A Review of the Impacts of ORVs on Soil

Healthy forest soils provide nutrients and the physical foundation for plants.  Soils are also home to many animals that burrow beneath the surface.  One important characteristic of forest soil is that it contains pore space or tiny cracks and crevices that fill with air and water.  Pore spaces allow rain and snowmelt to enter the soil, gases to escape, and tree and other plant roots to grow.  

Compaction and Erosion
Off-road vehicles can cause compaction of soil pore spaces.  Weighing several hundred pounds, ORVs can compress and compact soil (Nakata et al. 1976, Snyder et al. 1976, Vollmer et al. 1976, Wilshire and Nakata 1976), thus reducing its ability to absorb and retain water (Dregne 1983), and decreasing soil fertility by harming the microscopic organisms that would otherwise break down the soil and produce nutrients important for plant growth (Wilshire et al. 1977).  An increase in compaction decreases soil permeability, resulting in increased flow of water across the ground and reduced absorption of water into the soil.  This increase in surface flow concentrates water and increases erosion of soils (Wilshire 1980, Webb 1983, Misak et al. 2002).  Increased erosion due to ORVs also adds sediment to streams (Sack and da Luz 2003, Chin et al. 2004), which decreases water quality, buries fish eggs, and generally reduces the amount and quality of fish habitat (Newcombe and MacDonald 1991).

Erosion of soil is accelerated in ORV use areas directly by the vehicles and indirectly by increased runoff of precipitation, and by creating conditions favorable to wind erosion (Wilshire 1980).  Knobby and cup-shaped protrusions from ORV tires that aid the vehicles in traversing steep slopes are responsible for major direct erosional losses of soil.  As the tire protrusions dig into the soil, forces far exceeding the strength of the soil are exerted to allow the vehicles to climb slopes.  The result is that the soil and small plants are thrown downslope in a “rooster tail” behind the vehicle.  This is known as mechanical erosion, which on steep slopes (about 15o or more) with soft soils may erode as much as 40 tons/mi (Wilshire 1992).  The rates of erosion measured on ORV trails on moderate slopes exceed natural rates by factors of 10 to 20 (Iverson et al. 1981, Hinckley et al. 1983), whereas use of steep slopes has commonly removed the entire soil mantle, exposing bedrock.  Measured erosional losses in high use ORV areas range from 1.4-242 lbs/ft2 (Wilshire et al. 1978) and 102-614 lbs/ft2 (Webb et al. 1978).  A more recent study by Sack and da Luz (2003) found that off-road vehicle use resulted in a loss of more than 200 lbs of soil off every 100 feet of trail each year.  Some soils, such as those supporting biological soil crusts, require decades to centuries to recover (Belnap 2003).  

Most soils are vulnerable to compaction and erosion due to several factors.  An analysis of more than 500 soils at more than 200 sites found that virtually all types of soils are susceptible to ORV damage (Schubert and Associates 1999).  Some soils such as clay-rich soils, while less sensitive to direct mechanical displacement by ORVs, have higher rates of erosion than most other soil types, and when compacted can result in a strong surface seal that can increase rainwater runoff and increase gullying (Sheridan 1979). Sandy and gravelly soils are susceptible to direct excavation by ORVs, and when stripped of vegetation they are susceptible to rapid erosion processes – usually by rill and gully erosion. Compaction is also greater in wet, poorly drained soils than well-drained soils (Willard and Marr 1970, Burde and Refro 1986).  Finely textured soils are more prone to erosion than coarser soils (Welch and Churchhill 1986).  

In addition to the chemical make-up of soils, location of ORV routes is a determinant to whether soils erode.  Routes on steep slopes (about 15o or more) are more likely to cause erosion (Welch and Churchhill 1986), as are routes in higher elevation alpine areas (Willard and Marr 1970, Marion 1994).  Additionally, forests that receive higher precipitation are more susceptible to erosion than drier forests (Cole and Bayfield 1983, Burde and Renfro 1986).

ORV impacts on forest soils are compounded by the loss of vegetation following ORV use.  It is well known that stable vegetation keeps soil in its place (Wilshire 1983, Belnap 1995), and once anchoring vegetation is removed, soil erosion increases.  For example, soil exposure is increased when vehicles damage or uproot plants, thereby allowing the exposed soils to easily become wind blown or washed away by water.  Wilshire et al. (1978) report on both the direct effects of ORVs on vegetation such as crushing and uprooting of foliage and root systems, as well as the indirect effects caused by the concomitant erosion.  This includes undercutting of root systems as vehicle paths are enlarged by erosion, creation of new erosion channels on land adjacent to vehicle-destabilized areas due to accelerated runoff or wind erosion, burial of plants by debris eroded from areas used by vehicles, and reduction of biological capability of the soil by physical modification and stripping of the more fertile upper soil layers (Wilshire et al. 1978).

