Soil Loosening of Compacted Urban Tree Sites Using a Pneumatic Lance Injector: Quantifying Treatment Success Through Tree Morphology and Soil Structure Parameters via Terrestrial Laser Scanning

ABSTRACT Background Urban trees provide essential ecosystem services, but their growth and longevity are often limited by soil compaction. Although pneumatic lance injection (PLI) is increasingly promoted as a minimally invasive and efficient soil loosening technique, robust scientific evidence for its long‐term effectiveness remains limited. Aim This study evaluated the effectiveness of pneumatic lance injector for compacted urban tree sites with silt, sand, and clay soils. Three treatment variants were tested against reference variant: compressed air (CA), compressed air with injection of granules (CA+G), and compressed air followed by water injection (CA+W). Soil structure and tree morphological traits were monitored over 2 years after soil treatment. In addition, the soil surface was scanned using a terrestrial laser scanner (TLS) before and after the treatment. This approach was based on the assumption that the pore volume generated in the soil correlates with measurable changes in the soil surface. Results Results showed that PLI effects were highly soil‐ and variant‐dependent. The sand site exhibited the greatest surface uplift and pore volume increase (19.8 dm 3 within a 0.8 m radius around the injection point), followed by silt (12.5 dm 3 ) and clay (8.3 dm 3 ). The CA and CA+G treatments produced pore volumes two to three times greater than those generated by the variant with water injection (CA+W). However, most soil parameters, including bulk density, gas diffusion, and root density, showed only marginal or inconsistent improvements. Tree structural parameters showed no improvement in any treatment compared to the reference. Moreover, crown development nearly stagnated in treated trees, particularly under the CA+W treatment. Overall, the treatment failed to produce sustained improvements in soil or tree structure parameters. Due to its limited efficacy and high cost, pneumatic lance injector treatment cannot be recommended in its current form without significant methodological optimization.