Non-Point Source Pollution and Forests

Non point source pollution is found in all land use types: urban, agricultural, meadows and forests. Erosion is the mechanism in non point source pollution with runoff from precipitation causing the soil to wear away. Of the four land use types listed above, forests are the least polluting land cover of all. The forest canopy intercepts precipitation that falls, reducing the impact of the rain droplets. The branches and leaves catching the water droplets allow the water to slowly run down the stem or drip slowly to the ground water. Interestingly enough, the forest floor is the most crucial feature of the forest’s ability to prevent erosion. The forest floor is comprised of a complex mix of leaves, twigs and downed branches in various stages of decomposition, along with roots, fungal mycelium, soil fauna and microbes. This natural ‘mulch’ protects the soil from exposure to rain droplets. Forests can soak up the rainfall from up to a 6” rain storm, absorbing the water and releasing it to ground water without significant erosion. Erosion being a natural process, undisturbed forests still contribute some sediment and nutrients, but at a negligible rate, less than ½ ton per year.

The time when forest land is most vulnerable to erosion is during disturbances. When the trees and litter layer of the forest are disturbed, the soil that had been protected by the vegetation is exposed to the forces of wind and rain. Forests are subject to natural disturbance such as wildfires, wind storms and earthquakes. Wildfires that burn intensely which happens often during droughts, can burn off the forest floor leaving exposed soil with little to no cover to prevent precipitation and wind from eroding the soil away. Soil loss rates of up to 60 tons per acre have been recorded on post-fire land.

Human caused disturbances, such as timber harvest, are the other way forests become vulnerable to erosion. The actual cutting of a tree or even several trees in and of itself will not normally lead to erosion. However, building forest roads needed to haul the logs out to the mill can lead to erosion. Landing areas, which are areas cleared to allow the loading of the logs on Figure 1 Erosion after Wildfire trucks, are also subject to erosion as the soil is usually churned up and exposed during the harvest operation.

To prevent erosion during harvesting activities, methods and practices known as “Best Management Practices” or BMPs have been developed through careful research to minimize the movement of soil from the site. BMPs can consist of anything from placing waterbars across roads to constructing roads at the proper grades to using mats when crossing small streams. Other BMPs include seeding skid trails when they are no longer needed to stabilize the soil. Used in the appropriate place, BMPs can almost eliminate movement of soil from the harvest site. See the attached links for more information and detailed descriptions. MD Forest Harvest BMPs: http://www.na.fs.fed.us/watershed/pubs/bmp/09_md_bmp_report.pdf

More BMPs: http://www.na.fs.fed.us/watershed/bmp.shtm http://www.epa.gov/owow/watershed/wacademy/acad2000/forestry/forestry5.htm