Sodium chloride (NaCl) is a type of salt commonly used for ice removal. When applied, it is effective at melting ice, and preventing ice accumulation. It is often used in urban settings to improve the safety of roadways, sidewalks, and parking lots during the winter season. However, when applied in large quantities, salt can have a major detrimental effect on the environment, particularly regarding trees. The following is an overview of salt injury, and how it impacts trees.
What Causes Salt Injury?
Salt injury to trees generally occurs as a result of excessive soil salinization. Salinization is a process in which water-soluble salts accumulate in the soil. The increased soil salinity inhibits the ability of the tree to absorb water through its roots. This compromises the tree, causing it to become dehydrated, and susceptible to desiccation injury and other stressors.
Salt damage may also occur when salt is deposited on dormant stems and buds as a result of salt spraying. Salt spray injury often remains confined to individual branches. However, prolonged salt exposure can result in entire trees becoming affected. In severe cases, trees affected by salt spray injury may develop a withered or tufted appearance.
The Effects of Soil Salt Injury:
Soil salt injury occurs most frequently in urban areas where large quantities of salt are used to remove ice or snow from roadways. Trees that develop in these settings are exposed to salt as the result of water splashing from passing vehicles, snow that has been ploughed, or urban runoff. The water from these sources seeps into the soil, and increases soil salinity. This results in the soil becoming more compacted, which can impede soil drainage and restrict root growth. As the water saturates the soil, salt is absorbed into the roots and transported to the shoots. Salt then accumulates in the dormant twigs and buds, with concentrations peaking just before budburst.
For deciduous trees, soil salt injury becomes evident in the late summer following the growing season. It may also appear during periods of hot, dry weather. The first indication of soil salt injury is the failure of buds to open, resulting in entire branches being devoid of growth. In some instances, the symptoms of soil salt injury may be delayed, with buds sprouting fresh growth, only to wither shortly thereafter. This type of disorder is called post flushing dieback.
Leaves may also exhibit symptoms of salt damage, developing a marginal browning and necrosis at the tips. Necrosis will gradually extend into the inter-veinal tissue, resulting in further discoloration, and premature leaf fall. Severe salt damage can cause crown dieback, resulting in chronic limb death. This often occurs over the course of a few months, and can lead to total tree failure. This is especially common in younger trees, and newly planted trees. If salt damage is less severe, the buds may bloom and sprout fresh growth, but develop at a slower rate, resulting in a reduction in leaf, flower, and fruit size.
The Effects of Salt Spray:
Salt spray occurs most frequently on roads and motorways that feature a heavy flow of traffic. As vehicles pass along, they splash salt contaminated water onto nearby trees, coating buds and branches in a heavy concentration of salt. As salt is absorbed into the buds, it causes apical bud death and twig dieback. In order to compensate for apical bud loss, the lateral buds on older branches may be released. This results in the development of tuft-life growths on the branches, a disorder called witches broom.
Symptoms of salt spray damage become evident as growth resumes in the spring. Deciduous trees will bloom later than usual, while evergreens and conifers will exhibit severe needle browning. Browning develops in late February or early March, becoming more extensive through the spring and summer. Though it does not often result in outright tree failure, recurring salt spray damage can promote crown dieback and prevent stem growth.
Most salt spray injuries occur within sixty feet of the road. Injuries tend to be most significant on the side of the tree facing the road. Trees that develop on inclines or in depressions are more susceptible to soil salt injury resulting from runoff. Branches that are insulated by snow, or other physical barriers are less likely to be injured.