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/m2from 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.
Introduction
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 accel-
erated 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 revege-
tation of heavily used areas depends upon the condition
of the soil. Several studies show the erosion and compac-
tion of desert soil surfaces in ORV-use areas (Snyder and
KEYWORDS: Off-road vehicles, Soil properties, Erosion,Trail manage-
ment, Universal Soil loss equation, Traffic impact
Note:The unit valuesfor penetrationare givenas inchesin 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 proper-
ties 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 manage-
ment in ORV-use areas to reduce long term resource
damage.
Hollister Hills SVRA, operated 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 deter-
mined 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
219

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
Ten 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 different parent
materials. The soils southwest of the fault zone are of 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 of the fault zone are of the Diablo-
San Benito association and are underlain by Tertiary
sandstone, shale, and siltstone (T. H. Dibblee, unpub-
lished data). The major vegetative habitats present are
grassland and oak woodland.
Six of 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 mantle 25-50 cm deep, and has a vegetative cover
of chamise and coyote 15rush. The Sorrento series occurs
in and southwest of the San Andreas fault zone and is
higlJy variable texturally because of its alluvial nature.
Cometa sandy loam occurs on flat alluvial terraces (0-5 ~
slopes) and has a vegetative cover of grass.
Nacimiento sandy loam of 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 of chamise and grasses. San Benito
clay loam also occurs on steep slopes near the tops of
ridges, and has a vegetative cover of grass; this series is
the most common soi~ northeast of the fault zone. Diablo
clay occurs on ridgetops and in drainages in the northeast
corner of Hollister Hills, and has a vegetative cover of
grass.
Methods
Erosion transects were used to estimate soil removal
from hillsides with surfaces of minimum convexity or
concavity. This method assumed that the original surface
was approximately level across the trail. Stakes were
placed on either side of the trail flush with the undis-
turbed 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 of dis-
placed soil, and multiplication of this volume by the bulk
density determined tons of soil displaced from the trail.
Several standardized tests were used to determine the
properties of modified and undisturbed soil in each

50
40
~E
v
. 30
<
r,,-
(.D
Z
o 20
d
U.I
Z
~10o'?
n
I
>J
I
oo
-p
/,
q
"" ".9o
I
o
=42.9 ..,~
a~]--:o
r
o.
2x~
o
/o
ck.
L
1
O/
I
I
)
O~ -.
~ -
0 1.0 2_.0 5.0
PENETRATION (IN)
Figure 4. Results of a penetrometer transect on Nacimiento
sandy loam. Dashed lines represent modified samples and solid
lines represent undisturbed samples.
series. The surface strength was measured with a 75 cm
long penetrometer tipped with a 30~ cone, and was
recorded as the inches of penetration of the device with
75 kg of body weight applied. The bulk density was
determined 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 modi-
fied soil samples, and solid fines with solid points represent undisturbed
soil samples.
to determine the soil moisture present when the samples
were taken, Oven-dried samples were heated at 400~ for
1 hr to determine the amount of organic material pres-
ent. The diurnal temperature 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 pH 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 of all tests except soil temperature were
analyzed statistically to determine possible property
changes with modification, Penetrometer readings and
soil samples were taken for modified and undisturbed soil
along trail transects at intervals of 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 of the modi-
fied surfaces in the soil mantle was ignored because the
properties of 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 dis-
placed 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 from 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/m2 displaced from a 26~slope; a third
trail on Nacimiento sandy loam had 0.3 metric tons/ms
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).
The clay loam and clay had moderate erosion potentials
and eroded by rilling and sheetwash rather than by gully-
ing. No erosion transects were measured on clay or clay
loam.
Su rface Strength
Fig. 4 shows the results of a penetrometer transect on
Nacimiento sandy loam plotted as the inches of penetra-
tion for undisturbed and modified soil at each sample
point along the trail. These results show a decrease in
penetration for modified soil, which indicates an
increased surface strength. The paired-sample t-value of
42.9 indicates that modified soil has a significantly higher
surface strength beyond the 0.0005 alpha significance
level (probability), which disproves the null hypothesis of
no difference between modified and undisturbed surface
strengths. Similar results were obtained for San Benito
clay loam and Diablo clay (Fig. 5). Table 1 summarizes the
surface strength results; the surface strength of all 6 soil
series was found to increase significantly with
modification.
Bulk Density
Table 2 shows the results of bulk density measure-
ments for the 6 soil series. ORV-modification had the
greatest effect on the bulk density of Cieneba series (Fig.
6a); 2 transects on this soil series showed significantly
increased bulk density (compaction) to a depth of 30 cm.
Sorrento coarse sandy loam also showed significant
increases at all depths, although subsurface densities
showed less increase than those of Cieneba series.
Table 2 Summary of transect results for bulk density
Mean bulk density (g/cc)
Depth Undis-
Soil Series (cm) mrbed 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 2.8lit 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
***--signifiCantto p < 0.0005
**--significant to p < 0.005
*--significant to p < 0.01
#t--significant to p < 0.025
t--significantto 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
The 2 sandy loams showed variable results. One tran-
sect on Cometa sandy loam had an increased surface bulk
density with modification, although the subsurface bulk
density was unchanged. Similar results were obtained for
a transect on Nacimiento series (Fig. 6b); a third transect
on this series showed increased subsurface densities while
the surface density was not affected by vehicle use.
Bulk densities of modified 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;
although the transect site was subjected to 8500 motorcy-
cle laps, the bulk density remained unchanged to a depth
of 30 cm (Fig. 7).

