Sophie Joimel, Claire Chenu and Baptiste Grard, researchers at the laboratory of Functional Ecology and Ecotoxicology of Agroecosystems (ECOSYS/AgroParisTech/INRA), unveil their research on the biodiversity of green roof soil.
Green spaces, known for their environmental roles such as regulating water, combatting urban heat islands and providing food for local communities, are on the rise in cities. They have become key for local authorities, who use them to develop their cities and enhance their sustainability and liveability.
These spaces include green roofs, the number of which has soared in France, with 1.3 million square metres installed in 2014.
While this type of roof is drawing more and more attention, green roof soil and the biodiversity it harbours such as earthworms, beetles and other bacteria have not been widely studied, despite the fact biodiversity plays a crucial role for the environment, especially regarding soil fertility and water regulation.
To broaden our knowledge on this topic, we studied a number of green roofs, whether or not they were used to cultivate food. Do these spaces harbour high soil biodiversity? Where do their organisms come from?
The quest for urban biodiversity
There are different types of green roofs that vary according to the use, type and depth of the soil or greenery. We are interested in two in particular: ‘extensive’ green roofs and ‘intensive’ green roofs or rooftop gardens.
The layer of engineered soil used for extensive roofs is 5 to 15 cm deep. It is made up of non‑irrigated, raw mineral substrates (usually pozzolan, i.e., volcanic rock, mixed with compost) and generally covered with succulent ornamental plants such as sedums.
Productive, or intensive, green roofs were devised to produce food (generally vegetables) over more than 50% of their surface area, with a deeper layer of engineered soil (15 to 30 cm) mainly made up of organic substrates – compost or mushroom bed soil, for instance.
Extensive green roofs are the most common type, while rooftop gardens are gradually becoming more popular, mainly due to calls for projects. One such example is the Paris city council’s Parisculteurs, which is to lead to the installation of 100 hectares of green roofs and walls, 30 hectares of which will be dedicated to urban agriculture. The few studies carried out on green roof fauna focus only on extensive green roofs.
These studies revealed an abundance of invertebrates, mostly made up of a small number of species of arthropods and Collembola, small, 2 mm creatures that live in soil. Collembola, one of the largest taxonomic groups to live in soil, are well known indicators of soil quality (fertility, pollution, etc.). They also play a major role in soil fertility, as they recycle organic matter (nutrients) and create micropores (aeration and root development).
Collembola Planet, the Secret Life of Soil. (Philippe Lebeaux/Vimeo, 2015).
There have been several hypotheses on the role green roofs play in urban biodiversity, although data is still very limited. Green roofs could thus contribute to green infrastructure in cities - an urban development tool used to protect biodiversity while improving the connectivity between green spaces.
To learn more, we studied the collembolan communities of 15 green roofs – seven extensive roofs and eight rooftop gardens – in Paris and its closest suburbs in spring 2016. Our study, published in the scientific journal Ecological Engineering in 2018, also made it possible to compare the levels of biodiversity observed on the roofs with those of other types of green spaces: family gardens, forests and arable land.
Our research pertained to the taxonomic diversity of the communities – what species were there, and in what numbers? It also focused on their ‘functional traits’: size, body shape and pigmentation.
44 species of Collembola
44 species of Collembola were identified in all the roofs studied, with densities varying from 3,500 to 152,000 individuals per square metre. This high variability does not really depend on the type of roof, whether intensive or extensive, although extensive green roofs tend to have slightly less diverse, lower densities of Collembola. In France, density levels of Collembola in green roofs are respectively two and eleven times greater than those of family gardens and arable land. In terms of diversity, the number of species is twice as great for family gardens, but still three times greater in green roofs than in arable land.
The engineered soil used for green roofs is thus suited to hosting Collembola, be it in a rooftop garden or an extensive green roof. The presence of these species indicates that such spaces can harbour high soil biodiversity.
Engineered soil is home to a diverse mix of common species, as well as rarer species. The presence of rarer species is undoubtedly linked to engineered soil’s greater resistance to drought in extensive roofs, which are not irrigated, contrary to rooftop gardens, or its composition in rooftop gardens, rich in organic matter.
Rooftop garden on one of the RATP (Paris transport network)’s buildings in Paris. Sophie Joimel.
Green infrastructure in urban areas
In our study, we made two hypotheses to explain the Collembola settlements in green roofs.
These could be explained either by the compost added to rooftop gardens, or wind dispersal, the wind carrying Collembola to both types of roofs studied. Other possibilities exist, such as the involuntary addition of eggs contained in planting clods or phoresis, whereby Collembola use a larger organism, such as a ground beetle or even a human, to travel elsewhere.
Our study thus shows that green roofs can harbour a high biodiversity of Collembola. While these results confirm the major role green roofs can play as the supports for urban biodiversity, they also raise important questions regarding the connection of green roofs to other green spaces, which would enhance urban biodiversity.
As Collembola are often carried by the wind, other green spaces serving as refuges for these organisms should be close to green roofs to make settlements possible.
The study mentioned in this article was carried out in collaboration with Apolline Auclerc, Mickaël Hedde, Nolwenn Le Doaré and Sandrine Salmon. The authors thank the project leaders who opened the doors to their roofs: Association Espace, RATP, Topager and Veni Verdi.