Webb et al 1978

Environmental Effects of Soil Property Changes With Off-Road Vehicle Use

ROBERT H. WEBB, H. CRAIG RAGLAND, WILLIAM H. GODWIN, 3ENNIS JENKINS Department of Biology University of Redlands Redlands, CA 92373

ABSTRACT / The effects of off-road vehicles (ORVs) on the physical and chemical properties of 6 soil series were rlleasured at Hollister Hills State Vehicular Recreation Area in central California. Accelerated soil erosion and the alteration of surface strength, bulk density, soil moisture, temperature, and soil nutrients were quantified to gain an insight into the difficulty of revegetating altered, or modified, areas.

Erosion is severe at Hollister Hills, particularly in coarse grained soils on steep slopes. Erosion displaced 0.5 and 3.0 metric tons per square meter on 2 trails on gravelly sandy loam, and 0.3 metric tons/m 2 from a trail on sandy loam. The surface strength and bulk density increased while the soil moisture decreased in gravelly sandy loam, coarse sandy loam, sandy loam, and clay. Clay loam had an increased surface strength with variably increased bulk density and no decrease in soil moisture. Diurnal temperature fluctuations increased and organic material and soil nutrients decreased in soil modified by vehicles.

These property changes increase the erosion potential of the soil, impede germination of seedlings, and slow natural revegetation. Management methods in ORV-use areas should include planning trails by prior application of the universal soil loss equation and soil surveys, trail closure before complete loss of the soil mantle, and revegetation of closed areas.

I n t r o d u c t i o n

Several recent studies have shown the importance of developing stringent management plans for areas used by off-road vehicles (Stebbins 1974, Lnckenbach 1975, Berry 1977, Geological Society of America 1977). Off- road vehicles cause accelerated soil erosion (Snyder and others 1976, Wilshire and Nakata 1977), denudation and loss of floral species diversity (Keefe and Berry 1973, Davidson and Fox 1974, Duck 1977), and reductions in animal populations (Busack and Bury 1974, Hicks and others 1976, Bury and others 1977). These studies dem- onstrate that off-road vehicles cause severe effects on all components of the ecosystem. It is necessary to study ways to minimize these effects in areas set aside for ORV recreation.

The most important long term effect of ORVs is accelerated erosion because of the removal of life-supporting soil. The formation of topsoil to replace eroded materials takes 300-500 years per inch (Hudson 1971) and revegetation of heavily used areas depends upon the condition of the soil. Several studies show the erosion and compaction of desert soil surfaces in ORV-use areas (Snyder and

KEY WORDS: Off-road vehicles, Soil properties, Erosion, Trail management, Universal Soil loss equation, Traffic impact

Note: The unit values for penetration are given as inches in this paper because the instrument used recorded penetration in inches.

Environmental Management, Vol. 2, No. 3, pp. 219-233

others 1976, Wilshire and Nakata 1976 and 1977, Webb 1977). Snyder and others (1976) and Wilshire and Nakata (1976b) determined the effects of ORVs on the properties of soils in arid and semi-arid climatic zones. The purpose of this study is to measure and describe the effects of ORVs on the physical and chemical properties of distinctively different soils at Hollister Hills State Vehicular Recreation Area ($VRA), and to show the problems'caused by ORVs and the feasibility of management in ORV-use areas to reduce long term resource damage.

Hollister Hills SVRA, o p e r a t e d by the California Department of Parks and Recreation, is a heavily used ORV facility located in the Gabilan Range of central California 20 miles east of Salinas. The park consists of approximately 1350 hectares of land ranging in elevation from 221-739 m. Off-road vehicle use at Holfister Hills began in 1941 under the supervision of Howard Harris, the former owner of the property, and heavy use by the public began in 1969. When the State of California acquired the property in 1975, over 100 miles of trail were present on terrain varying from 0-40 ~ slopes. More than 25,000 motorcycles were used in the facility from October 1975 to September 1976 (Department of Parks and Recreation, unpublished data).

Detailed studies were carried out during January 1977, of sites located on 6 different soil series as determined from a U.S. Department of Agriculture Soil Sur- vey (1969). Soil textural data are available in Webb and others (1977).

0364-152X/78/0002-0219 $03.00 �9 1978 Springer-Verlag N~w York

2 1 9

220 Robert H. Webb, H.Craig Ragland, William H. Godwin, Dennis Jenkins

Figure 1. Accelerated soil erosion in Cieneba series, HoUister Hills SVRA.

