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

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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
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