...Vehicle tracking systems recorded position each vehicle every second. Instrumented vehicles were driven through courses of varying velocities and turning radii. GPS position data were used to calculate vehicle velocities and turning radii throughout the course. Vegetation damage along vehicle tracks was recorded immediately, 5 months (end of the first growing season) and 12 months after tracking. Vegetation damage was quantified by both the amount of vegetation lost and the area impacted. Vehicle type, turning radius (TR), velocity (V), and TR. V interaction were found to significantly affect all vegetation damage measures. The tracked M88 tank recovery vehicle caused more vegetation loss than either of the wheeled vehicles (M35A3 cargo truck and M 1009 utility cargo vehicle). Decreasing turning radius increased vegetation loss for all vehicles. Increased vegetation loss associated with turning was a function of both greater vegetation loss within the track and a wider tracked area. Power equations using only turning radius and vehicle type as independent variables predicted vegetation damage measures with [R.sup.2] values ranging from 0.822 to 0.933. A critical turning radius between 15-20 m differentiated turning radii with relatively high vegetation loss as compared to straight-line tracking. Recovery of vegetation cover to pretreatment levels ranged from approximately 6-12 months, depending on impact treatment.

Keywords: Vehicle impacts, off-road, vegetation impact, impact assessment

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The Department of Defense is responsible for administering more than 10 million hectares of federally-owned land in the United States. Military training, especially vehicular training, is an intensive land use that can negatively impact soil and vegetation (Goran et al. 1983; Demarais et al. 1999). Numerous studies have investigated the effects of vehicle traffic on soil and vegetation (Johnson 1982; Payne et al. 1983; Webb & Wilshire 1983; Prose 1985; Braunack 1986; Wilson 1988; Shaw & Diersing 1990; Ayers 1994; Trumbull et al. 1994; Demarais et al. 1999; Ayers et al. 2000; Hirst et al. 2000; Milchunas et al. 2000; Hirst et al. 2003). Potential consequences of vehicle traffic are loss of vegetation, exposed soil, increased erosion, soil compaction, soil puddling, displaced surface horizons, rut formation, decreased macropore space, restricted water movement, reduced soil strength and structure, and physical damage to root systems. The immediate physical disturbance affects not only vigor and mortality of current vegetation but also the rate of vegetation recovery (Thurow et al. 1995; Prosser et al. 2000; Lovich & Bainbridge 1999).

The amount of vegetation damage resulting from vehicle traffic is determined by vehicle characteristics and site conditions. Site conditions that are important in determining vegetation damage include soil type, soil moisture, slope, vegetation type, and plant growth stage (Payne et al. 1983; Wilson 1988; Thurow et al. 1995). Static vehicle characteristics important in determining vegetation damage include surface contact area, surface pressure, total weight, and track design (Ayers et al. 1994; Ayers et al. 2000). Dynamic vehicle properties important in determining vegetation damage include speed, turning radius, and driving pattern (Braunack 1986; Ayers et al. 2000; Halvorson et al. 2001).

A number of vehicle impact studies have assessed the impact of vehicle traffic on vegetation without characterizing the vehicles or activities that caused the disturbance (Johnson 1982; Shaw & Diersing 1990; Milchunas et al. 1999; Milchunas et al. 2000). Typically these studies compared vegetation on relatively large tracked and untracked sites. Usually vegetation impacts were assessed after an unknown period of time and/or by an unknown combination of vehicles using the study sites. Goran et al. (1983) reported a sequence of vehicle-induced effects on vegetation ranging from minor vegetation disturbance from apparent one-time only traffic, to increased bare ground and loss of sensitive plant species for occasional to frequent use, to complete vegetation loss and soil movement for frequently and intensely used areas. The authors also reported local site damage resulting from single turns as similar to intensely-used areas. While these studies are useful for quantifying the cumulative impact of tracking on vegetation, they provide little quantitative information that relates type and level of vehicle use to the amount of vegetation damage.

A number of studies related the impact of specific vehicles to a specified level of use (Payne et al. 1983; Wilson 1988; Thurow et al. 1995; Prosser et al. 2000; Grantham et al. 2001). Typically these replicated studies involved repeated tracking of study plots with a specific vehicle. Thurow et al. (1995) assessed the impact of 1, 4, and 10 straight-line passes of a 22.5 metric ton (t) tracked M2 Bradley Infantry fighting vehicle on vegetation during wet and dry soil conditions. Similarly, Payne et al. (1983) assessed the impact of 2, 8, and 32 straight-line passes of a 2.2 metric ton (t) wheeled Chevy Blazer on vegetation over a period of time to quantify temporal effects of tracking on vegetation. While dynamic vehicle properties like velocity were not always reported in these studies, the vehicle's dynamic properties generally maintained constant throughout the disturbance regime. However, it is not clear if the dynamic vehicle properties used in these studies are representative of actual site use or include the most damaging vehicle activities. For example, Grantham et al. (2001) quantified the impact of 1, 2, 4, and 8 straight-line passes of a 62.6 metric...

NOTE: All illustrations and photos have been removed from this article.



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