Table 3 Summary of temperature resultsfor 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
ns--not significant
Soil Moisture
Table 3 shows the results for the soil moisture mea-
surements. Only one soil type, San Benito clay loam,
showed no change in soil moisture with modification in
the upper 10 cm (Fig. 8a): All other spit types, including
df Diablo clay (Fig. 8b), showed significant decreases with
5 modification. The most pronounced changes occurred in
5 Cieneba and Nacimiento series, which showed significant
5 decreases in soil moisture to a depth of 30 cm in the
5
5 modified soil (Fig. 9).
5
5 Soil Temperature
5 Diurnal temperature curves were obtained for Cie-
5 neba gravelly sandy loam, Nacimiento sandy loam, and
5
4 San Benito clay loam. The 2 cm, 6 cm, and 12 cm curves
3 for Nacimiento series (Fig. 10) show that modification
caused the diurnal temperature fluctuation to become
more extreme to a depth of 12 cm; soil temperatures
were warmer during the day and cooler at night in modi-
fied soil. The 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
us--not significant

show a more extreme fluctuation in temperature with
modification (Fig. 1la). The results for San Benito series
(Fig. 1lb) were different in that the modified soil temper-
ature was warmer or about the same as undisturbed soil
at night. A summary of the temperature 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 of soil. The results show that the per-
centage of organic material decreased with ORV soil
modification (Fig. 12); the t-value of 4.09 obtained for
this transect verifies the decrease statistically (p < 0.01).
The 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 for the exchangeable calcium
and magnesium tests. The results for Cieneba series show
a significant decrease in exchangeable calcium for 2 tran-
sects and a significant decrease in exchangeable magne-
sium for one transect; the second transect showed no
difference between the amounts of exchangeable magne-
sium in modified and undisturbed soil.
Dicussion
The results show soil property changes for all 6 soil
series as a result of ORV use. ORV soil modification
increased the surface strength, increased the bulk den-
sity, 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
of the land surface at Hollister Hills SVRA. The diurnal
temperature fluctuation of gravelly sandy loam increased
in modified soil, while the daytime temperature increased
in modified clay loam. Decreases occurred in soil pH,
organic material, and nutrient content with modification.
These property changes contribute to accelerated erosion
and increase the environmental stress on plant seedlings,
and thus create management problems in ORV-use
areas.
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 undisturbed areas (Wis-
35 ~o.0
i 0..
30 o.-'o-
0
-
~ 23._.i!"'~
20 .o
~ 10
0 1.0 2.0 3.0 4.0
PENETRATION (IN)
30
v
._J
a: 20
(-9
Z
0
_.1
wlO
z
F-
O
b)
t=lZ8 ~--o.
I
O,~ ~ ~ ~-O"
e2r--~
0 1.0 2.0 5.0 4.0
PENETRATION (IN)
Figure5. 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.

