Resilience of stormwater trees to temporary flooding: The case of Acer platanoides ‘Globosum’

High levels of urbanisation, combined with the effects of climate change, are affecting meteorological phenomena, leading to an increase in global urban rainfall anomalies and more flooding. This phenomenon is exacerbated in urban areas by the increasing imperviousness. As a result, flooding is one of the most devastating and widespread natural disasters in the world, affecting regions on all continents. Sustainable Urban Drainage Systems (SUDS) have emerged as a practical solution to mimic natural drainage processes and mitigate the adverse effects of flooding while providing other co-benefits. This is the case, for example with stormwater trees, which contribute to the sustainable management of rainwater and surface water runoff by optimising the processes of infiltration, retention and transpiration. However, in the case of extreme rain events or a fast succession of rain events, the soil or substrate surrounding these trees can remain in saturated conditions for longer periods of time, undermining their capacity to provide the ecosystem services needed. In order to evaluate the resistance of urban trees and in particular to better assess/understand the physiological limits of the stormwater trees, soil saturation assays were carried out in 2023 and 2024 on maple trees (Acer platanoides Globosum), a common street tree in European cities. The assays consisted of evaluating the morphological and physiological responses of 3 young maple trees subjected to water saturation of the planting soil during 21 days and comparing them with 3 reference maple trees under normal drainage conditions. At the tree level, the transpiration changes and the trunk pulsations were continuously monitored with sap flow sensors (Implexx Sense) and dendrometers (Ecomatik), respectively. At the leaf leaves level, the physiological responses following prolonged soil saturation conditions were monitored by instantaneous fluorescence-based measurements of leaf pigments and the nitrogen balance index (DUALEX®, Force-A,) as potential stress biomarkers, and leaf stomatal conductance and transpiration (LI-COR). The soil compartment was monitored using continuous soil moisture measurements (Campbell Sci.) and punctual measurements of pore water oxygen level and redox potential (WTW). 

The results showed a rapid fall in soil pore water oxygen level and redox potential, while the physiological effects of saturation were delayed and appeared only after 7 days of soil saturation. The most impacted tree measured parameter was the transpiration rate, followed by leaf ecophysiological traits such as phaeopigments. Remarkably, the prolonged soil saturation profoundly affected tree health, showing effects even after the winter dormant period during the following growing season This questions the extent to which stormwater trees could provide ecosystem services in the future. The presentation will focus on the impact of soil saturation on the various tree parameters measured and propose the definition of a “tolerance threshold” for stormwater trees in the context of runoff management.