The Impact of Vehicles on Dune and Grassland Vegetation on a ...

0042127
Journal <strong>of</strong> Applied Ecology (1980), 17, 173-182
THE IMPACT OF VEHICLES ON DUNE AND GRASSLAND
VEGETATION ON A SOUTH·EASTERN NORTH CAROLINA
BARRIER BEACH
HY PAUL E. HOSIER AND THOMAS E. EATON
Department <strong>of</strong> Biology, <strong>The</strong> University <strong>of</strong> North Carolina at Wilmington,
Wilmington, North Carolina 28403
SUMMARY
(1) Two barrier beaches in southeastern North Carolina were compared with
respect to vegetation patterns and soil compaction. One had been widely used by
<strong>of</strong>f-road vehicles and the other had not.
(2) <strong>The</strong> vegetation cover and the number <strong>of</strong> species present on both dunes and
grassland were fewer on the area that had been subjected to vehicular traffic. It is
suggested that the reduction in vegetation cover may increase the intensity <strong>of</strong>
oceanic overwash at this site.
(3) <strong>The</strong> soil was more compact where vehicular traffic had been most intense.
It is suggested that this compaction may increase the area <strong>of</strong> salt flats in the
impacted area.
INTRODUCTION
<strong>The</strong>re has been an increasing use <strong>of</strong> <strong>of</strong>f-road vehicles (ORVs) on barrier beaches in
North Carolina in the past 10 years and these vehicles have made areas available for
recreational use. However, they may damage vegetation and their use has also resulted in
disturbance <strong>of</strong> other users, e.g., sunbathers, surfers and fishermen.
<strong>Vehicles</strong> may affect natural vegetation by reducing plant cover and height, lowering
species diversity, and altering community composition (Bates 1935; Chappell eta!. 1971;
Trew 1973; Liddle & Greig-Smith 1975b; Boorman & Fuller 1977). Godfrey, Leatherman
& Buckley (1978) found that growth <strong>of</strong> plants in berm and marsh areas <strong>of</strong> Cape Cod
National Seashore was inhibited by vehicular traffic and the faunal populations were
subjected to crushing and habitat alteration. Liddle & Greig-Smith (1975a) found that
vehicular traffic compacted sub-surface layers in sandy soils.
<strong>The</strong> present study was undertaken to study the effect <strong>of</strong> ORVs on barrier beaches in
the southeastern United States.
MATERIALS AND METHODS
<strong>The</strong> Cape Fear area <strong>of</strong> North Carolina is representative <strong>of</strong> the low narrow coastal barrier
island system <strong>of</strong> southeastern North Carolina. Sub-aerial portions <strong>of</strong> the islands vary in
width from 60 to 600 m. <strong>The</strong> islands are characterized by berm crest elevations <strong>of</strong> 3·0-
3·5 m above mean sea level (msl) with dunes as high as 5·5 m above msl. <strong>The</strong> Cape Fear
islands are highly unstable~ hurricanes and northeasters influence the area. Inlets are
common and the area has one <strong>of</strong> the highest washover records in the state (Cleary &
Hosier 1979). Three temporary inlets were formed by Hurricane Hazel in 1954.
0021-8901/80/0400--0173$02.00 ©1980 Blackwell Scientific Publications
173

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174 <strong>Impact</strong> <strong>of</strong> vehicles on dune and grassland vegetation
Fort Fisher Beach, north <strong>of</strong> New Inlet, is easily accessible via a hard surface road.
Conversely, Bald Head Beach is an undeveloped inaccessible section <strong>of</strong> the Cape Fear
barrier island system. New Inlet prevents access to the beach from Fort Fisher (Fig. 1 ).
Observers have recorded as many as fifty vehicles along the 5 km section <strong>of</strong> Fort Fisher
Beach on summer holidays, while ten to fifteen vehicles per day is a common number
during the autumn and spring fishing months. One or two vehicles per month may be
observed at Bald Head Beach. <strong>The</strong> beach between the two sites has been altered by inlet
migration since 1962 (Baker 1977) and, therefore, was excluded from the study.
<strong>The</strong> beach is composed <strong>of</strong> medium quartz sand with a large proportion <strong>of</strong> broken shell
material. Shell pavements, produced when fine sands are winnowed and blown into dunes,
develop after washover events.
<strong>The</strong> vegetation <strong>of</strong> the Cape Fear region is similar to associations described for other
washover dominated barrier islands in North Carolina (Godfrey & Godfrey 1973; Hosier
1973). Uniola paniculata, Spartina patens, and Iva imbricata are the most conspicuous
and characteristic perennial plant species within the dune system <strong>of</strong> the study area. A
number <strong>of</strong> annuals including Erigeron canadensis, Euphorbia polygonifolia, and Strophostyles
he/vola are common. <strong>The</strong> lower elevations toward the sound side are dominated
by a grassland which grades into the regularly flooded saltmarsh characterized by
Spartina alterniflora. <strong>The</strong> grassland is typified by luxuriant growth <strong>of</strong> Spartina patens,
Fimbristylis spadicea, and Solidago sempervirens. A number <strong>of</strong> other coastal plant species
were found in the grasslands (Table 1). Nomenclature is that <strong>of</strong> Radford, Ahles & Bell
(1968).
0
FIG. 1. Map <strong>of</strong> the study sites at Cape Fear, North Carolina. <strong>The</strong>re is unlimited vehicular
access to the Fort Fisher Beach site (A) via the nearby road. Access to Bald Head Beach
site (B) is prevented by New Inlet.

