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Since the delivery of the biodiversity school in 2015, the building has matured, the environments have evolved, students and teachers have appropriated the site and developed new uses for it. In the five years following completion, management was essentially focused, with success, on maintaining and perpetuating the ecological environments that had been put in place. In 2020, the challenge was to increase the biological potential of all the environments present (meadows, woods and facades) by using ecological engineering techniques applied in an innovative way to architecture. The operation carried out is a new experimentation, adapted to this unique project that is the Boulogne school.

For a new project built from scratch, the first years are devoted to the installation of a perennial prairie environment. The initial prairie was seeded in 2015 and has been maintained since then. Our goal is to encourage species diversification in the prairie and move the floristic background toward more perennial, floriferous, local, site-adapted species. To do this, we have identified, only 10 km away, magnificent historical dry marsh meadows in the national domain of Marly-le-Roi. With the agreement of Alain Baraton, head gardener of the Versailles estate, we selected a parcel as the "ideal donor" for Boulogne. The donor meadow was chosen for its exceptional floristic richness and compatibility with the conditions that will be encountered on the roof of Boulogne. In June and then July 2020, we successively collected by hand the seeds of the earliest species (meadow sage, upright brome, amourette) and then we mowed these same meadows by hand. The enrichment of the Boulogne prairie is achieved in one to two seasons of haying and should then never need to be reseeded. It is the monitoring of biological indicators, associated with a precise management plan, that will make it possible to evaluate the success of the operation and to specify possible adjustments in the practices.

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Originally, the trees were planted very close together and their lack of size made one think of a "toothpick" forest rather than a true afforestation. Today, when you arrive in the heart of the forest, you are in a magical space: you are in the middle of the forest at a height of 14 meters. The freshness that seizes you is really that of a small forest, the shade and the light play with the foliage and the top of the trees culminate at 7 or 8 meters, which gives them a true extent of young trees, dense and full of vigor. But the soil has settled and the undergrowth (thickets, ground cover, etc.) is struggling to establish itself in a "competitive" context.

Knowledge of soil life in an urban context has evolved since the building was delivered as have the distribution channels for native plants. We want to improve this environment in all its components to create a functional ecosystem at all strata that will benefit the species that frequent the tree tops, but also those that use the undergrowth to find refuge or feed on the ground as is the case of the blackbird or the familiar robin. These species, evolving at the height of the children's sight, will allow naturalist observations which will be the basis of an educational project.

The concrete block façade is a research that the office has conducted to promote all the conditions for the establishment of fauna and flora over time. The whole is a composition of blocks that shift, overlap, create overhangs, crevices ... A whole vocabulary of niches and cracks for the installation of spontaneous species. The concrete, very close to stone, has allowed this diversity which has even allowed us to include a large quantity of nesting boxes, some of which have been well and truly colonized by passerines such as blue tits and great tits or black redstarts. But above all, time at work on this wall will guarantee its colonization: it is a new way of seeing architecture, of apprehending it on the scale of the living.

On delivery, the pH of the concrete was very alkaline, incompatible with plant colonization. We had to wait several years for the decarbonation process to take place and make the conditions more favorable. In 2020, we observed for the first time the development of unicellular terrestrial algae veils on the blocks, as well as the implantation of the first encrusting lichens on the mortar, indications that the time of seeding can now come. As this building retains its function as a laboratory of living things, we have launched an experiment in the vegetation of the facades based on the observation of the flora of old walls. We have collected seeds of vegetation native to the Ile-de-France region on dozens of sites colonized by wall flora (wall marigolds, red Centranthes, small-leaved bellflowers, rooted Porcella, sedums, etc.). In parallel, we have collected and analyzed small quantities of microsols to know the conditions in which this flora develops and to create technosubstrates that mimic the degraded mortar of the old colonized walls. It is these substrates and seeds that we are trying to implant to accelerate the process of colonization of the building walls.

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At the beginning of the school year 2020, we reestablished the roof of "our school" for a week, and in summary, here is what we did in stages:

- Manual decompaction of the soil of the afforestation, bringing leaves and forest soil samples, planting of native ground covers from the label Végétal Local.

- Enrichment of the native plant palette of the meadow by the reseeding technique "Fleur de Foin" from Marly-le-Roi with the enthusiastic help of the school children.

- Seeding of the facades using a gondola: 600 liters of substrate seeded with thousands of seeds were inserted into the interstices of the blocks of three facades of the building.

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During this week of action, we questioned and mobilized the school's educational teams in order to participate concretely in the planned actions.

This approach was fortunately self-evident. Indeed, the roof is a place fully invested since its creation, it did not take much to trigger the desire and enthusiasm.

The students, accompanied by their teachers, actively contributed to the planting in the undergrowth: under the direction of the gardeners, the children dug, planted and watered the plants while being made aware of their fragility. The spreading of the hay was the object of a very festive collective action while remaining effective and the temperaments of each one were revealed on this occasion. We have observed a young generation of willing actors who are very conscious of the stakes linked to the necessity of managing and preserving biodiversity.

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The spaces of this roof do not correspond to the horticultural model, nor to that of the roofs with spontaneous vegetation. This is a unique approach, resulting from ecological engineering, to which neither conventional maintenance nor non-interventionist choices correspond. These spaces have now begun a dynamic that must be supported and enriched in relation to the expectations of users. There are still actions, ways of doing things and follow-up protocols to be instituted in order to move forward beyond this singular case.

It is a system of life in the city and of balances that we are experimenting with. The process is well established. We are happy about it. We wish to continue to bring a multiplicity of views and interests to this project which goes beyond the perimeter of the school. It is a first step, a first living laboratory and an experiment that is bearing fruit on the human, environmental and social levels.

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