Impacts of vehicles on natural terrain at seven sites in the San Francisco Bay area |
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Authors: | H G Wilshire J K Nakata Susan Shipley Karen Prestegaard |
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Institution: | (1) U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California;(2) University of California, Berkeley, California |
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Abstract: | The impacts of off-road vehicles on vegetation and soil were investigated at seven representative sites in the San Francisco
Bay area. Plant cover of grass and chaparral (with shrubs to 4 m tall) have been stripped by the two- and four-wheel vehicles
in use. Impacts on loamy soils include increased surface strength (as much as 275 bars), increased bulk density (averaging
18%) to depths of 90 cm or more, reduction of soil moisture by an average 43% to 30 cm depths, greatly reduced infiltration,
extension of the diurnal temperature range by as much as 12°C, and reduction of organic carbon by an average 33% in exposed
soils. Very sandy soils respond similarly to vehicular use except that moisture is increased and surface strength of beach
sand is decreased.
These physical and chemical impacts reduce the land's capability of restoring its vegetative cover, which in turn adversely
affects animal populations. Both the loss of plant cover and the physical changes caused by vehicles promote erosion. Measured
soil and substrate losses from vehicular use zones range from 7 to 1180 kg/m2. The estimated erosion rate of the Chabot Park site exceeds the rate of erosion considered a serious problem by a factor
30, it exceeds United States Soil Conservation Service tolerance values by a factor of 46, and it exceeds average San Francisco
Bay area erosion rates by a factor of 17. The resulting soil losses are effectively permanent. Neither the increased sediment
yield nor the increased runoff is accomodated on the sites of use, and both are causing adverse effects to neighboring properties. |
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