50
40
30
20
10
0
O-IOcm
Depth
t =7,22
I I
,
I
I

!
/
/
/
/
? -
/
/
50
40
30
20
10
0
I
O-IOcm
_ Depth
t=7.5
,(
I
1
6.

I
i/
/
/

b I
~50
_.1
~40
i---
Z
0
_.A
<20
w
zlO
co
! !
10-20 crn
Depth
t=9.88
~?--
/
/
/
q -

/
/
I

/
/
L~
50
40
50-
20-
10-
00
a
A
i i i
20 -30cm
Depth
t=6.15
t I i I_~
0.5 1.0 1.5 2.0
BULK DENSITY (g/co)
2.5
Figure 6. Resuks for bulk density transects on a) Cieneba
series, b) Nacimiento series. Dashed lines represent modified
samples and solid lines represent undisturbed samples.
226
5O
4O
30
2O
I0
0
50
40
3O
20
I0
0
b
I I J ty
10-20cm ~.. /
Depth o _
~_
t=3,9/ ~ _-
20-30crn ~
_ Depth ~ ~ _
_ ~.. /"/ _
t=1.24 l
0.5 1.0 1.5 20
BULK DENSITY (g/cc)
25

chmeier 1974). The universal soil loss equation describes
the rate of sheet and rill erosion as
A= Rx Kx LSx Cx 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 modi-
fication 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 manage-
ment factors change with modification of the undisturbed
site. The vegetative cover factor increases because vegeta-
tion is removed from ORV-modified areas. The soil ero-
dibility factor increases because of compaction and loss of
organic material from the surface soil; the erosion man-
agement 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 denu-
dation 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 penetra-
tion 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 gJcm3 for
sands. No roots penetrated layers with bulk densities
higher than 1.90 g./cm3. 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 mois-
ture 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 modifica-
tion. 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 conduc-
tivity 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

228 RobertH. Webb, H.Craig Ragland,William H. Godwin, DennisJenkins
Figure7. 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.
a
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 con-
tent 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 reduc-
tions 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 manage-
ment practices are necessary to minimize these property
changes and thus lessen the long term effects of ORVs on
40
50
20
10
0
40~
j 50
~- 20
o, I0
<
,,, 0
Z
4O
3O
2O
ot,o cm
Depth ~9 -
t =O.85 ~// _
10-2Ocrn
_ Depth
t=0"72 ~ -
t=0.95 ../'~ J
0
0 0.5 I.O 1.5 2.0
BULK DENSITY (g/cc)
b

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 vegeta-
tive 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, Soilmoisture results for the upper 10 cm of surface
soil a) San Benito series, b) Diablo series.
b
20 l i
15
t= O,50
10
5
0 '
5
% SOIL
4 , ~ ,
l
l
1

2 t=5.o9

I%J t ~,
15 20 25
% SOIL
"t"//
I d/ Ig
10 15 20
MOISTURE
30
MOISTURE
d
rr
t--
(.9
Z
0
d
bJ
r,.)
Z
I--
U)
50
4O
50
2O
I0
0
50
40
30
20
IO
0
50
40
I84 ~ I
O-IOcm /
_ Depth 6~
I
- i
t = 6.48 /
I
-- 0
I
I
-- 0
iI
"~'~x I
x
0-20 cnr
Depth )
I
I
- 6
t=5.52 //
/
- 6

- ;
I
,/
--'---'~ I I
i t
50-
I
I
t=7.15 i
20 ~

Deplth ; _
0 , i
0 5 I0 15 20
% SOIL MOISTURE
Figure 9, Soilmoisture results for Cieneba series at three
depths.

