MUFUWU Stadtbaum - Development, evaluation of multifunctional urban tree habitat in existing streets. Root zone, retention, microclimate

The project titled "MUFUWU Stadtbaum – Development and Evaluation of Multifunctional Urban Tree Locations in Existing Streets" aims to achieve a measurable, sustainable, and long-term enhancement of living conditions for urban trees in existing street spaces within densely built urban districts. This objective is anchored in the Sponge City for Trees system, increasingly gaining recognition in Austria. The system, fusing the benefits of urban trees with the water retention capabilities of an expansive underground catchment basin, was implemented on Leonhardgürtel street in Graz, accompanied by a comprehensive monitoring facility. On one hand, this novel system holds promise for fostering more robust and enduring tree growth, thereby facilitating numerous ecosystem services in urban areas. On the other hand, it piques interest in urban water management as a decentralized rainwater management measure, constituting a form of blue-green infrastructure.

Despite its introduction in Austria six years ago, pilot sites with scientific measurement equipment and data-driven analyses of the system's processes and errors are scarce. Moreover, prior to the project's initiation, the system had limited implementation in existing cities, resulting in a dearth of experience. The research project aims to address these knowledge gaps, focusing on dimensions like water retention during heavy rainfall, water storage in dry periods, tree vitality across different life phases, and assessing the relevance of various components in the overall system or hard criteria for the functionality of the construction method in view of the many degrees of freedom in planning and execution.

The research project involved redesigning parking areas on both sides of a section on Leonhardgürtel using the sponge city system. New plantings featured three different tree species, while root spaces for four existing chestnuts were cleared, rehabilitated with sponge city substrate, and tree substrate. Soil hydrological investigations of various fine substrate mixtures were conducted, leading to the selection and implementation of a chosen mixture. Noteworthy was the installation of a "first flush" and cleaner downstream surface water differentiation through an "outflow box," a unique aspect not seen in other projects. Direct discharge of roof water from neighbouring buildings into the sponge city system was also a distinctive feature.

In addition to the unique planning and execution of the sponge city system, emphasis should be placed on the comprehensive monitoring system. This system, installed in consultation with project executors, aims to observe root propagation, tree development, and water dynamics. Sensors measure soil water balance, sap flow, stem growth, and inflows/outflows. Plexiglas tubes for root detection and perforated vertical tubes for quantifying sponge city substrate filling were installed. With the aim of observing a manageable section of the entire site better and at a higher resolution, the southern area of the Leonhardgürtel West with four trees was defined as the main measuring area (approx. 5 x 32 m), in which a larger number of sensors were used.

After two years of observation, signs of rooting into the sponge city substrate were evident in new plantings. Data underscored the importance of nursery care in the initial years for optimal growth and drought stress prevention. Stem growth rates varied among new plantings, influenced by factors like the volume of surrounding tree substrate. Existing trees demonstrated resilience after root clearing and rehabilitation, tapping into the improved root space.

Roof water is generally a valuable resource for direct discharge into the sponge city, accumulates very quickly and follows the rainfall hydrograph and has the potential to provide water frequently and in sufficient quantity to replenish the soil water reservoir in the sponge city substrate, even during smaller rainfall events, if a well-designed distribution system is used. Challenges included limited water level increases in vegetated filtration basins during precipitation and varied catchment area sizes due to slope conditions. Water distribution issues in the Leonhardgürtel system and full-drainage pipe installation further complicated matters.

From the synthesis of all these observations, valuable conclusions can be drawn as to which elements of the complex overall system are particularly relevant and in which phase of its life cycle they are significantly influenced. Above all, a reliable distribution system, which distributes the rainwater towards the tree roots, is a critical component of the sponge city system for trees. In connection with this is the coordination of the connected catchment area in terms of water quantity and quality with the capacities of the built system. The latter could only be roughly estimated due to a lack of empirical values; the measured values from the monitoring essentially confirmed the planning but provide much more reliable figures for the function.

It can therefore be stated that after the project duration, which only covers a short period at the beginning of the hopefully long life of the new tree locations, a lot of knowledge has already been gained in terms of the project idea. The trees will now slowly move into the establishment phase, in which they grow their roots in the sponge city substrate. This phase has also not yet been scientifically monitored on a real scale, and further measurements should provide promising insights. Projects have already been conceptualized to ensure this continuation. Once this has been achieved, the measurement setup can be modified in order to adequately answer the most relevant sub-questions in the respective phase.

Project management

3:0 Landschaftsarchitektur, corporate partner

Project or cooperation partners

Verein Land schafft Wasser, scientific partner

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