0042129
P. E. HOSIER AND T. E. EATON
TABLE 1. Importance values for all species summed over four transects (Nonimpacted
= Bald Head Beach; vehicle-impacted = Fort Fisher Beach)
Species
Uniola paniculata
Spartina patens
Iva imbricata
Strophostyles he/vola
Oenothera humifusa
Erigeron canadensis
Panicum amarum
Heterotheca subaxillaris
Fimbristylis spadicea
Solidago sempervirens
Andropogon virginicus
Ammophila breviligulata
Euphorbia polygonifolia
Triplasis purpurea
Cakile harperi
Commelina erecta
Limonium carolinianum
Salicornia virginica
Spartina alterniflora
Borrichia frutescens
Distich/is spicata
Baccharis halimifolia
Hydrocotyle bonariensis
Myrica cerifera
Aster subulatus
Cynanchum palustre
Baccharis angustifolia
Agalinis maritima
Sabatia stellaris
Lythrum lineare
Triglochin striata
Setaria geniculata
Muhlenbergia capillaris
Juncus roemerianus
Suaeda linearis
Atriplex arenaria
Non-<strong>Impact</strong>ed
Dune Grassland
77-07
11·18
8·13
16·08
29·27
23·09
2-68
1·70
0·75
4·68
J.47
5·49
12·19
3·99
0·25
2·03
52-86
8·55
4·13
0·62
2·73
1·82
0·84
43·76
18·41
6·93
4·94
0·22
12·59
5·45
1·64
8·96
7-35
5·78
4·24
1·77
0·92
0·83
0·72
0·44
0·39
0·39
0·38
0·23
FIELD STUDIES
Physiographic-vegetational analysis
Vehicle-<strong>Impact</strong>ed
Dune Grassland
94·91
10·92
14·24
3-68
27·16
13·58
4·36
0·47
13·71
22·11
0·29
0·18
0·26
1·69
76·59
0·46
0·98
0·66
28·99
27·02
22·03
35·70
4·53
0·91
0·43
At each site, four transects were established perpendicular to the beach at 30m
intervals. Elevations were determined at 2m intervals using transit and stadia rod. A
50 x 50 em quadrat was placed at the point <strong>of</strong> each elevation measurement and the
coverage <strong>of</strong> each species present in the quadrat was visually estimated. A single observer
made all cover estimates. <strong>The</strong> transects were divided into grassland, dune, and marsh
vegetation based on elevation and species composition. Quadrats were pooled for all
transects in each vegetation type at each site; marsh quadrats are not reported. An
importance value (IV = relative frequency + relative cover) was determined for each
species in dune and grassland associations. <strong>The</strong> total sum <strong>of</strong> importance values for each
vegetation association was 200.
Mean plant cover, % unvegetated quadrats, species diversity (Pielou 1975), and estimated
aerial extent were derived from the transect data for each vegetation type at both
sites. In addition, the total number <strong>of</strong> species present at each site was determined.
175