O
O
v
Ld
n-
F-
<[
n-"
Ld
Q_
Ld
t--
30 i I i i
cm Depth p'~ ,~
1 - }
2
25- ~-- -~r -
20 - /~'~/ ~~ / , 9 -
/ q
15- b _
10-
5 /
0 iI I
2O
15
10
5
20
15
10
501 I I I
t 6 cm Depth
25
I I
5 I I I
2400 0600 1200
TIME (Hrs)
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 Vehicleson Soil 231
25
20
a
5
o 0
O
v
I,I
13d
H-
rr 5
LtJ
O._
I0
t--
u 5
I I I
/0%, 2 cm Depth
- d tr~ --
P .
I I f
2400 0600 1200 1800 2400
TIME (Hrs)
9c[o'~ ~ OO,o I I
2 cm Depth
I "o
-/~~ ~Oo
I I I I
0800 1200 1800 2400. 0600
TIME (Hrs)
Figure 11. a) Diurnal temperature curve for Cieneba series,
b) Diurnal temperature curve for San Benito series.
K
v
n,-
t--
(D
Z
0
d
<~
LLJ
Z
<~
Figure 12.
as --7: '~. '
zo -~,
' - , -
10- i -
5 6 -
/ I
Og I ~.~ J
4 6 8 I0 12
% ORGANIC MATERIAL
Organic material results for San Benito series.
~ 50
Ev
40
rr
F--
3O(_9
Z
O
J 20
I_d
r
:.10
co
I ..1,"~ 71
- ? T -
- { /
6.0
pH (units)
0
5.0
Figure 13. pH results for Cieneba series.
7.0

232 RobertH. Webb, H.Craig Ragland, William H. Godwin, Dennis Jenkins
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 imported 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 of time required for
revegetation of ORV-modified soils and to develop ways
of mitigating property changes.
One important means of ORV management is the
prevention of accelerated erosion before it occurs. As
mentioned 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 deter-
mine 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 from unnecessary damage
resulting from soil modification caused by off-road
vehicle~.
ACKNOWLEDGMENTS
The authors thankJ. Trynor, F. Meyer, and]. Hiehle
from the Resources Division, and G. McGowan, head
ranger at Hollister Hills SVRA, for the project funding
and for help during the field work. We also thank G. O.
Gates, E. Smith, and J. Roberts from the University of
Redlands for equipment and much needed advice. We
especially thank H. G. Wilshire of the U.S. Geological
Survey for equipment, advice, and for reviewing the
manuscript. This study was partially funded by the
Resources Division of the California Department 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 effectsof off-road vehicleson vegetation
in Dove Springs Canyon: in K. Berry, ed. Proc. of the sympo-
sium 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 76-999 45 pp.
Taylor, H. M., and H. R. Gardner9 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 plants9 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 appendices9
--. 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. 65:487-4939
Webb, R. H. 1978. The effects of off-road vehicles on desert soil
in Dove Springs Canyon: in K. Berry, ed. Proc. of the sympo-
sium on the physical, biological, and recreational impacts of
off-road vehicles on the California desert. So. Calif. Acad9
Scis., Spec. Pub., in press9
9 H. C. Ragland, W. H. Godwin. 1977. Soilerodibi!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 Recreation9
17 pp
Wilshire, H. G., and J. K. Nakata. 1976a. Off-road vehicle
effects on California's Mojave Desert: Calif. Geology 29:123-
132.
91976b. Erosional consequences of off-road vehicle rec-
reation in California. Geol. Soc. Amer., Abs. Prog. 8(6):1171-
1172.
91978. Erosion of off-road vehicle sites in southern Cali-
fornia, in K. Berry, ed. Proc9 of the symposium on the
physical, biological, and recreational impacts of off-road vehi-
cles on the California desert. So. Calif. Acad. Scis., Spec. Pub.,
in press9
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, SoilConserv.
8oc. of Amer., Syracuse, New York.

Webb et al 1978

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Viewers also liked

Viewers also liked (6)

Similar to Webb et al 1978

Similar to Webb et al 1978 (20)

More from William Godwin

More from William Godwin (8)

Webb et al 1978