Impacts of ORVs on Cryptobiotic Soils
While cryptobiotic soil crusts are more often associated with arid and semi-arid regions, they are important components of some western forests as well.  Cryptobiotic crusts, which were historically widespread in western U.S. arid lands, are being rapidly depleted across rangelands today.  These crusts increase the stability of otherwise easily erodible soils, increase water infiltration in a region that receives limited precipitation, and increase fertility of soils often limited in essential nutrients such as nitrogen and carbon (Johansen 1993, Belnap et al. 1994).  ORVs are highly destructive to these fragile cryptobiotic crusts.  A single pass of an ORV through cryptobiotic crusts will increase wind and water erosion of surface soils that were previously protected by the crusts (pers. Comm., Howard Wilshire, USGS-retired).  This in turn can trigger rapid loss of the underlying topsoil, which can take up to 5,000 years to reform naturally in arid regions (Webb 1983).

The destruction of cryptobiotic soils by ORVs can reduce nitrogen fixation by cyanobacteria, and set the nitrogen economy of nitrogen-limited arid ecosystems back decades.  Even small reductions in crust can lead to diminished productivity and health of the associated plant community, with cascading effects on plant consumers (Davidson et al. 1996).  In general, the deleterious effects of ORV use on cryptobiotic crusts is not easily repaired or regenerated.  The recovery time for the lichen component of crusts has been estimated at about 45 years (Belnap 1993).  After this time the crusts may appear to have regenerated to the untrained eye.  However, careful observation will reveal that the 45 year-old crusts will not have recovered their moss component, which will take an additional 200 years to fully come back (Belnap and Gillette 1997).

Additionally, radical reduction of soil biota, including bacteria and fungi, results from compaction.  Soil microorganisms in desert soils exposed to ORV use are typically reduced from about 4 to less than 1 million/g, which in turn reduces the bacterial oxidation that makes nitrates available to plants (Liddle 1997).   A severe loss of nitrates to plants is significant in typically nitrogen poor arid environments, and may even eventually lead to desertification (Belnap 1995).