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176 <strong>Impact</strong> <strong>of</strong> vehicles on dune and grassland vegetation
Sediment characteristics and penetration resistance
Surface samples <strong>of</strong> sediment (top 3 em) were collected at 10 m intervals along the
transects. Samples were sieved to remove the silt/clay fraction and materials greater than
2 mm. <strong>The</strong> silt-clay fraction was not measured. Following removal <strong>of</strong> organic debris with
sodium hypochlorite, the samples were analysed in a standardized settling tube and
classified according to Folk (1974).
Compaction was assessed using a Soiltest cone penetrometer. This instrument measures
the shearing resistance <strong>of</strong> soils. <strong>The</strong> instrument consists <strong>of</strong> a 30 degree cone (3·2 cm 2 base
area), a 46 em extension rod, a proving ring, and a dial indicator.
A total <strong>of</strong> 504 determinations were taken at 1 em, 5 em, and 15 em depths in each
vegetation type in order to establish site conditions. Depth was recorded as positive
distance from the surface. Also, determinations were made specifically in vehicle tracks
at the impacted beach site. <strong>The</strong>se were accomplished by taking 100 readings at various
depths along transects perpendicular to the track. Least squares regression analysis was
applied to soil resistance readings in the tracks.
RESULTS
Physiographic-vegetational analysis
Fort Fisher and Bald Head Beaches were subjected to severe oceanic overwash prior
to 1900, during 1954 (Hurricane Hazel), and most recently in 1962 (Ash Wednesday
Storm) (Cleary & Hosier, 1979). We attribute the present differences in vegetation and
physiography to the presence or absence <strong>of</strong> vehicles. According to local residents the
number <strong>of</strong> vehicles using the Fort Fisher Beach increased in the mid- to late 1960s.
At both locations, the beach-berm crests at approximately 3· 5 m above msl and the island
surface slopes gently to elevations where tidal marsh vegetation begins, approximately
40 em above msl. <strong>The</strong>re are more dunes at Fort Fisher Beach (Figs 2 and 3).
Importance values for species within each vegetation association suggest a similarity
<strong>of</strong> dune vegetation at each site and a dissimilarity between grassland systems. Eleven <strong>of</strong>
the nineteen species found in the dunes are common to both dune sites. Major differences
in importance values occur only with Triplasis purpurea, Erigeron canadensis, and
Strophostyles he/vola (Table 1). Within the grassland association, Fort Fisher Beach has
less than half the number <strong>of</strong> species found on Bald Head Beach. Only ten species are
common to both areas from a total flora <strong>of</strong> thirty-eight species. Nineteen species found
in the grassland at Bald Head are not found in the grassland at Fort Fisher, while only
two species are found in the latter and not in the former site. Grassland species such as
Fimbristylis spadicea, Solidago sempervirens, and Andropogon virginicus are lacking at
Fort Fisher. Fort Fisher Beach has a higher cover <strong>of</strong> Borrichia frutescens, Limonium
carolinianum, and Salicornia virginica, species usually common in salt flats or salt pans.
While Bald Head Beach has nearly equal amounts <strong>of</strong> grassland and dune type vegetation,
the dune association dominates at Fort Fisher. Also, the latter has lower mean cover
values (Fig. 2). If quadrats containing a track are removed from consideration, the
average cover <strong>of</strong> the dunes on Fort Fisher Beach (impacted) is increased to 16·4%, nearly
the same as Bald Head Beach (non-impacted). This is further illustrated by the high
number <strong>of</strong> unvegetated quadrats in the dunes (Fig. 3 and Table 2).
Table 2 shows that vehicles alter nearly all the parameters which were measured. <strong>The</strong>
total number <strong>of</strong> species and species diversity both decline in vegetation types where

0042131
m~L
- -- ... -- - ------- --
---
·-
•
I
0 5 10m
Scale
7 -·
• ~ 20% Cover
.........
--
- -
~
- -
Spartina a/termflara
Spartma patens
Distich/is sptcata
Ltmamum carolmianum
Ftmbnstylts spadicea
Agalims manttma
Aster subulatus
Triglochin strtata
Bacchans haltmtfolta
Soltdago sempervtrens
Mynca cerifera
Andropogon vtrgmtcus
Strophostyles he/vola
Iva tmbricata
Umola pamculata
Engeron canadensis
Oenothera humtfusa
Trtp/asts purpurea
Euphorbta polygonifolta
Ammophila brevtltgu/ata
FIG. 2. Transect <strong>of</strong> Bald Head Beach showing physiography and vegetation distribution. <strong>The</strong> beach is scarcely used by vehicles. Vertical exaggeration is 5:1.
:-c
~
~
~
~
;-3
I:I1
~
-.l
-.l

