A Review of the Impacts of ORVs on Vegetation

Trees, shrubs, and grasses hold soil in place and provide habitats for a broad diversity of wildlife.  Wildlife health is intricately connected with the integrity of its associated plant communities.   Off-road vehicles (ORV) can greatly impact vegetation through trampling and the introduction of invasive species.  Here we review how ORVs impact plant communities and propose methods for restoring areas degraded by ORV use.

Trampling Impacts

Riding a several hundred pound ORV off-route or cross-country can crush, break, and ultimately reduce overall vegetative cover (Wilshire 1983, Cole and Bayfield 1993).  Vehicular impacts on vegetation range from complete denudation of large staging areas to selective kill-off of the most sensitive plants. Ultimately, web-like networks of ORV trails can coalesce into broad areas largely denuded of vegetation.  Large shrubs and trees 15-20 feet tall have been killed by root exposure caused by adjacent ORV traffic, and at one locality 10-foot junipers were destroyed by direct impact (pers. comm., Howard Wilshire, USGS-retired).  Plants that survive are weakened, limiting their ability to grow upwards, and are more susceptible to disease and insect predation.  One study found that there was half as much vegetation in an ORV use area than in a similar undisturbed site (Misak et al. 2002).  

These trampling effects generally result in the simplification (e.g., decreased diversity) of vegetation communities either through direct mortality or by increasing seedling mortality, which can eventually lead to changes in species composition.  Studies have found that in areas with high ORV use and repeated trampling, forb and grass communities generally replace shrub communities (Leininger and Payne 1971, Stout 1992).  There is also an increased risk of local extinction of sensitive plant species in ORV use areas (Stensvold 2000, Brown and McLachlan 2002).

The compaction and erosion of soil can greatly impact vegetation.  Soil nutrient uptake by plants is decreased in compacted and eroded soils, root growth is reduced, and plant growth can be severely limited in compacted soils (Blackburn and Davis 1994).  Trampling of soils by ORVs can also damage germinating seeds, and even seeds in the soil seed bank (CEQ 1979).   Other indirect impacts on young plants include the reduction of water storage and soil infiltration rates, and alteration of thermal (temperature) characteristics of soils.  These are all ORV related deficiencies that can disrupt seed germination and seedling growth (Davidson and Fox 1974).  Moreover, soils left bare by the damage of ORVs offer excellent germination beds for aggressive weedy species. Lastly, when ORVs travel through exposed soil sites during dry periods, they often create dust, which settles on and can damage nearby plants.  The dust can affect the plants’ ability to photosynthesize, grow, and reproduce (MWLAP and GCC 2004).

Non-native Invasive Species

In addition to trampling effects, ORVs are a major vector for non-native (exotic) invasive plant species.  When non-native plants invade areas, they tend to crowd out and outcompete native vegetation, and as a result, multiple aspects of that ecosystem can be impacted.  The impact is so large that Forest Service Chief Bosworth in 2003 named the spread of invasive species as one of the four “great issues” facing the Forest Service (Bosworth 2003).  Weeds are spreading at an estimated 4,600 acres a day on western public lands (USDI 2000) and ORVs are a key cause of their spread.

With knobby tires and large undercarriages, ORVs can unintentionally take invasive non-native species deep into forestlands.  For example, one study found that in just one trip on a 10 mile course, an ORV dispersed 2,000 spotted knapweed seeds (MSU 1992).  In Wisconsin, a survey of seven invasive plant species along ORV routes found at least one of these (exotic) plant species on 88 percent of segments examined (Rooney 2005).  ORVs in roadless areas pose a particular risk of spreading invasive non-native species because roadless areas are often less weedy.  Gelbard and Harrison (2003) found that vehicles are the chief vector for invasive species infestation in roadless areas, which were shown to be very important refuges for native plants.

Plant Community Restoration

In some areas it may be determined that there are more routes than are necessary or wanted.  This may be due to illegal route creation, route redundancy, or the determination that the environmental or social cost is too great to continue ORV use in that area.  In these cases, it is essential that routes are closed and an appropriate restoration plan be implemented.  

