Salt damage
Cause and general information
Salt, in a variety of forms and through various types of exposure, can injure trees. The most common and widespread way in which trees are exposed to salt is through the application of road salt (also called de-icing salt). Road salt is used throughout much of Canada every winter to mitigate detrimental ice and snow conditions on roads and parking lots and on sidewalks in urban centres to improve safety. Almost all road salt applied to Canadian roads is sodium chloride (NaCl). Other de-icing salts include calcium chloride (CaCl2), magnesium chloride (MgCl2), and potassium chloride (KCl). About 4.75 million tonnes of NaCl road salt and 0.1 million tonnes of CaCl2 are used on roads in Canada each year. Only very small amounts of the other salts are applied.
A second way in which trees may be exposed to salt is through the application of CaCl2 and MgCl2 in the warm season to control dust on gravel roads and road shoulders. The volume of salt applied is generally low and usually only one application is required per season.
The third way trees can be exposed to salt is through accidental saltwater spills associated with oil and gas exploration and extraction. Saline water is found with oil and gas deposits. High volumes of salt water are produced by conventional and unconventional extraction techniques. Most extracted salt water is returned underground into the oil- and gas-bearing geological formations. Accidental spills can occur at well sites, storage facilities on well pads, and from pipelines and flowlines. These spills can potentially expose trees to salt.
A final mechanism for salt exposure is natural. In some parts of western Canada, Solonetzic soils develop naturally from parent geological materials that were salinized with salts high in Na or they develop in areas of saline groundwater discharge and poor internal drainage. Solonetzic soils are generally unfavourable for normal tree growth. This is because the salinity and the relatively impermeable claypan layer that develops close to the surface with Solonetzic soils limits aeration and water and root penetration. Trees planted in such soils are unlikely to grow well without soil ameliorations. Before agricultural settlement and development, many areas in western Canada with Solonetzic soils were covered with grasslands rather than forests because of the unfavourable chemical and physical properties of these soils (particularly the Bn horizon, which is diagnostic for Solonetzic soils and has elevated levels of Na). Trees occasionally grow naturally on Solonetzic soils, but they likely seeded in on these soils at a later stage of soil development when solodization (leaching of Na) was dominant, permitting tree establishment.
There are a wide range of negative environmental effects associated with exposure to salt. Trees and other vegetation near salted roads and walkways can be directly damaged. Alternatively, road salt and saltwater spills can also affect soil and soil micro-organisms, thereby indirectly affecting vegetation. Finally, salt can migrate through soils and groundwater, and with surface run-off, to waterways where freshwater aquatic plants and animals are negatively affected.
Distribution and species affected
Although road salt is applied directly to road, parking lot, and sidewalk surfaces, it migrates to trees in a variety of ways: splash; mist or spray (aerosolization); infiltration into the soil (to tree roots) and water table; runoff; and physical removal of snow and ice (plowing of roads and parking lots and plowing or shovelling of sidewalks). Migration of road salt associated with roads is higher than other surfaces.
As the concentration of road salt dissolved in water increases, the freezing point of the solution decreases. However, there is a limit to how much the freezing point can be decreased. For NaCl, this limit is –21 °C; but it is difficult to reach this limit in practice on roads. Therefore, in Canada, NaCl road salt is not used at temperatures below about −10 °C, which restricts the use of this type of road salt to more southern regions where winters are less extreme. Magnesium chloride road salt can be used at colder temperatures than NaCl road salt. Expected milder winters with climate change may increase the number of regions and municipalities in Canada using NaCl road salt in the future.
Damage from road salt is prevalent in Canada wherever it is used heavily and where traffic volume is high. Therefore, trees damaged from road salt can be found in most heavily populated urban centres across the country and along freeways, expressways, and other high-traffic highways in Ontario, Quebec, and the Atlantic provinces, where high volumes of road salt are used. Although road salt is generally used less frequently in western Canada, major urban centres, such as Calgary, Edmonton, Regina, Saskatoon, and Winnipeg, still use road salt frequently and damage to trees can be found there too.
Solonetzic soils are located mainly in parts of the grasslands and parkland ecoregions of Alberta, Saskatchewan, and Manitoba. In British Columbia, some Solonetzic soils occur near Dawson Creek and near Kamloops. There are about 6 to 8 million hectares of Solonetzic soils in western Canada.