0042132
--.1
00
-- ----. ·- -.
•
•
--
-
•
.......
•
•
-
m ~L 0 5 10m ORV track .......................
Scale
- -
..&..
I
-
•
---
•
•
•
.. -·
• •
•
- = 20% Caver
Spartma alterntflara
Spartina patens
Barnchia frutescens
Saltcorma virgmica
Ltmonium carolmtanum
• Umola paniculata
Iva frutescens
Pamcum amorum
Euphorbta po/ygomfolta
Oenothera humifusa
Trtplasis purpurea
Erigeron canadensis
FIG. 3. Transect <strong>of</strong> Fort Fisher Beach showing physiography and vegetation distribution. <strong>The</strong> beach is frequently used by vehicles. Vertical exaggeration is 5:1.
~
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~
~
~
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;:::..
;::;·
1r
0
~
~
~
(1:>
§
l:l...
~
iS"'
~
l:l...
~
~
(1:>
~
§·

0042133
P. E. HOSIER AND T. E. EATON
TABLE 2. Summary statistics and % change for dune and grassland vegetation
in non-impacted (B.H.) and impacted (F.F.) sites
Dunes
Parameter
B.H. F.F. %Change
Species
15 13 -13·3
Aerial extent (%)
51·28 82·87 +61·6
Vegetation cover(%)
19·39 10·58 -45·4
Unvegetated quadrats (%) 4·30 22·73 -428·6
Species diversity
0·9482 0·8386 -11·6
* Includes two species observed, but not found in quadrats.
B.H.
31*
48·72
53·29
0·00
1·0320
179
Grassland
F.F. %Change
12 -61·3
17·17 -64·8
38·53 -27·7
0·00 0·0
0·7436 -27·9
vehicular activity is significant. Dune vegetation cover is decreased along the tramects by
over 45%, while grassland cover is reduced by 27·7%. <strong>The</strong> % <strong>of</strong> unvegetated quadrats
shows the greatest% decline, 428·6%.
Sediment characteristics and penetration resistance
Except for grassland areas, sand grain size distribution was similar at both sites. <strong>The</strong>
grassland at Bald Head had finer sands, the modal sand size was 2·0 cp at Bald Head
Beach and 1·0 cp on Fort Fisher Beach grasslands (Fig. 4). While neither site had measurable
amounts <strong>of</strong> silt or clay, the dune field and grassland areas <strong>of</strong> the Fort Fisher Beach
had nearly twice the accumulation <strong>of</strong> surface shell material ( > 2 mm) when compared to the
other beach.
Compaction near the surface (1 em depth) was twice as high in the non-impacted areas
as that recorded at the impacted beach (Table 3). At the 5 em and 15 em depths, the
impacted site had higher values in both communities, except in the dunes at 5 em depth.
<strong>The</strong> differe~ces are significant (P = 0·05).
<strong>The</strong> relationship between compaction and depth is further illustrated by the compaction
pr<strong>of</strong>iles made in actual tracks. Figure 5 shows increased compaction at more shallow
depths in. the track area (B and C) as compared to adjacent undisturbed areas (A).
Values taken in the upper strata <strong>of</strong> the hump (C), however, were lower than those <strong>of</strong> the
control (A).
40~a) /~
2:t ~b ___
!::[) ~
~0 t9~~----
40(c) ~
J . .-------,
oE
• ....----=-= ---'----'
-1 0 I 2 3 4
I V. coarse I Coarse I Medium I Fine I V. fine I
cf> Size (sand)
FIG. 4. <strong>The</strong> size distribution <strong>of</strong> sand-sized particles at Fort Fisher Beach (impacted) (e)
and Bald Head Beach (non-impacted) (0). Distributions were determined for beach
(a), dunes (b), and grassland (c).

0042134
180 <strong>Impact</strong> <strong>of</strong> vehicles on dune and grassland vegetation
TABLE 3. Soil penetration resistance (kg em- 2) at three depths in dunes and
grassland vegetation associations at Bald Head and Fort Fisher Beaches,
North Carolina
Bald Head Beach (non-impacted)
Parameter
Depth (em)
1
Mean
0·96
S.d.
0·29
n
50
Dunes
5
2-95
0·63
50
Fort Fisher Beach (impacted)
Depth (em) 1 5
Mean 0·44 2·83
S.d. 0·11 0·30
n 50 50
N
I
E
u
~
"' u
c
0