Figure 2. "The Chute" on Cieneba series. This hiUclimb had 3.0 metric tons of soil per square meter of trail displaced.

Effects of Off-Road Vehicles on Soil 221

Figure 3. A trail on Nacimiento series showing erosion and compaction of the surface soil.

Description of Soils

T e n soil series occur at Hollister Hills SVRA (U.S.D.A. 1969). The San Andreas fault zone divides these series into 2 general associations underlain by di f ferent parent materials. The soils southwest o f the fault zone are o f the Cieneba-Sheridan-Auberry association and are underlain by Mesozoic granites and pre-Mesozoic limestone and dolomite (T. H. Dihblee, unpublished data). Chaparral and oak communities are the dominant vegetative haM- tats. Soils northeast o f the fault zone are of the Diablo- San Benito association and are underlain by Tertiary sandstone, shale, and siltstone (T. H. Dibblee, unpublished data). The major vegetative habitats present are grassland and oak woodland.

Six o f the soil series at Hollister Hills were studied. Cieneba gravelly sandy loam, the most common soil series at the SVRA, occurs on steep hillsides (to 40 ~ slopes), has a soil mant le 25-50 cm deep, and has a vegetative cover o f chamise and coyote 15rush. T h e Sorrento series occurs in and southwest o f the San Andreas fault zone and is higlJy variable texturally because o f its alluvial nature. Cometa sandy loam occurs on flat alluvial terraces (0-5 ~ slopes) and has a vegetative cover o f grass.

Nacimiento sandy loam o f the Diablo-San Benito Asso- ciation occurs on steep hillsides (20-35 ~ slopes), has a shallow soil mantle 20-50 cm deep (U.S.D,A. 1969), and

has a vegetative cover o f chamise and grasses. San Benito clay loam also occurs on steep slopes near the tops o f ridges, and has a vegetative cover o f grass; this series is the most common soi~ northeast o f the fault zone. Diablo clay occurs on ridgetops and in drainages in the northeast corner o f Hollister Hills, and has a vegetative cover o f grass.

M e t h o d s

Erosion transects were used to estimate soil removal f rom hillsides with surfaces o f min imum convexity or concavity. This method assumed that the original surface was approximately level across the trail. Stakes were placed on either side o f the trail flush with the undisturbed soil at 10 m intervals, and a llne was stretched tightly across the trail, The distance between this hypo- thetical former surface and the present soil surface was measured at 20 cm intervals along the line. These dis- tances were plotted for each interval and a planimeter was used to measure the cross-sectional area. Subsequent calculations were used to obtain the total volume o f displaced soil, and multiplication o f this volume by the bulk density de termined tons o f soil displaced f rom the trail.

Several s tandardized tests were used to determine the propert ies o f modified and undis turbed soil in each


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series. The surface strength was measured with a 75 cm long p e n e t r o m e t e r t ipped with a 30 ~ cone, and was recorded as the inches o f penetrat ion of the device with 75 kg of body weight applied. The bulk density was de termined for depths of 0-10 cm, 10-20 cm, and 20-30 cm by weighing cores of known volume taken with a soil auger. These cores were then dried at 105~ for 6 -24 hrs

Note: In Figures 4-13, dashed lines with hollow points represent modified soil samples, and solid fines with solid points represent undisturbed soil samples.

to de termine the soil moisture present when the samples were taken, Oven-dried samples were heated at 400~ for 1 hr to de termine the amount of organic material present. The diurnal tempera ture fluctuation was recorded in half hour intervals at depths of 2 cm, 6 cm, and 12 cm with thermister probes attached to a portable tele-ther- mometer . The soil pH was measured with a Beckman digital p H meter after soaking the samples in distilled water for 10 minutes. Coulometric titration methods were used to measure the exchangeable calcium and magnesium content.

The results o f all tests except soil tempera ture were analyzed statistically to determine possible property changes with modification, Penetrometer readings and soil samples were taken for modified and undis turbed soil along trail transects at intervals o f 1 m and 5 or 10 m intervals, respectively. Modified and undisturbed sample points for each interval were approximately 5 m apart, which allowed for natural variation in soil texture along the transect while maintaining the same soil type for each pair. The differences between the measurements for each sample pair were compared using a one-sided paired-sample t-test to verify changes statistically. Two assumptions were necessary for the t-test comparison: the results for each property measured were assumed to lie in a normal distributiori for each soil series, and the null hypothesis assumed that no property changes occurred with ORV soil modification. Also, the depth o f the modified surfaces in the soil mantle was ignored because the propert ies o f the material exposed at the surface were of interest.

