# The suitability of structural soil for the development of trees growing in urban areas

## TL;DR

Structural soil supports healthy tree growth in cities by mimicking natural soil conditions and promoting photosynthesis.

## Contribution

Demonstrates structural soil's effectiveness in urban tree growth by comparing physiological responses across different substrates.

## Key findings

- Structural soil supports photosynthetic efficiency similar to natural soil.
- Trees in structural soil showed lower stress indicators compared to harsh urban conditions.
- Flavanol levels suggest no significant stress in structural soil and control groups.

## Abstract

Tree survival in urbanized areas is increasingly challenged by dense city infrastructure, including underground systems. Engineered planting systems, such as structural substrates, offer more effective solutions for tree planting in urban sites where natural soil is unavailable. This study aims to assess the physiological responses of trees to different substrates in the context of long-term growth in modified urban habitats.

Eight Tilia tomentosa Moench trees were planted in four substrate types: structural soil (SS), a mixture of crushed stone and soil; compacted soil (CS), simulating urban soil conditions; a mixture of soil and rubble with impermeable pavement (AS), representing harsh urban conditions; and natural soil (C), serving as the control. In the fifth and sixth years after planting, chlorophyll a fluorescence, relative chlorophyll content (Chl), and epidermal flavanols (Flv) were measured.

The highest values Fv/Fm were observed in the control and SS samples (average Fv/Fm: 0.78–0.85), indicating good tree health, in contrast to the AS group. Energy dissipation per reaction center (DI0/RC) showed no significant differences between SS and C, except in September 2022. Indicators such as energy transfer into the electron transport chain (ET0/TR0), performance index (PIABS), and chlorophyll content declined slightly in SS compared to C, but remained higher than in AS and similar to CS. Flavanol content was lower in both the control and SS groups, suggesting no evident stress response.

This study confirms that structural soil provides favorable habitat conditions for urban trees, supporting undisturbed photosynthetic processes over the long term, comparable to natural soil. Structural substrates show strong potential as effective solutions for improving urban tree soil conditions, particularly in areas with soil compaction or limited root space beneath pavements. Future research should explore potential nutrient limitations, especially nitrogen availability, in structural soils.