Buffer Strips for Riparian Zones
Iowa is a landscape of fertile soil, agriculture crops, and networks of streams and rivers for drainage of the abundant rainfall. The woodland resource of the state at the time of settlement was concentrated, in part, along these streams and rivers. With settlement and the increased mechanization of agriculture, many of these natural woodland corridors were removed to provide more lands for crop production.
These losses resulted in reduced ability of the riparian forest ecosystem to assimilate chemical and soil losses from agriculture lands and because of reduced diversity of the ecosystem, less wildlife habitat. In addition, farm chemical and fertilizer use has increased fourfold since 1960. The present farming practices have resulted in increased loss of soils and agriculture chemicals from their intended site of action. Today's concerns about soil loss, global warming, ground and surface water contamination must be addressed by both the agriculture and non-agriculture communities of our state.
Management practices of the riparian corridors along creeks and streams can be modified for improved benefits. The riparian zone is the last area for interception and biological processing of up-hill materials produced by agriculture practices before they enter the stream. If these zones are managed to maintain a continuous cover crop with an extensive and dynamic root system, large quantities of water, associated chemicals, and moving sediments from adjacent fields can be kept on site before they are lost and contribute to water pollution.
These riparian buffer strips of perennial vegetation perennial vegetation contribute to sustainable agriculture by reducing soil loss, improving water quality and stabilizing the banks of the drainage system. Combinations of perennial grasses, forbs and woody vegetation along stream corridors improve aesthetics and wildlife habitat providing increased number and diversity of wildlife populations. Trees are especially valuable because they reduce atmospheric CO2 through carbon storage and provide alternative sources of renewable energy.
Design of Buffer Strips
The basic design component is multiple rows of trees and shrubs adjacent to the stream with an additional filter of perennial grasses on the outside of the trees. As size of the stream and/or effected watershed increases, the width of the buffer strip must be increased to maintain its effectiveness.
Minimum total width of a filter strip is 33 feet; spacing between rows and trees within a row varies with species and objectives. Common plantings will be 8 to 10 feet between rows and 4 to 6 feet between trees within the row; shrubs will be planted at closer spacings.
Tree and shrub species and planting design will vary depending on the site and the ultimate use of the woody vegetation produced. Fast growing hardwoods such as cottonwood and hybrid poplars, silver maple, willows, and green ash can be used on a variety of bottomland sites. Their advantage is that they grow fast and coppice after being harvested. These trees can harvested for biofuels within 4-6 years or can be left longer to produce small dimension lumber and biofuels.
These trees are harvested in the winter and resprout vigorously in spring producing even more growth than the original stems. As such they continually function in taking up agricultural chemicals during the growing season. It is also possible to produce high quality sawlogs (oak and walnut) by planting the oak and walnut in the middle row of trees with the faster growing species listed above in the outside rows.
The faster growing species will not become over competitive because they can be harvested on a short-rotation. The high quality species are more site specific and should not be planted in poorly drained soils. The choice of shrubs can include species such as ninebark, red-osier dogwood, Nanking cherry, serviceberry and others. The selection of shrubs should depend on site conditions and wildlife attraction.
It is recommended that several different species be used on one site. The up-hill grass strip can also be planted to several different permanent cover species. Switchgrass is a good species because it can be harvested for biofuels. But any permanent prairie mixture that fills the rooting zone with roots could be used. Once again selection might be determined by wildlife habitat requirements.
The establishment of buffer strips will reduce soil loss and ground and surface water pollution, provide improved wildlife habitat, increase diversity of agricultural operations by producing biofuels and timber products, improve the aesthetics of the agricultural landscape and reduce the effects of global warming.