0042135
P. E. HOSIER AND T. E. EATON 181
available sand supply has been completely mobilized into low foredunes. At Fort Fisher
Beach, however, constant churning <strong>of</strong> the substrate continually renews the windblown
grain size and increases the amount <strong>of</strong> sand available for dune building. While this
hypothesis is supported by grain size analysis, it is presently being tested by a sand
trapping experiment. Visco (1977) similarly concluded that vehicles influence the movement
<strong>of</strong> sand in the beach environment. He found that when vehicles form humps and
ruts in the s<strong>of</strong>t beach sand, increased sand movement occurred via both wind and littoral
vectors.
Traffic appears to affect dunes more than grasslands in respect to vegetation cover.
<strong>The</strong> reduced cover results in an increased number <strong>of</strong> blowouts in the dunes. Vehicular
activity alone does not create blowouts in the dunes; wind action following denudation
is required.
Traffic also appears to decrease the relative acreage <strong>of</strong> grassland vegetation, mean
cover values, the number <strong>of</strong> species present, and to alter species composition. Several
workers have found that impacts, similar to those <strong>of</strong> vehicles, such as crushing and
compaction lower cover and density indices (Liddle & Greig-Smith 1975a, b; Chappell
et al. 1971; Bates 1935). Liddle & Greig-Smith (1975b) also suggest that such impacts
tend to make sites more uniform. <strong>The</strong> dominance <strong>of</strong> open dune fields at the Fort Fisher
site tends to support this conclusion.
Compaction readings taken away from tracks at Fort Fisher Beach showed that the
area had less compacted surface sediments than did Bald Head Beach; however, Fort
Fisher Beach was more compacted at the 5 and 15 em depths. <strong>The</strong>se results conflict with
those reported by Liddle & Greig-Smith (1975a), but they admit their data may be
atypical. Trew (1973) found that trampling broke up the top layer <strong>of</strong> sandy soils, but
compacted it slightly with depth. This trend is also evident within tracks at the Fort
Fisher site and is illustrated by the track pr<strong>of</strong>iles (Fig. 5). Within the track an average
resistance to penetration <strong>of</strong> 15·8 kg em- 2 was found at the 5 em depth. Adjacent to the
track at the same depth, an average resistance <strong>of</strong> only 2·8 kg em - 2 was found.
<strong>Vehicles</strong> also scour within the track. Since there is a layer <strong>of</strong> shell fragments that
resists penetration (formed in the 1962 washover), part <strong>of</strong> the compaction may be a
result <strong>of</strong> decreasing the distance to this layer.
Compaction tends to increase the water content <strong>of</strong> dry, sandy soils (Liddle & Greig­
Smith, 1975a). Compaction <strong>of</strong> substrates by vehicles may be the causal factor in the
formation <strong>of</strong> poorly drained sites at Fort Fisher Beach. <strong>The</strong> formation <strong>of</strong> salt pans on
Core Banks, North Carolina, is considered to be a result <strong>of</strong> the presence <strong>of</strong> a sub-surface
impermeable layer (Hosier 1973). This suggests that the importance <strong>of</strong> the halophytes at
the impacted site is a function <strong>of</strong> this effect.
<strong>The</strong> differences between the two sites, particularly the extent <strong>of</strong> grassland and reduced
vegetation cover, have implications beyond conservation or aesthetic values. Dolan &
Godfrey (1973) have shown that grassland vegetation is important in reducing the effects
<strong>of</strong> oceanic overwash by acting as a brake which decreases the velocity <strong>of</strong> water and traps
sand. <strong>The</strong> high concentration <strong>of</strong> fine material in the non-impacted grassland (Fig. 4) is
evidence that the grasses collect windblown sediments. At Fort Fisher Beach, the lack <strong>of</strong>
extensive grassland cover increases the vulnerability <strong>of</strong> the beach to the impact <strong>of</strong>
wash over.
We conclude that vehicular traffic on Fort Fisher Beach, North Carolina, is detrimental
to the maintenance <strong>of</strong> the island system. Historically, the area has been influenced by the

0042136
182 <strong>Impact</strong> <strong>of</strong> vehicles on dune and grassland vegetation
process <strong>of</strong> oceanic overwash. Vegetation alterations and devegetation resulting from
unrestricted use <strong>of</strong> vehicles on the beach seems likely to cause future overwash to be
unusually severe.
ACKNOWLEDGMENTS
<strong>The</strong> research was funded by Grant No. 04-0-M01-66 from the University <strong>of</strong> North
Carolina Sea Grant Program. Appreciation is extended to the North Carolina Department
<strong>of</strong> Natural Resources and Community Development and research assistants,
Stanley Boc, Cathy Horrell, and Carol Snyder.
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(Received 4 June 1979; revision received 10 October 1979)