Riparian Forest Buffers Function and Design for Protection and Enhancement of Water Resources
Streamside forests are complex ecosystems vital to the protection of our streams and rivers
Streamside forests are crucial to the protection and enhancement of the water resources of the Eastern United States. They are extremely complex ecosystems that help provide optimum food and habitat for stream communities as well as being useful in mitigating or controlling nonpoint source pollution (NPS). Used as a component of an integrated management system including nutrient management and sediment and erosion control practices, streamside forests can produce a number of beneficial effects on the quality of water resources. Streamside forests can be effective in removing excess nutrients and sediment from surface runoff and shallow groundwater and shading streams to optimize light and temperature conditions for aquatic plants and animals. Streamside forests also ameliorate the effects of some pesticides and directly provide dissolved and particulate organic food needed to maintain high biological productivity and diversity in the adjoining stream.
The removal of streamside forests has adversely affected the vitality of our water resources
In natural conditions, streamside forests protected most of the rivers and streams of our nation, but deforestation associated with agricultural and urban expansion has drastically reduced the extent of streambank protected by forest. The result has been an adverse effect on the quality of water and aquatic habitats. In many of our streams and estuaries, water is unfit for human consumption, industrial use or recreation. Shellfish and finfish production is also reduced. These problems are linked, in part, to contamination from nutrients, sediment, animal waste, and other pollutants associated with agricultural and urban runoff.
Continued strengthening of the Clean Water Act reflects the publicís concern for clean water
The Water Pollution Control Act of 1948 or "Clean Water Act" and its subsequent amendments through 1987 demonstrate strong congressional determination to improve the quality of our water resources. These laws have done much to clean up point source contaminants by requiring states to establish and enforce water quality standards, by requiring specifications and licensing for the discharge of effluents, and by funding the installation of municipal sewage treatment plants. As a result of the cleanup of concentrated pollution from specific sites, nonpoint source pollution from specific sites, nonpoint source pollutants, which are typically dispersed in origin, have increased in relative importance and now account for more than 50% of the pollution in our nationís waters. Nonpoint source pollutants include sediment, nutrients, pesticides, animal wastes and other substances which enter our water supply as components of runoff and groundwater flow.
Streamside forests remove pollutants in several ways
Recent research has shown that streamside forests can:
Streamside forests function, often simultaneously, as filters, transformers, sinks and sources
- improve the quality of water resources by removing or ameliorating the effects of pollutants in runoff
- increase the biological diversity and productivity of stream communities by improving habitat and adding to the organic food base.
The streamside forest removes sediment and sediment-attached phosphorus by filtration
The streamside forest functions as a filter by removing sediment and other suspended solids from surface runoff. Sediment is probably the most common and most easily recognized of the nonpoint source pollutants.
- Cropland erosion accounts for about 38% of the approximately 1.5 billion tons of sediment that reach the nationís waters each year. Pasture and range erosion accounts for another 26%.
- Sediment suspended in the water can reduce or block the penetration of sunlight, adversely affecting the growth and reproduction of beneficial aquatic plants.
- Sediment deposited on the stream bottom can interfere with the feeding and reproduction of bottom dwelling fish and aquatic insects, weakening the food chain. Large deposits of sediment can overfill stream channels and floodplains, greatly increasing the potential for flooding.
- Several mechanisms of sediment removal are at work in the streamside forest. Some sediment settles out as the speed of the flow is reduced by the many obstructions encountered in the forest litter. Additional sediment is filtered out by the porous soil structure, vegetation and organic litter as the runoff flows over and into the floor or the streamside forest.
- Phosphorus is also reduced by the filtering action of the streamside forest because about 85% of available phosphorus is bonded to the small soil particles comprising the sediment. Approximately 4% of the phosphorus is attached to soil particles too small to be filtered by these processes resulting in a removal of about 80% of phosphorus by the riparian forest filter. The minor amount of ammonium which is bound to sediment can be filtered out in the same way.
- However, dissolved phosphorus and nitrate must be removed by either microbial or biochemical transformation processes.
