erosion
Soil erosion is often a result of forest clearing. When there are no trees covering the soil, rainfall lands directly on the ground, rather than dripping gradually through the tree branches and falling much more softly to the forest floor. This means that when it rains, more water hits the ground with more force, washing soil away. On the ground in most forests, there is a layer of water-absorbing organic material such as decomposing leaves and wood. Rainfall can soak into this layer instead of running off over the soil.

New housing developments are built on bare land where the soil is very susceptible to erosion. Photo credit Corel Photo Clipart CD.

new development
tree roots
Tree roots hold the soil in place. Photo by Maya Walters.
Extensive erosion causes landslides. Many landslides begin on clearcut slopes or from the downhill side of steep logging roads. Run-off rainwater collects in these areas and this concentrated water flow begins to wash away the soil. Also, these areas have already been cleared of trees, and this means the soil is already unstable. Tree roots are responsible for holding soil in place, providing a framework that prevents it from eroding away. When trees are cut, the roots often remain in the soil, but eventually they begin to rot. This leads to a great increase in the number of landslides that occur 10 to 20 years after an area has been logged.
When topsoil has eroded, it is much harder for new plants to grow, and the lack of stabilizing plant roots leads to more erosion. Once erosion begins, it is very hard to repair the damage. In major landslides, all the soil right down to the bedrock is carried downslope, taking all the trees and other vegetation with it. Since no soil is left for new plants to grow, the bare tracks of landslides remain visible for hundreds of years. Erosion is therefore a serious long-term threat to forests -- without soil, vegetation cannot grow. After a landslide, hundreds of years are required before enough new soil accumulates to replace the former layer of topsoil and organic material.
landslides Above and Right: Landslides that began from a logging road partway up a mountainside. The soil slid right down into the lake below, leaving the bedrock exposed. Photos courtesy Al Walters.
landslide
Eroded soil is often washed into streams and rivers. Fish and other freshwater animals need clear water, and this eroded soil destroys their habitat. Many forest streams that were once spawning areas for salmon have been ruined by silt. Often these streams run near clearcuts, which can be areas of extensive erosion because of their loss of tree cover.
related topics
[soil] [water] [roots] [forests through time] [riparian zones] [fish] [amphibians]

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