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

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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