The streamside forest transforms nitrate to nitrogen gas
The streamside forest functions as a transformer when chemical and biological processes occurring within it change the chemical composition of compounds. For example, under well oxygenated soil conditions, bacteria and fungi in the streamside forest convert nitrogen in runoff and decaying organic debris into mineral forms NO3. These forms can then be synthesized into proteins by plants or bacteria. When soil moisture is high enough to create anaerobic conditions in the litter and surface soil layers, denitrifying bacteria convert dissolved nitrogen into various nitrogen gasses, returning it to the atmosphere. Studies have shown that the amount of nitrogen in runoff and shallow groundwater can be reduced by as much as 80% after passing through a streamside forest.
The streamside forest can also function as a transformer when toxic chemicals such as pesticides are converted to non-toxic forms. Because of continued improvements in the formulation and management of pesticides, only very small amounts manage to leave the area of application. These residues, borne by runoff, are converted to non-toxic compounds by microbial decomposition, oxidation, reduction, hydrolysis, solar radiation and other biodegrading forces at work in the soil and litter of the streamside forest. While scientists have long understood the biological processes at work in the streamside forest, additional data are necessary to fully quantify their importance with respect to pesticide degradation.
The streamside forest acts as a sink by storing nutrients for extended periods of time
The streamside forest can function as a sink when nutrients are taken up by plants and sequestered in plant tissue. Some estimates indicate that 25% of the nitrogen removed by the streamside forest is assimilated in tree growth which may be stored for extended periods of time in woody tissue and possibly removed as logs or other forest products. Nitrogen and other nutrients may also be passed up the food chain when plant tissues are consumed by animals and converted to animal tissues. In wetter areas, nutrients in leaf litter can be stored for longer periods as peat. Sediments filtered out by the streamside forest remain to become incorporated into the forest soil.
The streamside forest provides a source of energy for aquatic life
The streamside forest functions as a source when it provides energy to streams in the form of dissolved carbon compounds and participate organic detritus. These materials are critical to processes within the stream itself, helping to restore and maintain natureís equilibrium. In small, well-shaded upland streams, as much as 75% of the organic food base may be supplied by dissolved organic compounds or detritus such as fruit, limbs, leaves and insects that fall from the forest canopy. Benthic detritivores (the stream bottom bacteria, fungi and invertebrates that feed on the detritus) form the basis of the aquatic food chain. They pass on this energy when they are, in turn, consumed by larger benthic fauna and eventually by fish. Thus the streamside forest functions as an important energy source for the entire aquatic food chain from headwaters to estuary.
Simple removal of nonpoint pollutants is not enough to improve the quality of water resources. A balanced, integrated, adaptive community of riparian and aquatic organisms comparable to the natural systems of the region with stability and capacity for self repair must be reestablished. The restoration of a healthy aquatic ecosystem from the headwaters to the estuaries to the oceans requires the reestablishment of significant amounts of riparian forest.
Control of point source pollutants was a start; control of nonpoint pollutants and repair of the aquatic ecosystem through reestablishment of the streamside forest is a logical next step in improving the quality of our water resources.
Specifications for such a statewide forest should consider the following:
- Streamside forests should be used in conjunction with sound land management systems that include nutrient management and sediment and erosion control.
- Sediment removal - The streamside forest must be wide enough to filter sediment from surface runoff. Maximal effectiveness depends on uniform shallow overland flow. Percent removal of total suspended solids is a good indicator of effectiveness.
- Nutrient removal - Periodic flooding and the presence of forest litter contribute to conversion of nitrate to gaseous nitrogen by denitrification. Plant uptake also accounts for significant removal of nitrogen. Trees must be removed periodically to remove nutrient sequestered in woody biomass and to maintain system efficiency.
- Periodic minor ground shaping may be necessary to encourage dispersed flow and prevent concentrated flow.
- A portion of the riparian forest immediately adjacent to the stream should be managed to maintain a stable streamside ecosystem and to provide detritus and large stable debris to the stream.
- Crown cover should be managed to minimize fluctuations in stream temperatures within the range necessary for instream aquatic habitat.
- Instream slash and debris removal practices should be revised to conserve existing large stable debris by retaining useful stable portions of jams whenever possible. Unstable tops and smaller debris with potential to form problem jams should be removed a sufficient distance to prevent re-entry during storm events.
For additional information on specifications/Riparian Forest Buffers please contact the Maryland Department of Natural Resources Forest Service.
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