Title : Enhancing urban tree establishment with technosoils: A comparative study of growth and physiological adaptation under water stress of hackberry and linden
Abstract:
The overall goal of the clim-arbre project is to develop innovative solutions to improve the establishment and resilience of trees in urban environments in the face of climatic constraints. Techno soils, produced by recycling organic and mineral materials, represent a promising approach that both recycles waste and supports tree growth in cities. Our first objective is therefore to select a techno soil that meets the physiological needs of trees while ensuring a high survival rate under water stress conditions. To achieve this objective, two tree species— the hackberry (Celtis australis) and the small-leaved linden (tilia cordata)—were grown in a greenhouse in different techno soils (T1: crushed concrete + green waste compost; T2: crushed concrete + organic waste compost; T3: T1 enriched with biochar), and then subjected to two irrigation regimes (100% and 50% irrigation) to evaluate their growth and physiological response to water stress using phenotyping tools (Rapid Scan, Multiplex, and Pocket PEA sensors). The hackberry exhibits superior growth on techno soil T2 with a better leaf-to-stem ratio compared to T1 and T3, regardless of the water regime. Under water-stress conditions, hackberries grown on the T2 techno soil show very good resistance and produce more biomass than those observed on T3 and T1. Water stress was detected by phenotyping tools through an increase in the flavanol index (an indicator of oxidative stress) and a decrease in chlorophyll indices, nitrogen status indices (NDRE, NDVI, NBI), and photosynthetic activity (PI index, which indicates the ability to use light energy for photosynthesis). δ¹³C, considered an integrative proxy for water stress experienced over long periods, reflects water-use efficiency and indicates well-optimized transpiration across all techno sols.Conversely, in linden trees, techno soil T3 appears overall to be the most favorable under water-stress conditions, ensuring better maintenance of nutritional status particularly nitrogen status (NDRE index)—as well as relative stability of chlorophyll indices, whereas techno soil T1 remains more suitable under non-stressed conditions. This results again highlights the effect of biochar added in T3 vs T1. IRMS analyses show that δ¹³C— which reflects water-use efficiency—is higher in T2, indicating more severe water stress and stronger regulation of stomatal conductance. These initial findings suggest that techno soils may be a promising alternative for tree planting in urban environments, even under water-stress conditions, and that the composition of the techno soil must be tailored as function of considered tree species.