Results

Erosion

Accelerated soil erosion is widespread at Hollister Hills SVRA because of past land management and ORV use (Fig. 1). Soil on steep slopes shows severe gullying in firebreaks and trails denuded by ORVs; also, ORVs mechanically erode the soil and bedrock. The soil displaced by gullying and direct mechanical erosion was measured for 2 trails on Cieneba gravelly sandy loam. One trail, called "The Chute," had 3.0 metric tons/m ~ displaced f rom a 28 ~ slope (Fig. 2); the trail had one gully 200 cm deep and was 90 cm into bedrock at one point because vehicle impacts caused mechanical destabilization of the parent material. Another trail on Cieneba series had 0.5 metric tofis/m 2 displaced f rom a 26 ~ slope; a third trail on Nacimiento sandy loam had 0.3 metric tons/m s displaced from a 30 ~ slope (Fig. 3), Cieneba, Cometa, and


Nacimiento series had severe erosion potentials and were highly susceptible to gullying (Webb and others 1977). T h e clay loam and clay had modera te erosion potentials and e roded by rilling and sheetwash ra ther than by gullying. No erosion transects were m e a s u r e d on clay or clay loam.

Su rface Strength

Fig. 4 shows the results o f a pene t rome te r transect on Nacimiento sandy loam plotted as the inches o f penetra- t ion for und i s tu rbed and modif ied soil at each sample point a long the trail. These results show a decrease in penet ra t ion for modif ied soil, which indicates an increased surface s t rength. T h e pai red-sample t-value o f 42.9 indicates that modif ied soil has a significantly h igher surface s t rength beyond the 0.0005 alpha significance level (probability), which disproves the null hypothesis o f no di f ference between modif ied and und i s tu rbed surface s t rengths. Similar results were obtained for San Benito clay loam and Diablo clay (Fig. 5). Table 1 summar izes the surface s t rength results; the surface s t rength o f all 6 soil series was found to increase significantly with modification.

Bulk Density

Table 2 shows the results o f bulk density m e a s u r e - ments for the 6 soil series. ORV-modif icat ion had the greatest effect on the bulk density o f Cieneba series (Fig. 6a); 2 transects on this soil series showed significantly increased bulk density (compaction) to a dep th o f 30 cm. Sorrento coarse sandy loam also showed significant increases at all depths , a l though subsurface densities showed less increase than those o f Cieneba series.

Table 2 Summary of transect results for bulk density

Mean bulk density (g/cc)

Depth Undis- Soil Series (cm) m r b e d Modified t-value df

Cieneba (I) 0-10 1.43 1.91 5.82** 5 10-20 1.64 1.96 20.4*** 5 20-30 1.76 1.97 2.8l i t 5

Cieneba (2) 0-10 1.52 2.20 7.72*** 5 10-20 1.70 2.25 9.9*** 5 20-30 1.96 2.32 6.13"* 5

Sorrento 0-10 1.67 2.17 5.41"* 5 10-20 1.68 2.12 3.63* 5 20-30 1.61 1.94 1.73ns 5

Cometa 0-10 1.70 1.96 3.76* 5 10-20 1.82 1.85 0.64ns 5 20-30 1.85 1.83 -0.26ns 5

Nacimiento (1) 0-10 1.46 1.78 7.15"** 5 10-20 1.46 1.80 3.91" 5 20-30 1.45 1.67 1.24ns 5

Nacimiento (2) 0-10 1.84 1.66 -1.15ns 5 10-20 1.70 1.92 1.26ns 5 20-30 1.65 1.89 2.74:~ 5

San Benito 0-10 1.35 1.43 2.04ns 4 (before-after) 10-20 1.19 1.26 0.64ns 4

20-30 1.37 1.24 -0.79ns 4 San Benito 0-10 1.52 1.76 2.42t 4

10-20 1.59 1.64 0.58ns 4 20-30 1.40 1.76 4.20* 4

Diablo 0-10 1.15 1.41 3.58tt 3 10-20 1.00 1.63 3.02t 3 20-30 1.29 1.69 3.62t# 3

***--signifiCant to p < 0.0005 **--significant to p < 0.005 *--significant to p < 0.01

#t--significant to p < 0.025 t--significant to p < 0.05 m--not significant

Table 1 Summary of transect results for surface strength

Soil Series

Mean Penetrance (in.)

