Session C2 - Vegetation monitoring in SCMs

Stormwater tree pits (STP) are a nature-based solution for managing urban water flood risk and pollution. They reduce-road runoff, by re-directing it to infiltrate into the soil of a street tree. However, runoff has many pollutants, especially high concentrations of de-icing salt, sodium chloride in cold climate cities.  Nordic cities are concerned with the impact of street runoff on trees in STP. We will present the preliminary results from the 3rd year of a ongoing pilot study in Montreal, Quebec aiming to determine growth conditions of street trees in STP, and how they are affected by the level of blockage of street runoffs access for different periods. We measure the effect of the blockage (3 levels) on ground water height, the soil chemical environment (especially salinity), and the health and growth of two common street tree species planted in STPs (total 18 stp). We found that during and after heavy rainfall, rises and falls in water levels were more pronounced in open or semi-open pits than in closed pits. This suggests that infiltration, percolation and transpiration are more significant there than in closed pits. We also determined that conductivity levels (a proxy for salinity) peaked during spring, but and that during the tree’s growth period, conductivity did not differ significantly between treatments. Further analysis will be presented about tree growth, soil chemistry, conductivity between treatments and multi-year analysis.

Urbanization profoundly disrupts the water cycle through soil sealing. To mitigate these impacts, Nature-Based Solutions such as green roofs, permeable pavements, rain gardens and stormwater trees are increasingly being proposed to restore water retention, infiltration and evapotranspiration processes. This study investigates the effects of prolonged soil saturation in stormwater trees focusing on the ecophysiological and hydrological functioning of maple tree, frequently found in urban environments in Europe. Therefore the stormwater trees data of the Sense-City experimental facility (Paris metropolitan area) were analysed, indicating that a saturation period lasts on average half a day, but that periods of around ten days occurred up to once a year. Based on these data, pot experiments were then conducted in the fall of 2023, consisting of artificially saturating the substrates for three weeks. After seven days of exposure, a significant decrease in stomatal conductance and sap flow, as well as changes in the biochemical parameters of the leaves, were observed, illustrating the damaging effects of waterlogging. In addition, certain alterations in leaf traits in exposed trees were still observable the following year, suggesting a kind of “memory effect” or adaptation process to the stress previously experienced. These data should contribute to improving guidelines for stormwater management using trees.

Nature-based solutions such as bioretention systems are increasingly being used for urban stormwater management. However, plant communities, one of the components of these systems, change over time, potentially influencing their performance. This study compares the evolution of the number of species recorded in 437 permanent quadrats located in green stormwater management infrastructure on Papineau Avenue in Montreal (Canada) over four years, based on IV conditions, i.e., water retention level (2), substrate type (4) and orientation (3). The data showed that there has been a steady increase in the number of species over the years. In 2025, vegetated (well-drained) basins had higher values than bioretention basins, which retain more water. The type of substrate alone does not seem to have an effect. A west-facing orientation also seems to promote greater species diversity. The bottom of the basins has, on average, 5 to 7 species in 2025 compared to no species in 2017. A more detailed analysis of environmental factors and functional traits (morphological, physiological or phenological characteristics measurable at the individual level) of dominant species will help identify the traits that enable species to survive and thrive in these environments.

Nature-based solutions (NbS) are increasingly valued in public policy, particularly for improving citizens' quality of life, enhancing attractiveness, adapting to climate change by reducing overheating and improving stormwater management, and, of course, preserving and restoring biodiversity. One solution is based on the development of tree-lined areas in cities, but it is important to evaluate the actual performance of these solutions. To meet this objective, an experimental study site located on the Doua campus (Villeurbanne, France) has been set up. Through experimental and theoretical studies, the aim is to better understand how urban microclimatic conditions influence tree functioning through ecophysiological measurements, microclimatological measurements, and soil moisture measurements. This work presents the first measurements obtained at this site in 2025 and the prospects for the coming years.