The objectives of a plant community restoration plan should be to stabilize the area, prevent it from further degradation, and return it to its previous native condition.  First the route must be effectively blocked or obscured to prevent further ORV use.  Blocking the entrance of the route could include fencing, placing barriers or boulders, laying woody debris, planting trees, and/or fully recontouring the entrance of the route.  In certain situations it even helps if not only the entrance is blocked, but the view of the actual line of sight is blocked.  Once access is prevented, native seed should be used for revegetation.  Incorporating local plant materials, duff, and woody material will help retain moisture, provide native plant seed, and speed the revegetation process.  Lastly, some sort of educational and enforcement component is helpful as well – revegetation efforts tend to fail if there is further damage from ORV use while the plants are germinating and growing.

— Adam is Science Coordinator for Wildlands CPR and Allison is Conservation Biologist for the Wild Utah Project.

Literature Cited

Blackburn, J., and M. Byrd Davis.  1994.  Off-Road Vehicles: Fun and/or Folly. Livingston, Kentucky. ASPI Publications. 56 pp.
Bosworth, D.  2003.  Managing the National Forest System: Great Issues and Great Diversions.  Speech presented to the San Francisco Commonwealth Club and Berkeley University on Earth Day, April 22.
Brown, A.C., and A. McLachlan. 2002.  Sandy shore ecosystems and threats facing them:  some predictions for the year 2025.  Environmental Conservation.  29(1): 62-77.
CEQ (Council on Environmental Quality).  1979.  Off-Road Vehicles on Public Land.  Council on Environmental Quality, Washington, DC.  PrEX 14.2: V53.                                                           
Cole, D.N., and N.P. Bayfield. 1993. Recreational trampling of vegetation: Standard experimental procedures. Biological Conservation 63(3): 209-215.
Davidson, E. and M. Fox.  1974.  Effects of off-road motorcycle activity on Mojave desert vegetation and soil.  Madrono 22: 381-412.
Gelbard, J.L., and S. Harrison.  2003.  Roadless habitats as refuges for native grasslands: interactions with soil, aspect, and grazing.  Ecological Applications 13(2): 404-415.
Leininger W.C., and G.F. Payne.  1971.  The Effects of Off-Road Vehicle Travel on Rangeland in Southwestern Montana.  Research Report 153, Agricultural Experiment Station, Montana State University, Bozeman, MT. 47 pp.
Misak R.F., J.M. Al Awadhi, S.A. Omar, and S.A. Shahid.  2002.  Soil degradation in Kabad area, southwestern Kuwait City. Land Degradation and Development 13(5): 403-415.
MSU (Montana State University, Extension Service).  1992.  Controlling Knapweed on Montana Rangeland.  Circular 311, February 1992.
MWLAP and GCC (Ministry of Water, Land and Air Protection and Grasslands Conservation Council).  2004.  Best Management Practices for Recreational Activities on Grasslands in the Thompson and Okanagan Basins. Ministry of Water, Land and Air Protection, Victoria, B.C.
Rooney, T.P.  2005.  Distribution of ecologically-invasive plants along off-road vehicle trails in the Chequamegon National Forest, Wisconsin.  The Michigan Botanist 44: 178-182.
Stensvold, M.C.  2000.  The conservation status of Ophioglossaceae in southern Alaska. Proceedings of Botany 2000.  August 6-10.  Portland, OR.
Stout, B.M., III.  1992.  Impact of Off-Road Vehicle Use on Vegetative Communities of Northern Canaan Valley, West Virginia.  Final Report of The Canaan Valley Task Force.  Wheeling Jesuit College, Department of Biology, Wheeling, WV.
USDI (U.S. Department of the Interior, Bureau of Land Management).  2000.  Use of weed-free forage on public lands in Nevada.  65 Federal Register 545444 (September 8, 2000).
Wilshire, H.G.  1983.  The impact of vehicles on desert soil stabilizers. In: Webb RH, Wilshire HG (eds.), Environmental Effects of Off-Road Vehicles, Springer-Verlag, New York, NY. pp 31-50.