Undisturbed Modified t-value

Degrees of Freedom

(dr)

Cieneba (1) 3.71 1.51 9.4*** 50 Cieneba (2) 3.72 0.85 13.6"** 30 Sorrento 1.94 0.91 16.2"** 25 Cometa 2.28 1.49 6.2*** 10 Nacimiento (1) 2.22 0.93 29.7*** 20 Nacimiento (2) 2.10 0.50 42.9*** 50 San Benito (1) 1.87 0.45 30.7*** 30 San Benito (2) 3.43 1.24 10.0"** 30 San Benito (3) 3.36 1.68 17.8"** 30 Diablo 2.90 0.70 23.2*** 35

***--significant to p < 0.0005

T h e 2 sandy loams showed variable results. One transect on Cometa sandy loam had an increased surface bulk density with modification, a l though the subsurface bulk density was unchanged . Similar results were obtained for a transect on Nacimiento series (Fig. 6b); a th i rd transect on this series showed increased subsurface densities while the surface density was not affected by vehicle use.

Bulk densities o f modif ied San Benito and Diablo series increased at the surface but were variable in the subsurface. Bulk density results were obtained for the San Benito series before and after a motocross race; a l though the transect site was subjected to 8500 motorcycle laps, the bulk density r emained u n c h a n g e d to a dep th o f 30 cm (Fig. 7).


Table 3 Summary of temperature results for paired thermister probes

Maximum Daytime Temperature (C ~ )

Minimum Nighttime Temperature (C ~

Soil Series Depth (cm) Modified Undisturbed Modified Undisturbed

Cieneba (1) 2 22.0 19.5 6.5 7.0 6 23.0 22.0 6.5 7.0

12 15.0 15.0 9.0 10.0 Cieneba (2) 2 22.5 19.0 - - - -

12 17.0 13.0 8.0 9.5 Nacimiento (1) 2 28.5 24.0 3.0 5.0

6 25.8 22.0 5.0 7.0 12 19.5 15.5 9.0 10.0

Nacimiento (2) 2 29.0 22.0 4.0 5.5 12 18.0 13.5 10.0 9.5

San Benito (1) 2 15.5 10.0 2.0 2.0 6 11.0 8.0 3.0 3.0

12 9.5 6.0 3.5 3.0 San Benito (2) 6 11.0 6.5 3.0 4.0

12 9.0 6.0 3.5 3.0

Table 4 Summary of soil moisture results

Mean Hydroscopic Moisture (%)

Depth Undis- Soil Series (cm) turbed Modified t-value

Cieneba (1) 0-10 13.9 8.9 5.51"* Cieneba (2) 0-10 9.8 4.9 8.9***

10-20 8.7 5.5 10.1"** 20-30 8.1 5.3 5.62**

Sorrento 0-10 15.4 8.0 9.7*** Cometa 0-10 13.2 10.5 4.42** Nacimiento (1) 0-10 17.4 11.8 9.2*** Nacimiento (2) 0-10 12.4 8.2 6.48**

10-20 12.3 8.0 5.28** 20-30 12.8 7.3 7.13'**

San Benito 0-10 16.3 15.4 0.50ns Diablo 0-10 28.7 21.2 5.09**

***--significant to p < 0.0005 **--significant to p < 0.005 ns--not significant

Soil Moisture

Table 3 shows the results for the soil mois ture measurements . Only one soil type, San Benito clay loam, showed no change in soil mois ture with modification in the uppe r 10 cm (Fig. 8a): All o ther spit types, including

df Diablo clay (Fig. 8b), showed significant decreases with 5 modification. T h e most p ronounced changes occurred in 5 Cieneba and Nacimiento series, which showed significant 5 decreases in soil moisture to a dep th o f 30 cm in the 5 5 modif ied soil (Fig. 9).

