Assessment of Road Deicing Impacts to Roadside Pines in the Black Hills
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Assessment of Road Deicing Impacts to Roadside Pines in the Black Hills

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      Final Report May 2006 to March 2018
    • Abstract:
      During the early to mid-2000s the public expressed concern regarding the poor appearance of the ponderosa pines (Pinus ponderosa) along highways in the Black Hills. Deicing road salts were thought to be the cause for the change in appearance. The objectives of this study were to define the extent of discoloration and dieback of pines along the state highways in the Black Hills, to determine if the decline and discoloration of the pines were due to deicing salts and investigate other stressors that may have been responsible for the symptoms. A total of 346 miles of state highways were surveyed and observations were made at one-mile intervals along the roads. Pines occurred at approximately two-thirds of the points and the majority of these trees exhibited little dieback during the 2007 survey, but this increased to the majority for the 2011 survey. A reverse trend was noted with foliage color, where the majority of pines on the points were discolored in 2007 but exhibited normal color by 2011. Dieback is due to the accumulation of past stresses while discoloration is an indicator of current stress. Soil and pine foliage samples were collected from four sites with roadside pines expressing canopy discoloration and dieback and another four sites where the pines were asymptomatic. Soil samples were collected from the sites at three feet and 20 feet from the pavement edge and 60 feet from the road centerline and analyzed for chloride, magnesium, and sodium. Foliage samples were collected from the nearest pine at the 20 and 60-foot soil sampling locations. There were no pines at three feet from the road edge. There were significant differences in foliage concentrations of sodium and chloride between sites containing symptomatic or asymptomatic trees at 20 feet from the road edge but not at 60 feet from the road centerline. The pines on symptomatic sites at 20 feet from the road showed foliage concentrations of chloride high enough to cause injury. A similar trend was found in the soil with a significant difference in the concentration of all three ions with respect to distance. While sodium and magnesium decreased with distance, chloride increased. Although there was a significant difference between soil amounts of these three ions at 20 feet and 60 feet, the amounts at all sites were too low to result in injury to the trees, pointing to aerial spray being the vector of the ion into the trees. Ozone injury was investigated as a possible additional stressor for these trees. Active and passive ozone monitors were deployed for two years along state highways in the Black Hills to quantify ozone concentrations and determine if this pollutant may have been responsible for the discoloration observed in foliage. Ozone concentrations were not sufficient to be a contributing factor in the decline of roadside ponderosa pine trees. While deicing salts were a primary stress, it appears that the drought that occurred during much of the past decade was a contributing factor in pine discoloration and decline.
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