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

References
Belnap, J.  1993.  Recovery rates of cryptobiotic crusts: inoculant use and assessment methods.  Great Basin Naturalist 53:89-95.
Belnap, J.  1995.  Surface disturbances—their role in accelerating desertification.  Environmental Monitoring and Assessment 37: 39-57.
Belnap, J.  2003.  The world at your feet:  desert biological soil crusts.  Frontiers in Ecology and the Environment 1(5): 181-189.
Belnap, J. and D.A. Gillette.  1997.  Disturbance of biological soil crusts: impacts on potential wind erodibility of sandy desert soils in SE Utah.  Land Degradation and Development 8: 355-362.
Belnap, J., K.T. Harper, and S.D. Warren.  1994.  Surface disturbance of cryptobiotic soil crusts: Nitrogenase activity, chlorophyll content, and chlorophyll degradation.  Arid Lands Research and Rehabilitation. 8: 1-8.
Burde, J.H., and J.R. Renfro.  1986.  Use impacts on the Appalachian Trail. Pages 138-143 in R.C. Lucas, editor. Proceedings: National Wilderness Research Conference: Current Research. USDA Forest Service, Intermountain Research Station, General Technical Report INT-212, Ogden, UT.                                                                   
Chin, A., D.M. Rohrer, D.A. Marion, and J.A. Clingenpeel.  2004.  Effects of all terrain vehicles on stream dynamics.  Pages 292-296 in Guldin, J.M. technical compiler, Ovachita and Ozark Mountains Symposium: Ecosystem Management Research.  USDA, Forest Service, Southern Research Station.General Technical Report SRS-74, Ashville, NC.
Cole, D.N., and N.P. Bayfield 1993. Recreational trampling of vegetation: Standard experimental procedures. Biological Conservation 63(3): 209-215.
Davidson, D.W,  W.D. Newmark, J.W. Sites, D.K. Shiozawa, E.A. Rickart, K.T. Harper, and R.B. Keiter.  1996.  Selecting Wilderness areas to conserve Utah’s biological diversity.  Great Basin Naturalist 56: 95-118.
Dregne, H.E. 1983.  Physical effects of off-road vehicle use.  Pages 15-30 in R.H. Webb and H.G. Wilshire.  Environmental Effects of Off-Road Vehicles: Impacts and Management in Arid Regions. Springer-Verlag, New York.  
Hinckley, B.S., Iverson, R.M. and  B. Hallet.  1983.  Accelerated water erosion in ORV-use areas.  Pages 81-96 in Webb, R.H. and H.G. Wilshire, editors, Environmental Effects of Off-Road Vehicles.  Springer-Verlag, New York.
Iverson, R.M., Hinckley, B.S., and R.H. Webb.  1981.  Physical effects of vehicular disturbance on arid landscapes.  Science 212: 915-917.
Johansen, J.R.  1993.  Cryptogamic crusts of semiarid and arid lands of North America. Journal of Phycology 29: 140-147.
Liddle, M.  1997. Recreation Ecology. Chapman & Hall, London. 639 pp.
Marion, J.L.  1994.  An assessment of trail conditions in Great Smoky Mountains National Park. U.S. Department of the Interior, National Park Service, Southeast Region, Research/Resources Management Report, Atlanta, GA.
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 & Development.  13(5): 403-415.
Nakata, J.K., H.G. Wilshire, and G.G. Barnes.  1976.  Origin of Mojave Desert dust plumes photographed from space.  Geology 4(11): 644-648.
Newcombe, C.P., and D.D. MacDonald.  1991.  Effects of suspended sediments on aquatic ecosystems.  North American Journal of Fisheries Management 11: 72-82.
Sack, D., and S. da Luz.  2003.  Sediment flux and compaction trends on off-road vehicle (ORV) and other trails in an Appalachian forest setting.  Physical Geography 24(6): 536-554.
Schubert and Associates.  1999.  Petition to enhance and expand regulations governing the administration of recreational off-road vehicle use on National Forests.  Published by Wildlands CPR, Missoula, MT 188p.  www.wildlandscpr.org/orvs/ORVpetition.doc
Sheridan, D.  1979.  Off-road vehicles on public land.  Council on Environmental Quality, U.S. Government Printing Office.  Report No. 041-011-00041-6.  Washington, D.C.
Snyder, C.T., D.G. Frickel, R.E. Hadley, and R.F. Miller.  1976.  Effects of off-road vehicle use on the hydrology and landscape of arid environments in central and southern California.  U.S. Geological Survey Water-Resources Investigations Report #76-99.  45p.
Vollmer, A.T., B.G. Maza, P.A. Medica, F.B. Turner, and S.A. Bamberg.  1976.  The impact of off-road vehicles on a desert ecosystem.  Environmental Management 1(2):115-129.
Webb, R.H..  1983.  Compaction of desert soils by off-road vehicles.  Pages 51-79 in: Webb, R.H. and Wilshire, H.G., editors, Environmental Effects of Off-Road Vehicles. Springer-Verlag, New York.
Webb, R.H., Ragland, H.C., Godwin, W.H., and D. Jenkins.  1978.  Environmental effects of soil property changes with off-road vehicle use.   Environmental Management 2: 219-233.
Welch, D.M., and J. Churchill.  1986.  Hiking trail conditions in Pangnirtung Pass, 1984, Baffin Island, Canada.  Parks Canada Report, Ottawa, Canada.
Willard, B.E., and J.W. Marr.  1970.  Effects of human activities on alpine tundra ecosystems in Rocky Mountain National Park, Colorado.  Biological Conservation 2:257-265.
Wilshire, H.G.  1980.  Human causes of accelerated wind erosion in California’s deserts.  Pages 415-433 in D.R. Coates and J.B. Vitek, editors, Thresholds in Geomorphology.  George Allen & Unwin, Ltd., London.
Wilshire, H.G. 1992.  The wheeled locusts. Wild Earth 2: 27-31.
Wilshire, H.G.  1983.  The impact of vehicles on desert soil stabilizers.  Pages 31-50 in Webb, R.H. and Wilshire, H.G., editors, Environmental Effects of Off-Road Vehicles. Springer-Verlag, New York.
Wilshire, H.G. and J.K. Nakata.  1976.  Off-road vehicle effects on California’s Mojave Desert. California Geology 29(6):123-132.
Wilshire, H.G., G.B. Bodman, D. Broberg, W.J. Kockelman, J. Major, H.E. Malde, C.T. Snyder, and R.C. Stebbins.  1977.  Impacts and management of off-road vehicles.  The Geological Society of America.  Report of the Committee on Environment and Public Policy.
Wilshire, H.G., Nakata, J.K., Shipley, S., and K. Prestegaard.  1978.  Impacts of vehicles on natural terrain at seven sites in the San Francisco Bay area.  Environmental Geology 2: 295-319.