All species of trees can be affected by salt, but the degree of damage varies with different tree species (see Prevention and management section) and the degree of salt exposure. Some species with thick cuticular wax on their foliage and covering their buds (i.e., Colorado spruce) are more resistant to damage from salt mist or spray. Bud scales also provide some protection. Additionally, some species are more tolerant to salt that they take up through their roots and to the chemical and physical changes that occur to the soil in which they grow.
Tree parts affected
Foliage and buds on twigs and branches of most conifers and broadleaf trees are the parts most affected.
Symptoms and signs
Road salt (or de-icing agents) is applied to roads, parking lots, and walkways as dry granular crystals, moistened salt, or in a liquid solution. Road salt is also sometimes mixed with sand or relatively fine aggregates as abrasives. Heavily salted surfaces appear whitish or covered in white dust when dry.
Almost all road salt is applied when trees are dormant during the fall, winter, and very early spring when ice and snow occur. Therefore, on broadleaf trees, damage is usually limited to twigs and buds, which becomes evident in the spring when the flush of new growth begins. A common symptom on broadleaf trees is tufted twig growth or brooms resulting from repeated twig dieback, and failure of vegetative buds and flower buds. Flower buds (e.g., on fruit trees) are more susceptible than vegetative buds. Vegetative buds near the ends of branches are more exposed and vulnerable than those in the interior of the tree crown. Damage to broadleaf trees is more evident on the side of the tree closest to the road and on trees on the downwind side of the road. On conifers, foliage can become discoloured and appear brown. Browning begins at the needle tip and progresses to its base (needle sheath). The branches and crowns of conifers with repeated injury appear bare or lopsided, with more injury on the side of the crown closest to the road or highway and on conifers on the downwind side of the road. Small trees and branches covered by snow during the winter are usually unaffected or less affected by salt mist.
Symptoms associated with the buildup of road salt in soils (chronic exposure) or from accidental saltwater spills (acute exposure) are usually faster to appear and more evident in broadleaf trees than conifers. Chronic exposure can lead to weakened tree crowns that appear chlorotic (yellowing) and have slow growth or even twig and branch dieback. For acute exposure, the foliage of broadleaf trees becomes chlorotic, beginning at the leaf tip and leaf margins but advancing relatively quickly to the central leaf vein or mid-rib. With severe acute exposure, foliage will die. For conifers, crowns of severely affected trees will be reddish brown or magenta in colour. Such colouration appears first in current and 1-year-old needles of the upper crown and then gradually extends downward. Older needles can also accumulate salts, but they are much slower to do so. Salts can also accumulate in the cambium, damaging it and even killing it with severe exposure. When the cambium is killed, the tree dies.
Damage
Damage from salt is usually localized regardless of the type of exposure. The level of damage to individual trees is related to the amount of salt reaching the various parts of the tree. Damaged foliage and buds will most often result in reduced tree growth. Highly damaged or stressed trees are susceptible to additional damage from insects and pathogens. Prolonged and severe exposure to salt can kill trees.
The extent of damage to trees from road salt is limited to relatively close proximity to roads. Studies have shown that damage usually follows a gradient with maximum damage at road edge and decreasing to no effects at about 40 metres from the road. About 90% or more of total salt deposition occurs within 13 to 20 metres of the road edge. From 20% to 63% of applied salt is spread as mist in the air. Winds can distribute salts a few thousand metres from major roads, but deposition is generally low and decreases rapidly beyond a few tens of metres. Prevailing winds also affect deposition with the downwind side of the road having an increased and more extensive deposition (and increased levels of crown damage). Roadside effects extend farther from the most heavily salted roads and roads with high traffic volumes and higher speeds. Trees sheltered from the salt mist are also usually unaffected.
In soils affected by salts, studies have found that chloride (Cl-), calcium (Ca2+), and sodium (Na+) ions are generally elevated in soils adjacent to roads, other paved areas such as parking lots, and in urban settings adjacent to salted surfaces. However, Na+ readily leaches in soils. Magnesium ions (Mg2+) are also increased in areas where salts containing Mg are applied. Increased levels of these elements generally translate into increased levels in trees tissues, which can cause physiological changes and result in damage or stress.
