Green roof biodiversity and aesthetics

Both vegetation abundances and community compositions play important roles for the functions of green roofs. Examples are e.g. stormwater retention, but, given the multi-functional character of green roofs, also urban biodiversity, aesthetic appearance, etc. However, green roof vegetation can change significantly over time, which consequently also may affect the functions related to them.

Thus, within DRIZZLE, we performed a study aiming on evaluating the vegetation on green roofs in northern Sweden, in terms of presence/absence of plant species. This included evaluating to which extent the originally intended (i.e. delivered by the industry) species, as well as the colonising unintended species where present on the roofs and which percentage of the roof area they covered. Further, relationships between biotic factors (vegetation diversity, composition and abundance) and abiotic factors (e.g. roof age, substrate depth, slope) were explored. Due to the short growing seasons and long, cold winters in northern Scandinavia, vegetation planted on green roofs in regions with a Nordic climate may have low survival rates since the industry is relatively immature delivering similar solutions to over whole Europe which often are not adapted to local/regional conditions.

Results showed that on average 76% of the intended plant species were absent. This shows the demand for using better adapted species selections to improve survival and cover in demanding climates. Under the specific cold climate conditions of this study, originally intended vegetation is of great importance for the vegetation cover, but the majority of the species consist of colonizing unintended species. Therefore, when considering installing a green roof in a similarly demanding region, it would help to know the main objective (e.g., promotion of urban biodiversity, aesthetic appeal or stormwater management) from the start.

These results are valuable for the green roof industry to provide more diverse solutions which are (better) adapted to the specific conditions in different parts of Scandinavia/Sweden. While specific vegetation mixtures already are available for Norwegian conditions/regulations, a mixture for northern Sweden was und development in 2019/20 by the industry. The results of these studies supported that development.

Joel Lönnqvist, Godecke-Tobias Blecken och Maria Viklander