5 5 Soil Temperature

5 Diurnal t empera tu re curves were obtained for Cie- 5 neba gravelly sandy loam, Nacimiento sandy loam, and 5 4 San Benito clay loam. T h e 2 cm, 6 cm, and 12 cm curves 3 for Nacimiento series (Fig. 10) show that modification

caused the d iurnal t empera tu re fluctuation to become more ex t reme to a dep th o f 12 cm; soil t empera tu res were wa rmer du r ing the day and cooler at n ight in modified soil. T h e curves obtained for Cieneba series also

Table 5 Summary of exchangeable calcium and magnesium results Exchangeable Calcium Mean (ppm) Exchangeable Magnesium Mean (ppm)

Soil Series Modified Undisturbed t-value Modified Undisturbed t-value df

Cieneha (1) 600 1040 6.20** 137 195 5.13"* 5 Cieneba (2) 686 2000 5.07** 185 236 1.18ns 5

**--significant to p < 0.005 us--not significant


show a more ext reme fluctuation in tempera ture with modification (Fig. 1 la). The results for San Benito series (Fig. 1 lb) were different in that the modified soil temperature was warmer or about the same as undis turbed soil at night. A summary of the tempera ture changes with modification appears in Table 4.

Organic Material and Nutrient Content

One transect on San Benito clay loam was tested for the change in the amount of organic material present in the upper 10 cm o f soil. The results show that the per- centage of organic material decreased with ORV soil modification (Fig. 12); the t-value o f 4.09 obtained for this transect verifies the decrease statistically (p < 0.01). T h e results for a transect on Cieneba series show that the soil pH also decreased with modification (Fig. 13); again, this decrease was statistically significant (p < 0.005). Table 5 shows the results f o r the exchangeable calcium and magnesium tests. T h e results for Cieneba series show a significant decrease in exchangeable calcium for 2 transects and a significant decrease in exchangeable magnesium for one transect; the second transect showed no difference between the amounts of exchangeable magnesium in modified and undis turbed soil.

Dicussion

The results show soil proper ty changes for all 6 soil series as a result o f ORV use. ORV soil modification increased the surface strength, increased the bulk density, and decreased the soil moisture content in gravelly sandy loam, coarse sandy loam, sandy loam, and clay. San Benito clay loam showed increased surface strength with variable increased bulk density and no decrease in soil moisture. Although this series shows some resilience to ORV soil modification, it occurs on only about 15 percent o f the land surface at Hollister Hills SVRA. The diurnal tempera ture fluctuation o f gravelly sandy loam increased in modified soil, while the daytime tempera ture increased in modified clay loam. Decreases occurred in soil pH, organic material, and nutr ient content with modification. These proper ty changes contribute to accelerated erosion and increase the environmental stress on plant seedlings, and thus create management problems in ORV-use a r e a s .

Soil Modification and Accelerated Erosion

The causes of soil erosion are known to the extent that an equation has been developed to quantify erosion rates in agricultural, construction, and undis turbed areas (Wis-

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Figure 5. Results of pene~rometer transects for a) Diablo clay, b) San Benito clay loam. Dashed lines represent modified samples and solid lines represent undisturbed samples.

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chmeier 1974). The universal soil loss equation describes the rate of sheet and rill erosion as

A = R x K x L S x C x P

where A is the erosion rate, R is the rainfall intensity factor, K is the soil erodibility factor, LS is the slope angle and length factor, C is the vegetative cover factor, and P is the erosion management practice factor. ORV soil modification has several important effects on these factors, which increase the erosion potential of modified soil.

A comparison can be made for the change in the erosion rate of a site before and after modification. The soil erodibility, vegetative cover, and erosion management factors change with modification of the undisturbed site. The vegetative cover factor increases because vegetation is removed from ORV-modified areas. The soil erodibility factor increases because of compaction and loss of organic material from the surface soil; the erosion management factor also increases because of compaction and because ORV trails follow straight paths that allow no dissipation of runoff energy (U.S.D.A. 1975). These changes will increase the erosion rate of the site after the soil is modified. The accelerated erosion brought about by ORV use is the same as that which occurs with other land uses when the parameters of the universal soil loss equation are not taken into consideration.

