Forest roads can significantly reduce site productivity by removing and displacing topsoil, altering soil properties, changing microclimate, and accelerating erosion. The direct effects of taking land out of production by removing trees and displacing soil has been estimated to range from 1 to 30 percent of the landscape area in managed forests (Megahan 1988). Studies in eastern U.S. forests have consistently found that 4 to 5 percent of the total forested area is taken out of forest production by building roads during logging operations. Although more than 50 percent of this area may be reforested within 8 years, growth rates and productivity are reduced. Total road length required to support logging operations depends on the harvesting and silvicultural system and topographic configuration, but the area disturbed may be surprisingly consistent (Douglass and Swift 1977, Robinson and Fisher 1982, Swank and others 1982, Swift 1988).
Causes of Reduced Rree Growth Near Roads
Trees can grow on any portion of a closed road, but they can grow only on cut and fill slopes on open roads. Sites are harshest and soils are poor or nonexistent on road cuts and the cut portion of roadbeds. Measurable declines in tree growth are common where soil is excavated to build the road crown. Causes for these declines may be a combination of the following factors:
- Changes in soil physical properties. Road building changes soil physical properties including depth, density, infiltration capacity, water holding capacity, gas exchange rate, nutrient concentrations, and microclimate. Fertile topsoils, often containing most of the organic matter and plant nutrient capital of a site, frequently are buried under road fills or sidecast and may be rendered inaccessible to plant roots.
- Nutrient loss. Loss of nutrient capital is inevitable with soil disturbance from road building (Swanson and others 1989), but isolating this effect from other site changes has proved difficult. An indirect indication of nutrient loss is the marked growth response of plants on road fills after fertilizer is applied. Fertilizer applied to a granitic road fill in Idaho increased growth of vegetation by 32 to 116 percent (Megahan 1974a), but such increases are not documented after fertilizer is applied on undisturbed soils.
- Increased erosion. Both surface and mass-erosion rates increase after road building, and roads often accelerate erosion on the slope below. Downslope damage generally is associated with mass erosion when a landslide originates from a road and causes scour on lower slopes or gullies related to concentrated road drainage (Megahan 1988). This problem is widespread on steep slopes of the Pacific States and in the northern Rocky Mountains (Burroughs 1985, Swanson and others 1981), but Megahan (1988) estimates that productivity is reduced on only about 0.3 percent of forested land at a broad scale. The persistence of these effects may range from decades (Ice 1985) to more than 85 years (Smith and others 1986).
- Soil compaction. Roadbeds are highly compacted compared to natural soils, but soil compaction is not a productivity issue so long as roads are open and the running surface is bare. Road decommissioning must take compaction into account in restoring productivity, and various "ripping" treatments are routinely applied to decompact road surfaces.
Growth is sometimes enhanced on or below fill portions of roads because of reduced competition and greater soil depth. Improper fill placement and drainage can cause upslope groundwater to rise, and kill some trees (Boelter and Close 1974, Stoeckeler 1965), but this effect is not common.