Green Horizons

A gully passes through a wooded area. (Photo courtesy Iowa State University.)

Research sponsored by the University of Missouri Center for Agroforestry has shown that forest buffers with less tree density and more ground cover result in more effective buffering of surface runoff from crop- or pastureland by preventing the creation of deep gullies.

Riparian forest buffers - living filters comprised of trees, shrubs, forbs and grasses, established in distinct zones - are an important agroforestry practice that reduce runoff, stabilize stream banks and increase food and cover for wildlife. The Iowa State University Riparian Ecosystem Management Team has been evaluating the performance of riparian forest buffers and developing management plans that will maintain the benefits of the buffers over time. While many buffers have been planted over the past 18 years, little research has been conducted on their effectiveness over time and on the long-term management needed to maintain their functionality.

There is evidence that some of these buffers (i.e., narrow strips of natural forest), without adjacent warm-season grass filters, are not effectively slowing and spreading surface runoff from adjacent crop fields as it enters the buffer. As a result, surface runoff and pollutants make their way to the stream channel by creating gullies that begin in the field as ephemeral gullies, but become classic gullies as they pass through the buffers. They remain ephemeral gullies in the field because the farmer can cultivate through them each season, creating a gentle depression that carries the water in lieu of a more deeply incised classic gully that develops without annual management - the problem as it passes through the forest. Identifying the most effective species, composition, distribution and overall width of riparian forest buffers has become a major focus of the Iowa State Team as these attributes can be controlled by riparian forest buffer design and long-term management. Many "natural" and "planted" riparian forest buffers do not support much year-round perennial ground cover with their present tree densities.

Research showed ephemeral gullies that ended in wellmaintained warm-season grass filters did not continue through the grass filters, nor did they create gullies in adjacent forest buffers. However, width of grass filter and density of grass stems were important in determining how far into the grass filter sediment was deposited. In some cases, sediment was found choking out portions of the grass filter, making the filter less effective. Many cool-season grass filters may not be wide enough to handle the sediment load that is delivered by many of the ephemeral gullies. The results of this study should help refine the ideal ratio of gullyshed to buffer width to assure trapping of most of the gully sediment.

These studies are providing the information required for developing an assessment tool that will allow conservation professionals to quickly identify potential sites that could benefit from a grass filter planting or a timber stand improvement of an existing buffer.