The universal soil loss equation should be used in the planning and management of ORV-use areas. Planning the location of ORV trails with the universal soil loss equation can minimize the rate of erosion from ORV use. Since the factors of the equation depend on the site location, they can be manipulated to determine trail sites that will erode at a minimum rate. Choosing trail sites on hillsides with low slope angles and making upslope trail lengths shorter will reduce the slope length and angle factor. The use of soil surveys in choosing soils with a low erosion potential will reduce the soil erodibility factor. The vegetative cover and erosion management factors cannot be manipulated because o f compaction and denudation in active ORV trails. The rainfall intensity factor cannot be changed for a given location.

Property Changes and Revegetation Many "studies have documented the relationship

between seedling germination and soil physical property changes. The strength of the soil is the most important limiting property to root growth (Taylor and Gardner 1963). Increased surface strength impeded the penetration of corn seedling roots in clay (Phillips and Kirkham

1962), decreased elongation of wheat and pea roots in loam (Barley and others 1965), and decreased taproot growth of cotton seedlings in sandy loam (Taylor and Gardner 1963, Taylor and Burnett, 1964). Grimes and others (1972) showed that high soil strength in sandy loam impeded both cotton and corn root development; Taylor and Burnett (1964) found that surface-strength impedance-of root growth was species independent.

High bulk densities impede plant growth in the same manner as increased surface strength. Veihmeyer and Hendrickson (1948) found that plants and trees become shallow-rooted in high density soils and extract little or no moisture from high density subsoils; high bulk densities decreased sunflower taproot penetration in clay, clay loam, and sandy loam. Veihmeyer and Hendrickson (1948) also determined limiting bulk densities to root growth of 1.6-1.7 gJcm ~ for clay and 1.75 gJcm 3 for sands. No roots penetrated layers with bulk densities higher than 1.90 g./cm 3. Corn and cotton taproot growth also decreased with increased bulk density in clay and sandy loam (Phillips and Kirkham 1962).

The surface strength is dependent upon the soil moisture present in the surface layer, and drying of surface soils causes a seal to form, which impedes seedling growth (Arndt 1965a, b). Also, increased bulk density decreases the interstitial pore spaces, which decreases soil permea- bility to rainfall and the soiPs water-holding capacity (Buckmar/and Brady 1969). Taylor and Gardner (1963) indicate that increased surface strength and bulk density, and decreased soil moisture cause significant decreases in root penetration and growth.

An increase in the bulk density increases the heat conductivity and decreases the heat capacity of soil (Shurgin 1965). More importantly, ORVs expose the soil surface during soil modification, which decreases the thermal insulation provided by the vegetative cover (Rus- sell 1953, Shurgin 1965). These thermal property changes increase the diurnal temperature fluctuation of modified soils, as the results obtained for Cieneba and Nacimiento series show. The San Benito series results are explained also because the bulk density of the soil at the recording site was found to be unchanged with modification. The daytime temperature increased because the vegetative cover insulating the undisturbed soil was miss- ing from the modified soil, and the nighttime tempera- tures remained about the same because the heat conductivity and heat capacity were unchanged. The increase in heat transferred to modified soil causes soil moisture losses; conversely, decreased soil moisture causes soil tem- peratures to increase because less heat is required to


Figure 7. Bulk density for San Benito clay loam: a) Transect location after 8500 motorcycle laps, b) Comparison of transect bulk density before and after the race.

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vaporize the water present (Buckman and Brady 1969). The soil temperature is important to plant growth as a triggering mechanism for seedling germination (Shul'gin 1965, Luckenbach 1975).

Changes in the soil organic material and nutrient content have an effect on both revegetation and soil physical properties. The soil organic material affects the supply and availability of nutrients, and the presence of organic material increases the water-holding capacity (Buckman and Brady 1969). Calcium is a necessary nutrient for root-tip growth and functioning; magnesium is a constit- uent of chlorophyll (Russell 1953). Both magnesium and calcium are exchangeable bases, and a decrease in the exchangeable base content and drying can cause reductions in soil pH (Buckman and Brady 1969). This is consistent with the results for Cieneba series, which show decreased soil moisture, exchangeable calcium, and pH, and variable decreased exchangeable magnesium in modified soil.

