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A land area cleared for agricultural, residential, or
commercial use contributes more to nutrient loading than a naturally
vegetated area, such as forested land (Dennis1986). The combination of vegetation removal
and soil compaction involved in the clearing of land results in a
significant increase in surface runoff, which amplifies the erosion of
sediments carrying nutrients and pollutants of human origin.
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Naturally vegetated areas offer protection against
soil erosion and surface runoff (Firmage, pers comm). The forest canopy reduces erosion by
diminishing the direct physical impact of rain on soil. The root systems of trees and shrubs
reduce soil erosion by decreasing the rate of runoff, allowing water to
percolate into the soil. Roots
decrease the nutrient load in runoff through direct absorption of nutrients
for use in plant structure and function. Due to these features, a forested area acts as a
buffering system by decreasing surface runoff and absorbing nutrients before
they enter water bodies.
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Evidence of increased surface runoff due to development
and consequent effects on nutrient transport is presented in a study
concerning phosphorus loading in Augusta, Maine (Figure IIB2a). The study revealed that surface
runoff from a residential area contained ten times more phosphorus than
runoff from an adjacent forested area.
The study concluded that the surface-runoff flow rate of residential
area can be in excess of four times the rate recorded for forested land.
Top: volume of immediate runoff over a 12 hour period; Middle: phosphorus
concentration in the runoff; Bottom: total amount of phosphorus exported
into local streams and lakes from the storm (Dennis 1986).
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Activities that remove the
cover of the canopy and expose the soil to direct rainfall increase
erosion. Two studies by the
Land Use Regulation Commission on tree harvesting sites noted that erosion
and sedimentation problems occurred in 50 percent of active and 20 percent
of inactive logging sites selected (MDC 1983). Skidder trails may pose a problem when they run adjacent
to or through streams
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