Arboriculture & Urban Forestry 34(4): July 2008 and 42.6 cm/year in Larimer County), although Grand County appeared to receive a larger proportion of its precipitation during nonsummer months than Larimer County in the form of snowfall (23.9 cm/winter in Grand County compared with 18.9 cm/winter in Larimer County). Summer precipitation (May through Sep- tember) was negatively related to the percentage of vegetation showing severe damage symptoms, and an increase in precipi- tation was related to a decrease in the amount of severely dam- aged vegetation when control and treated roads were combined (P0.05). When only treated roads were included in the analy- sis, an increase in summer precipitation was not related to a decrease in severely damaged cover (P0.27) or an increase in healthy cover (P 0.55). Summer, winter, and total precipita- tion were always higher on nontreated roads compared with treated roads (all analyses P < 0.0001), as was elevation on nontreated roads (P < 0.0001), indicating some relationship among elevation, precipitation, and the probability that a road will or will not be treated with dust suppressants. DISCUSSION Habitat and Species Composition We observed some variation of roadside habitat types and spe- cies composition between counties, although many commonali- ties occurred. Species within forested and wooded habitats pro- vided the best means to measure health conditions of dominant vegetation along nonpaved roads because of their prevalence along both treated and nontreated roads surveyed. Also, large woody vegetation proved to be easier to identify and estimate crown conditions compared with smaller, seasonal-dependent graminoids, forbs, and shrubs in nonforested habitats. Although most tree species’ percent cover accurately represents coverage in county roadside forested areas, it may not accurately reflect percent coverage along all nonpaved county road mileage. Vegetation Health Conditions A major objective of this survey was to determine the health conditions of roadside vegetation throughout both counties and quantify the percentage of each species with no damage, mild, moderate, or severe visible damage to the crown or stem. Using multiple regressions, we determined the major site factors that related to the health conditions of dominant roadside vegetation. Treatment of nonpaved roads with MgCl2-based dust suppres- sion products correlated to the increase in foliar damage, hence the decline in health condition of several roadside species (Table 3). Several species had significantly higher proportions of se- verely damaged individuals along treated roads, including major components of Colorado forests such as lodgepole pine, ponder- osa pine, and aspen. In addition, Engelmann spruce, Salix spe- cies, common rabbitbrush, and Juniperus scopulorum (Rocky Mountain juniper) all had significantly lower percentages of healthy cover along treated roads (Table 3). Generally, damage to roadside vegetation was observed as dieback of crown or the entire plant in deciduous species and discoloration as necrotic or marginally burned needles in conifers. Crown defoliation, die- back, and foliage discoloration are important biologic diagnostic tests of vegetation health (Stravinskiene 2001). The higher se- verity of damage seen in many dominant roadside species along roads treated with MgCl2 dust suppression products is indicative of the declining health condition of these individuals. Influence of Application Rates and Slope Position The two major site factors that frequently related to the health conditions of roadside vegetation in multiple regression analyses were the average MgCl2 application rate and slope position from the road edge. Although the rate of increase varied between species, the application rate of MgCl2 was directly correlated with increases in the proportion of damaged individuals ob- served on several roadside species. Runoff of chloride salts is known to move through the soil matrix downslope with water movement through mass displacement (Westing 1969; White and Broadley 2001). Many previous research efforts have fo- cused on differences in soil and foliage properties based on slope direction from the road. These studies have shown that environ- ments upslope from the road base do not receive the amount of salt compared with downslope sides and soils and foliage do not display as much damage, although symptomatic foliage upslope from the road affected by deicing salts has certainly been ob- served, presumably through spray and aerial drift from the road (Hofstra and Hall 1971; Piatt and Krause 1974; Fleck et al. 1988). We also observed severely damaged vegetation in up- slope areas, which we speculate may be the result of upslope trees with extended roots under the road or roadside drainage ditches. We speculate that MgCl2 moves with water through the soil matrix into roadside soils and is taken up by plant roots. We do not believe that aerial spray of salts or dust caused damage to roadside trees, because no symptoms specific to spray were noted, including necrotic specks, crystallized salt deposits, or dust particles on foliage (Strong 1944; Trahan and Peterson 2007). Precipitation Precipitation was a significant factor in the analysis but was confounded by several parameters, including elevation, road treatment, and the vegetation types at different levels of precipi- tation. In general, vegetation health increased with precipitation, although species were not stratified over all precipitation levels to accurately model these results. Precipitation may also influ- ence the movement of MgCl2 into roadside environments by moving ions further from the road or diluting ions in roadside soils. More extensive surveys with similar vegetation types, el- evations, roads, and accurate precipitation and soil chemistry data are required to deduce the effects of precipitation and MgCl2 interactions on roadside vegetation health. Other Potential Stress Agents to Roadside Vegetation Surface erosion of road material should move downslope from the road in the same manner as runoff (Kahklen 2001). Forest roads can be major sources of accelerated soil erosion along roadsides as a result of the removal of surface cover and modi- fications or compaction to natural soil structure. Erosion of sur- face particles from nonpaved roads is influenced by traffic, pre- cipitation incidence and intensity, and road maintenance proce- dures such as grading (Kahklen 2001). Sedimentation, although a possible explanation for declining tree health downslope from the road, does not explain the increase observed in severely damaged vegetation upslope from the road (in aspen and pon- derosa pine) with an increase in MgCl2 application rates to the extent that exposure to MgCl2 ions in the soil matrix does. ©2008 International Society of Arboriculture 257
July 2008
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