When Quiet Streets Are More Toxic: The Paradox of Low-Traffic Urban Canyons

In this matched two-street comparison, canyon geometry, used as a proxy for street-level ventilation, was more closely associated with the observed biological stress patterns in trees and soils than traffic intensity alone. Urban air pollution studies often attribute vegetation stress to vehicle emissions, yet the influence of street structure and ventilation remains underexplored. We investigated how canyon-related stagnation may alter coupled plant-soil responses by comparing Ginkgo biloba trees of identical age and municipal management along a well-ventilated, high-traffic avenue and a poorly ventilated, low-traffic street in Budapest. Leaf morphology (surface area, stomatal traits), physiology (chlorophyll, peroxidase), and soil enzyme activity (acid/alkaline phosphatase, β-glucosidase) were analysed together with foliar and soil concentrations of Ni, Pb, Cd, Fe, and Li. Across the two sites, trees in the narrow, poorly ventilated canyon exhibited a stronger, integrated response pattern—higher stomatal density, elevated peroxidase activity, and reduced β-glucosidase in soils—while elevated chlorophyll may reflect stress-compensation and/or microclimatic acclimation; this pattern is consistent with the interpretation that constrained air circulation and reduced dispersion (as inferred from canyon geometry) can outweigh emission intensity under certain street configurations. Integrating leaf and soil indicators provided a coherent, low-cost framework for diagnosing urban tree health. Our findings indicate that street ventilation is an important contextual parameter to consider in urban planning. Optimising canopy spacing and maintaining airflow pathways may help reduce pollutant residence time and support vegetation resilience under intensifying urban stagnation and heat stress. • Street ventilation outweighs traffic in shaping urban tree and soil stress • Poorly ventilated canyons accumulate more Cd, Ni, Pb on foliage and soils • Limited airflow increases oxidative stress and suppresses soil enzymes • Ventilation corridors sustain tree health and enhance soil resilience • A dual leaf–soil diagnostic supports climate-adaptive street design