Most would agree that biodiversity is important for a sustainable world. On a much smaller scale it also applies to green roof and walls. Biodiversity can be enhanced by providing environmental conditions that will support a diversity of plant species and then in turn these plant communities may provide habitat for wildlife.
One of the main factors that influences plant diversity is substrate moisture which is directly related to substrate depth. The importance of depth was evident in a plant community study conducted on the MSU Molecular Plant Sciences Building (Vandegrift et al (2019), Ecological Engineering 138:264-273, https://dx.doi.org/10.1016/j.ecoleng.2019.07.032). Seventeen species of native perennials and grasses were planted at two depths, 10 cm and 20 cm, irrigated for two years until established, and evaluated over a period of eight years. Only seven of the original species survived the entire time and Allium cernuum was by far the dominant species at both depths. Sporobolus heterolepis and Koeleria macrantha were also present in the 20 cm zone, but died off in the 10 cm zone. Substrate moisture was likely the main cause as many species did not survive a moderate drought that occurred during 2016. The study emphasizes that many herbaceous perennials may require supplemental irrigation during drought periods and it points out the importance of long-term studies to accurately evaluate green roof plant communities.
Although the lack of substrate moisture contributed to plant death, a decrease in plant diversity is not uncommon. A similar decrease in diversity was observed on an irrigated roof over six years in Sheffield, UK (Dunnett et al (2008) Urban Ecosystems 11:373–384, https://doi.org/10.1007/s11252-007-0042-7). Numerous other studies have found similar decreases in diversity over time with the main cause usually attributed to drought. In contrast, a study conducted on an intensive roof in Germany found that diversity increased over time before reaching an equilibrium (Catalano et al (2016) Landscape and Urban Planning 149:11-19, https://doi.org/10.1016/j.landurbplan.2016.01.003). The main differences between this roof and the others were the deeper substrate (26 cm) and the fact that ruderal colonizers were not weeded. Colonizing species could become established, whereas on the shallower extensive roofs they were either removed or died because of the shallower depth. If species diversity is desired then removal of colonizing species should be avoided. In addition, designing a roof with various depths will increase plant diversity as different species find their niche where they can compete the best.