiological traits, NDVI and EIV reflected drought and heat stress • Substrate depth variation and deadwood piles increased plant species richness