An onsite test was carried out at the Schwamendingen enclosure (EHS) to compare three different substrate compositions. Three mixtures were tested: a single-layer substrate according to FLL substrate construction method 2 (substrate 1), a multi-layer structure according to the Wädenswil installation method (substrate 2) and a multi-layer structure according to the Stockholm construction method (substrate 3). Measurements of root development, scoring and sap flow measurements (transpiration) were carried out on the test trees. The final report summarises the investigations from 2019 to 2023 and describes the findings.

1. Regardless of the substrate mixture, the trees were able to root under the road section. In combination with the foundation layer, they met the load-bearing capacity and frost resistance requirements and can therefore be used without hesitation as substrates that can be built over.
2. The sap flow measurements, an indicator of the vitality of the trees, showed that the willows on the substrates were well adapted to the given wet conditions. Scoring and stem growth rates underpin these observations.
3. The scoring and relative stem growth rates show that Salix alba ‘Liempde’ and Ulmus ‘New Horizon’ are well suited to wet to alternately wet/dry conditions. They were able to develop well in the structurally stable substrate, although they suffered from waterlogging, limited root space and high utilisation pressure. The oaks, on the other hand, performed poorly regardless of the substrate
4. The calculations of the ecosystem services with iTree make it clear that although the oaks developed poorly, they still have a high economic value. It is therefore advisable to use a mixed plantation with a combination of hardwoods and softwoods or fast and slow-growing trees.
5. During the root excavation, it was found that many roots were growing along the support strips and into the structurally stable substrate, while the foundation layer was avoided due to its low mechanical resistance or air capacity. This indicates that structurally stable substrates (FLL2 construction method compliant) can positively influence and possibly direct root growth.

It should be noted that due to the small number of trees and unforeseen influences such as different planting qualities or wind breakage, the results cannot be analysed statistically. Nevertheless, it was possible to show that the substrates tested are suitable for areas that can be built on. The structurally stable substrates can therefore be used for development in Schwamendingen. It is important that the substrates are used to create sufficient rootable space (ideally 36m3). Mixed planting of bare-root tree species with different strategies would promote diversity and preserve the tree population in the long term.

The demands on urban tree substrates are diverse, whereby structurally stable/stable systems are sought that can guarantee tree vitality for as long as possible. In this context, the test design conceived at the Schwamendingen enclosure serves to compare three different substrate compositions. The aim is to determine which substrate composition proves to be the most suitable. This substrate composition is to be used as a substructure for the joint path at the future enclosure and thus guarantee the root space even under heavy traffic. The substrate properties and their effect on three different tree species were observed over a period of four years. Ideally, the source materials for the substrates should be available locally, in consistent quality and sustainably.

At the enclosure in Schwamendingen, three mixtures (substrate construction method 2 of the FLL, Stockholm substrate adapted, Stockholm substrate) were compiled on the basis of existing structural substrates and planted with trees (Ulmus ‘New Horizon’, Salix alba ‘Liempde’, Quercus frainetto ‘Trump’). Measurements of the development of the root systems, assessments and measurements of sap flow (i.e. transpiration) were carried out on the experimental trees. The trees were dug up again at the end and measured.