Implications for Management of ORV-Use Areas

This study has shown that off-road vehicles change the basic properties of soil, and these property changes have adverse effects on the soil's stability to erosion and its ability to support natural revegetadon. Strong management practices are necessary to minimize these property changes and thus lessen the long term effects of ORVs on

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Effects of Off -Road Vehic les on Soil 2 2 9

the environment. To accomplish this, accelerated erosion must be minimized in ORV-use areas because of the slow formation rate of soil. According to the universal soil loss equation (U.S.D.A. 1975), the rate of soil loss increases with increasing slope length and angle, long, straight trails on steep slopes should not be used for ORV trails if erosion is to be minimized. Also, since vegetation cannot withstand vehicle impacts, ground cover such as wire netting could be applied to lower the erosion potential (Rasor 19t6).

Areas modified by ORVs will require management after closure to minimize erosion. The most effective way to minimize erosion is to establish and maintain a vegetative cover; because of the increased environmental stress on seedlings, certain steps might be necessary to mitigate ORV-induced property changes. Trails should be closed

Figure 8, Soil moisture results for the upper 10 cm of surface soil a) San Benito series, b) Diablo series.

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% SOIL MOISTURE Figure 9, Soil moisture results for Cieneba series at three depths.


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T I M E ( H r s )

Figure 10. Diurnal temperature curves for Nacimiento series at three depths. Dashed lines represent modified samples and solid lines represent undisturbed samples.

I 1800 2200

Effects of Off-Road Veh ic leson Soil 231

25

20 a

5

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Figure 11. a) Diurnal temperature curve for Cieneba series, b) Diurnal temperature curve for San Benito series.

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Organic material results for San Benito series.

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Figure 13. pH results for Cieneba series.

7.0


before the soil mantle is removed or else the soil should be removed and stockpiled for later replacement. If trails are eroded to bedrock, soil should be impor ted and stabilized to replace the displaced soil mantle. Mulches such as hay can be applied to minimize erosion and provide a seedbed, or the modified soil can be loosened by plowing or discing and seeded for revegetation; fertil- izer applications may be necessary. More studies are needed to determine the amount o f time required for revegetation of ORV-modified soils and to develop ways of mitigating proper ty changes.

One important means of ORV management is the prevention of accelerated erosion before it occurs. As ment ioned before, the universal soil loss equation should be used to plan trails on proper slopes and angles. ORV- use area planning should involve soil surveys to determine which areas can support such use; soils with severe erosion potentials should not be used. As noted in this study, only one soil series showed any resilience to soil property changes. ORV-use management is the first step in protecting the environment f rom unnecessary damage resulting from soil modification caused by off-road vehicle~.

ACKNOWLEDGMENTS

The authors thankJ . Trynor, F. Meyer, a n d ] . Hiehle f rom the Resources Division, and G. McGowan, head ranger at Hollister Hills SVRA, for the project funding and for help dur ing the field work. We also thank G. O. Gates, E. Smith, and J. Roberts f rom the University of Redlands for equipment and much needed advice. We especially thank H. G. Wilshire o f the U.S. Geological Survey for equipment , advice, and for reviewing the manuscript. This study was partially funded by the Resources Division o f the California Depar tment of Parks and Recreation.

References Cited

Arndt, W. 1965a. The nature of mechanical impedance to seedlings by soil surface seals. Aust. J. Soil Res. 3:45-54.

- - . 1965b. The impedance of soil seals and the forces of emerging seedlings. Aust. J. Soil Res. 3:55-68.

Barley, K. P., D. A. Farrell, and E. L. Greacen. 1965. The influence of soil strength on the penetration of a loam by plant roots. Aust. J. Soil Res. 3:69-79.

Berry, K., ed. 1978. Proc. of the symposium on the physical, biological, and recreational impacts of off-road vehicles on

the California desert. So. Calif. Acad. Scis., Spec. Pub., in press.

Buckman, H. O., and N. C. Brady. 1969. The nature and properties of soil. Third edition. The Macmillan Co., New York. 653 pp.

Bury, R. B., R. A. Luckenbach, and S. D. Busack. 1977. The effects of off-road vehicles on vertebrates in the California desert. Wildlife Res. Repts.~ No. 8, U.S. Fish and Wildlife Service, Washington D.C. 23 pp.

Busack, S. D., and R. B. Bury. 1974. Some effects of off-road vehicles and sheep grazing on lizard populations in the Mojave Desert: Biol. Conserv. 6:179-183.

Davidson, E., and M. Fox. 1974. Effects of off-road motorcycle activity on Mojave Desert vegetation and soil. Madrofio 22:381-390.

