Diverse responses of urban street trees to conventional and eco-friendly deicers under long-term exposure

In urban environments, roadside trees contribute to improved quality of life but are vulnerable to complex stresses, particularly physiological damage, and mortality from repeated winter deicer applications. Although eco-friendly deicers (EFDs) have emerged as alternatives, long-term field assessments of their impacts on trees remain limited. To address this, three types of deicers (CaCl₂, NaCl, and an EFD) were applied at three concentration gradients (200, 400, and 800 g m⁻² yr⁻¹) over three years. The treatments targeted major landscaping tree species—Zelkova serrata, Ginkgo biloba, Pinus densiflora, and Pinus thunbergii—to assess their physiological and visual responses. Among the deicer types, CaCl₂ caused the most severe damage, while the EFD resulted in levels of damage comparable to those of NaCl. Notably, G. biloba, which is known for its high salt tolerance, failed to adapt to prolonged deicer stress, whereas P. densiflora and Z. serrata, which initially exhibited substantial damage, demonstrated long-term adaptability. P. thunbergii exhibited the highest initial salt tolerance and sustained adaptability. Deicer stress reduced the leaf area in deciduous species, whereas in evergreens, it caused leaf browning without significant leaf loss, reflecting differences in their leaf retention strategies. The leaf greenness index (LGI) was a strong predictor of growth indicators, suggesting it may be useful for early stress detection. These findings indicate that even EFDs require careful dosage management and that the selection of urban tree species should consider both initial salt tolerance and long-term adaptability.