In
only four years, a buffer strip has transformed this stretch of Bear Creek in
central Iowa into a thriving ecosystem that provides wildlife habitat, improves
soil and water quality, decreases flood severity, and replenishes groundwater
supplies.
Natural and restored buffer areas serve critical functions for nature and humans. The landowner benefits from production of biomass for forage, energy, timber, native prairie seeds, or berries and nuts from trees and shrubs. The landowner also benefits from improved fishing, hunting, and wildlife habitat. All residents in a watershed, and society in general, benefit from improved water quality, lower costs of cleaning sediment from major reservoirs and rivers, and increased diversity for wildlife. Properly functioning riparian zones also can sequester, or hold, more carbon than annually cropped fields or cool-season pastures. Natural and restored riparian areas improve the quality of life for rural and urban citizens alike.
Soil quality and water quality are improved.
The most important function of a buffer system is to reduce NPS pollution. Above ground, the dense stems of native prairie grasses, shrubs, and trees physically slow surface runoff from fields and out-of-bank floodwater, which causes sediment to be dropped on the soil rather than in the waterway. Sediment from adjacent fields that ends up in the buffer strip also keeps phosphorus and pesticides, which are bound to soil, from entering the stream. Below ground, roots improve soil porosity that allows more surface runoff to soak into the soil. Native trees, shrubs, and prairie grasses develop significantly deeper and greater root masses than crop plants and cool-season grasses.
The result is improved soil quality and streambanks that resist collapse. The extensive root system of natural plant communities adds organic matter to the soil as roots die and are replaced. This organic matter acts as a food source for microbes that reduce nitrogen, break down pesticides, and help build large pores to allow water to percolate into the soil.
Woody roots provide strength against streambank collapse. The combined activity of nutrient uptake and storage by the plants and microbial breakdown of chemicals is the "living filter" that reduces NPS pollutants. Water that by-passes the soil system's "living filter," such as drainage from a tile system, can be filtered if it enters a constructed wetland.
Streambanks are stabilized.
Streambanks in healthy riparian zones are stabilized by permanent woody roots and, to a lesser extent, by deep roots of prairie grasses. All streams tend to move across a floodplain, cutting outside corners of stream bends. This cutting causes the bank to erode, which is a major source of sediment. Removing permanent vegetation and replacing it with row crop agriculture or intensively grazed cool-season grasses accelerates the streambank cutting and slumping process. Native riparian zone species such as willows, silver maple, cottonwood, and green ash thrive under these conditions, and can reduce streambank erosion. Use of Eastern red cedar, bundles of other cut trees, and willow plantings on the bank itself can rapidly reduce streambank erosion and limit stream meandering.
Flood severity is decreased.
Massive flooding in 1993 showed that changes in the Midwestern agroecosystem had accelerated flow of water to streams and actually increased flooding. Cultivation of more than 90 percent of the land surface, tile drainage, and intensive grazing of riparian zones all contributed to increased flow of water to streams. Channelization of many streams reduced the length of previously meandering streams, cut water storage capacity of those streams, and caused water to flow more rapidly and flood more extensively.
Healthy riparian zones can reduce severity of flooding during heavy rains in three ways:
Wildlife habitat is improved.
Natural vegetation in a riparian zones improves habitat for both wildlife and fish, and may be the only habitat in an agroecosystem. A recent Iowa study compared songbird use of a four-year-old restored riparian buffer strip system with a neighboring stretch of channelized stream with row crop production within 10 to 15 feet of the channel bank. Over a 10-day sampling period, 30 different songbird species used the restored buffer area, whereas, only eight used the channelized stretch of the stream. The restored buffer system also provided excellent habitat for pheasants and Hungarian partridges, valuable game species in the Midwest.
The width and length of a buffer strip area is important because it provides corridors of travel for different wildlife species. A landowner can encourage wildlife by planting a variety of plant species. Species should be selected on their potential for cover and food, as well as the flowering and fruiting seasons. A diversity of size, shape, and species of plants will ensure the greatest variety of wildlife.
Riparian vegetation also improves in-stream habitat. Vegetation reduces the amount of sunlight that reaches the stream, creating cooler temperatures required by many aquatic species. Leaves, branches, and other plant litter fall into the stream and provide an essential source of food, hiding places, and reproductive sites for aquatic species.
In cold-water streams, overhanging prairie grasses provide hiding places for fish while allowing sunlight into streams. Along such streams, trees might be set back from the edge of the bank to allow native grasses to flourish.
Other products may be harvested.
Riparian buffer strip areas can be designed to harvest products such as hay from switchgrass; sawtimber from oaks, black walnut, and ash; chip material for pulp; biomass for energy; landscape mulch from fast-growing species such as willow, cottonwood, or silver maple; salable native grass and forb seeds; nuts from species such as walnut and hazelnut; and berries from chokecherry, Nanking cherry, and elderberry. Hunting rights also could be leased for game species to provide annual cash income from the area.