Duck, T. A. 1978. The effects of off-road vehicles on vegetation in Dove Springs Canyon: in K. Berry, ed. Proc. of the symposium on the physical, biological, and recreational impacts of off-road vehicles on the California desert: So. Calif. Acad. Scis., Spec. Pub., in press.

Geological Society of America, Committee on Environment and Public Policy. 1977. Impacts and management of off-road vehicles. Geol. Soc. America, 8 pp.

Grimes, D. W., R.j. Miller, V. H. Schweers, B. Smith, and P. L. Wiley. 1972. Soil strength modification of root development and soil water extraction. Calif. Agri. 26:12-14.

Hicks, D., A. Sanders, and A. Cooperrider. 1976. Impacts of Barstow-Las Vegas motorcycle race on wildlife habitat. Bui'eau of Land Management, Washington, D.C. Unpubl. Rept., 46 pp.

Hudson, N. 1971. Soil conservationl Cornell Univ. Press, Ithaca, New York. 320 pp.

Keefe, J., and K. Berry. 1973. Effects of off-road vehicles on desert shrubs at Dove Springs Canyon. Pages 19-44 in K. Berry, ed. Preliminary studies on the effects of off-road vehicles on the northwestern Mojave Desert: a collection of papers. Privately published, Ridgecrest, Calif.

Luekenbach, R. A. 1975. What the ORVs are doing to the desert. Fremontia 2:3-11.

Phillips, R. E., and D. Kirkham. 1962. Mechanical impedance and corn seedling root growth. Soil Sci. Soc. of Am. Proc. 26:319-322.

Rasor, R. 1976. Fair share. AMA News, August 1976, pp. 16- 17.

Russell, E.J. 1953. Soil conditions and plant growth. Longmans, Green, and Co., London. 635 pp.

Shul'gin, A. M. 1965. The temperature regime of soil. Jerusa lem, Israeli Prog. for Scientific Translations (translated from Russian). Available from NTIS. 218 pp.

Stebbins, R. C. 1974. Off-road vehicles and the fragile desert. Amer. Biol. Teacher 36:203-208, 294--304.

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 Cali-


fornia. U.S. Geological Survey Water-Resources Investiga- tions 76-99�9 45 pp.

Taylor, H. M., and H. R. Gardner�9 1963. Penetration of cotton seedling taproots as influenced by bulk density, moisture content, and strength of soil. Soil Sci. 96:153-156.

Taylor, H. M., and E. Burnett. 1964. Influence of soil strength on the root-growth habits of plants�9 Soii Sci. 98:174-180.

U.S. Department of Agriculture. 1969. Soil survey of San Benito County, California. U.S. Government Publishing Office, Washington, D.C. 110 pp. with appendices�9

- - . 1975. Guides for erosion and sediment control in Carl- fornia. Soil Conservation Service, Davis, Calif. 32 pp. with appendices.

Veihmeyer, F.J., and A. H. Hendrickson. 1948. Soil density and root penetration. Soil Sci. 65:487-493�9

Webb, R. H. 1978. The effects of off-road vehicles on desert soil in Dove Springs Canyon: in K. Berry, ed. Proc. of the symposium on the physical, biological, and recreational impacts of off-road vehicles on the California desert. So. Calif. Acad�9 Scis., Spec. Pub., in press�9

�9 H. C. Ragland, W. H. Godwin. 1977. Soil erodibi!ity and erosion control recommendations, final report on Holfister Hills State Vehicular Recreation Area. Unpublished report to the Resources Division, Calif. Dept. of Parks and Recreation�9 17 pp

Wilshire, H. G., and J. K. Nakata. 1976a. Off-road vehicle effects on California's Mojave Desert: Calif. Geology 29:123- 132.

�9 1976b. Erosional consequences of off-road vehicle recreation in California. Geol. Soc. Amer., Abs. Prog. 8(6): 1171- 1172.

�9 1978. Erosion of off-road vehicle sites in southern Cali- fornia, in K. Berry, ed. Proc�9 of the symposium on the physical, biological, and recreational impacts of off-road vehicles on the California desert. So. Calif. Acad. Scis., Spec. Pub., in press�9

Wischmeier, W. H. 1974. New developments in estimating water erosion. Pages 179-186 in Land use, persuasion or regulation?. Proc. of the 29th Annual Meeting, Soil Conserv. 8oc. of Amer., Syracuse, New York.

Webb et al 1978

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