Exploration of potential ecosystem service trade-offs when establishing trees in different urban substrates

Urban trees are expected to contribute to increased urban resilience, as part of multifunctional installations, and under a wide range of environmental conditions. Ideally, such installations should be beneficial both for trees and cities. The aim of this experiment was to increase our understanding of the effects of different substrates used in urban tree plantings. Two stone-based structural soils, one pumice-based planting substrate and a traditional horticultural mix were installed in 1 m³ containers, with one tree of Tilia x europea 'Kristina' in each container, in a factorial design with eight replicates per substrate. Half of the trees were treated with a mycorrhizal inoculum. Total height, shoot growth, leaf area and colour, and stem growth were monitored the first two years after planting, and leaf C, N, δ¹³C and δ¹⁵ were analysed in the second year. Soil characteristics were determined using measurements of runoff water quality as well as water infiltration capacity and drainable porosity. The study revealed a trade-off between plant performance and multifunctionality, with the horticultural mix resulting in mean shoot growth rates up 2.5 times higher, and a 64 % greater stem base after two growing seasons. It also had the healthiest leaf colours and a significantly higher mean total height (349 cm) than both structural soils (304 and 257 cm, respectively) but slower infiltration rates. The pumice-based substrate had a mean total height of 320 cm and the highest field capacity. Awareness of substrate characteristics and consequences for plant vitality, is important for building sustainable and resilient cities. The results indicate that installations of multifunctional substrates can have stormwater management benefits but might demand more management to maintain tree vitality.