Soil pH often increases with exposure to salts. Increased soil pH can affect the availability of other elements important to various physiological processes relevant to tree growth, vigour, and health. Soil microbial activity can also be negatively affected, reducing nitrogen (N) availability to trees. Nitrogen is critical to tree growth, vigour, and health.
Prevention and management
Preventive and management measures that can limit the negative effects of salt include:
- preventing or minimizing salt deposition, especially near ornamental trees in residential areas, by using less salt and more abrasives (i.e., sand or ash) and avoiding the deposition of salt-laden snow and ice near such ornamental trees;
- planting tolerant tree species where exposure cannot be prevented or minimized;
- protecting young trees from road splash or mist using burlap or temporary fencing in winter;
- planting trees in raised areas and avoiding areas where salt-laden runoff water might accumulate;
- implementing soil amendments to improve the physical and chemical soil properties that favour tree growth and soil microbial activity before planting into soils high in salt; and
- remediating sites where salts have accumulated, or spills have occurred.
Various studies have shown that inorganic and organic matter amendments can improve both physical (e.g., soil structure, permeability, water-holding capacity) and chemical (e.g., pH, cation exchange capacity) soil properties when such soils have been exposed to salt. Such amendments can improve conditions for tree growth and soil microbial activity, thus reducing stress on trees.
The relative salt tolerance of a wide range of native and non-native trees and shrubs, most of which grow in Canada, is provided by Dirr (1976), Jull (2009), and Lumis et al. (1975) (see Selected references section). Salt tolerance in trees appears to be associated with their ability to limit salt entering or accumulating in sensitive tissues, either through reduced uptake by roots (via salt accumulation in the soil), buds or foliage (via exposure to salt mist), or reduced transport of chemical constituents of salt in tree tissues.
The application of road salt in Canada is regulated. Because the negative environmental effects of salt are well known, all jurisdictions and levels of government have taken steps to minimize damage from road salt. Best management practices are shared and implemented across the country. Further information on road salts can be found at https://www.canada.ca/en/environment-climate-change/services/pollutants/road-salts.html.
Selected references
Blomqvist, G.; Johansson, E.-L. 1999. Airborne spreading and deposition of de-icing salt—a case study. Science of the Total Environment 235(1–3): 161–168. https://doi.org/10.1016/S0048-9697(99)00209-0
Bouslama, R. 2021. Surface spills associated with oil and gas activities in Saskatchewan and potential impacts on shallow groundwater resources. M. Sc. thesis, College of Graduate and Postdoctoral Studies, Department of Civil, Geological, and Environmental Engineering, University of Saskatchewan. Saskatoon, Saskatchewan. 124 p. https://harvest.usask.ca/bitstreams/c9b9b888-7aca-4269-b05b-c348a1da8209/download
Cunningham, M.A.; Snyder, E.; Yonkin, D.; Ross, M.; Elsen, T. 2008. Accumulation of deicing salts in soils in an urban environment. Urban Ecosystems 11: 17–31. https://doi.org/10.1007/s11252-007-0031-x
Dirr, M.A. 1976. Selection of trees for tolerance to salt injury. Arboriculture & Urban Forestry (formerly Journal of Arboriculture) 2(11): 209–216. https://doi.org/10.48044/jauf.1976.053
Environment Canada. 2004. Best management practices for salt use on private roads, parking lots and sidewalks. Environment Canada. Ottawa, Ontario. 18 p.
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Fehrenbacher, J.B.; Wilding, L.P.; Odell, R.T.; Melsted, S.W. 1963. Characteristics of Solonetzic soils in Illinois. Soil Science Society of America Journal 27(4): 421–431. https://doi.org/10.2136/sssaj1963.03615995002700040020x
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Jull, L.G. 2009. Winter salt injury and salt-tolerant landscape plants. Cooperative Extension Publishing, University of Wisconsin-Extension, University of Wisconsin. Madison, Wisconsin. UW Extension A3877. 12 p. Available at: https://hort.extension.wisc.edu/articles/winter-salt-injury-and-salt-tolerant-landscape-plants/ [Accessed February 2025]
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