The ‘salinification’ of urban soils is an unintended but very real side-effect of salting the roads in winter to prevent black ice build-up. Part of the salt mixture (4–13% of the total amount of chemical – the main components of which are NaCl and CaCl²) permeates from roads to adjacent lawns and green areas and causes damage to the road-side vegetation (Czerniawska‐Kusza et al., 2004). The higher the concentration of Na and Cl is in the soil, the higher accumulations there are within the road-side vegetation and, as a consequence, the greater the damage is.
The most significant symptoms of salts on roadside trees are reduction in biomass, chlorosis and necrosis, and, in extreme cases, death of the plant. Direct contact to de-icing salts can also harm foliage, just as high soil concentrations can.
De-icing salts essentially cause osmotic (cell plasmolysis) and ionic stress to plants – they interfere with both water and nutrient uptake by roots, and also interfere with leaf processes. Sodium and chloride ions in the soil that are absorbed by the roots are then transported to leaves via the vascular system, though there is little evidence that the transported ions return to the roots via the phloem. Therefore, it can be stipulated that salt ions accumulate at greater concentrations in leaves than in roots (Munck et al., 2010). In addition, salt ions from aerial deposition or salt spray enter leaves directly via the cuticle.
The resultant effect of de-icing salt ions on cells is that of causing dehydration through plasmolysis, the inhibition of enzymes involved with carbohydrate metabolism, and the subsequent impairment of photosynthesis. De-icing salts therefore disrupt the tree’s metabolic processes, and may cause localised areas of leaf cell mortality (Munck et al., 2010; Pedersen et al., 2000; Strouts & Winter, 1994). De-icing salts can also impact the dormant buds of deciduous species, with moderate to severe inner tissue discoloration occurring in cases of higher de-icing salt usage (Zimmermann & Jull, 2006).
The effects of de-icing salt are particularly harmful if compounded with other urban-induced stresses, such as soil compaction. However, serious cases of alkified soil due to de-icing salt application can (and do) significantly impact trees without any other significant attributing factor (Czerniawska‐Kusza et al., 2004).
Czerniawska‐Kusza, I., Kusza, G., & Dużyński, M. (2004) Effect of deicing salts on urban soils and health status of roadside trees in the Opole region. Environmental Toxicology. 19 (4). p296-301.
Munck, I., Bennett, C., Camilli, K., & Nowak, R. (2010) Long-term impact of de-icing salts on tree health in the Lake Tahoe Basin: Environmental influences and interactions with insects and diseases. Forest Ecology and Management. 260 (7). p1218-1229.
Pedersen, L., Randrup, T., & Ingerslev, M. (2000) Effects of road distance and protective measures on deicing NaCl deposition and soil solution chemistry in planted median strips. Journal of Arboriculture. 26 (5). p238-245.
Strouts, R. & Winter, T. (1994). Diagnosis of Ill-Health in Trees (Research for Amenity Trees 2). London: HMSO.
Zimmerman, E. & Jull, L. (2006) Sodium chloride injury on buds of Acer platanoides, Tilia cordata, and Viburnum lantana. Arboriculture & Urban Forestry. 32 (2). 45-53.