252 Goodrich et al.: Roadside Vegetation and MgCl2 Dust Suppressants Arboriculture & Urban Forestry 2008. 34(4):252–259. Roadside Vegetation Health Condition and Magnesium Chloride (MgCl2) Dust Suppressant Use in Two Colorado, U.S. Counties Betsy A. Goodrich, Ronda D. Koski, and William R. Jacobi Abstract. Many abiotic and biotic factors affect the health of roadside vegetation, including the application of magnesium chloride (MgCl2) dust suppression products. Three hundred seventy kilometers (230 mi) of forested, shrubland, meadow, rangeland, riparian, and wetland roadside habitats were surveyed along major nonpaved roads in two Colorado counties. Dominant species composition and visible damages of woody roadside vegetation were quantified. The majority (72.3% to 79.3%) of roadside vegetation surveyed was considered healthy (less than 5% damage to crown or stem), depending on slope position from the road. Severely damaged (greater than 50% damage) vegetation ranged from 6.4% to 11.4% of roadside cover, with the most severely damaged vegetation occurring downslope from the road. Percent of plants with severe or moderate damage increased with increasing MgCl2 application rates for roadside aspen, Engelmann spruce, and lodgepole and ponderosa pines. Further research is needed to determine the distribution of MgCl2 ions, nutrients, and interactions between MgCl2 and incidence of potential biotic damage agents in roadside soils and plants. Key Words. Aspen; lodgepole pine; magnesium chloride (MgCl2); ponderosa pine; road dust control; road stabilization. Biotic and abiotic stressors may influence the health of roadside vegetation individually or by interacting with one another. Col- orado, U.S. roadside vegetation may be exposed to stresses analogous to off-road vegetation, including fungal pathogens, parasitic plants, insects, or drought (Cranshaw et al. 2000). Roadside vegetation may also be exposed to more intense abiotic factors than off-road vegetation that include pollution (Lager- werff and Specht 1970; Monaci et al. 2000), erosion of road materials (Kahklen 2001), and road maintenance practices such as dust suppressant application and a concomitant increase in soluble salts in roadside soils and vegetation (Strong 1944; Westing 1969; Stravinskiene 2001; Hagle 2002; Piechota et al. 2004). Woody vegetation, particularly coniferous trees, is a re- liable bioindicator of abiotic stress because of its perennial na- ture, needle retention, and large surface area and biomass per tissue weight (Monaci et al. 2000). Magnesium chloride (MgCl2)-based dust suppression prod- ucts are applied to nonpaved roads during spring and summer months for dust suppression and road stabilization purposes. Chloride-based dust suppressants are used to control erosion and fugitive dust and reduce maintenance costs on nonpaved roads by stabilizing soil and drawing moisture from the atmosphere to keep road surfaces damp (Addo et al. 2004; Piechota et al. 2004). Dust from nonpaved roads can contribute significantly to atmo- spheric particulate matter, which has numerous environmental and human health effects (Sanders et al. 1997; Environment Canada and Health Canada 2001; Singh et al. 2003). The U.S. Environmental Protection Agency has established air quality standards for fine particulate matter (PM-10). Municipal road and bridge departments in arid climates can suppress PM-10 emissions on nonpaved roads by applied chemical dust suppres- sion products (Singh et al. 2003). Investigations of sodium chloride (NaCl)-based deicing salts and roadside tree health began as early as the 1950s because of ©2008 International Society of Arboriculture concern over injured roadside trees, and thus the negative im- pacts of NaCl salts on roadside vegetation are well documented (French 1959; Shortle and Rich 1970; Hall et al. 1972; Hall et al. 1973; Dirr 1976; Hofstra et al. 1979; Viskari and Karenlampi 2000; Kayama et al. 2003; Czerniawska-Kusza et al. 2004). The most significant symptoms of NaCl damage to roadside trees are reduction in biomass, marginal or full leaf chlorosis and necro- sis, withered leaf margins, branch dieback, or plant mortality (Westing 1969; Shortle and Rich 1970; Hofstra and Hall 1971; Hall et al. 1972, 1973; Dirr 1976; Hofstra et al. 1979; Czerni- awska-Kusza et al. 2004; Trahan and Peterson 2007). Various biochemical and physical problems occur in plants growing in saline soils, attributable singly or in combination to ion toxicities and osmotic changes in the plant (Ziska et al. 1991; Munns 2002). Both sodium and chloride ions have been indicated as the causal agents of roadside tree dieback and other symptoms (Hof- stra and Hall 1971; Hall et al. 1972; Lumis et al. 1973; Hofstra et al. 1979; Viskari and Karenlampi 2000; Czerniawska-Kusza et al. 2004). Compared with available information on NaCl, there is sub- stantially less published research that documents the impact of dust suppressant constituents such as MgCl2 on roadside envi- ronments. The use of chemical dust suppressants is increasing in the United States as a result of increases in population and traf- fic, especially in arid regions (Piechota et al. 2004). Potential impacts of chloride-based dust suppressants to roadside soils and vegetation may vary slightly from those related to deicing salt exposure, primarily through the timing of dust suppressants, which are applied to roads when roadside trees are actively growing and transpiring; the limitation of aerial drift and spray from dust suppression products (Strong 1944; Hofstra and Hall 1971; Trahan and Peterson 2007); and the absence of snowmelt to dilute soil salts (Trahan and Peterson 2007). The detrimental effects that high concentrations of soluble salts in roadside soils
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