Spiraling: how a stream uses nutrients

or the past few years in southern Chester County, young trees have been appearing beside streams that had flowed for years across open land. Near the Stroud Center, many meadow reaches are now surrounded by dozens or even hundreds of plastic tubes, each containing the sapling of a species once common to the stream bank. The idea that a forest is the proper habitat for a stream has recently begun to catch on, but its origins go back almost two decades, and Stroud scientists have been centrally involved in its development from the outset.

It had become well known by the mid-1980s that a streamside forest — also called a riparian forest because the area near a stream is the "riparian zone" — can filter out various pollutants which would otherwise enter the stream in ground water or overland runoff. As a result, Dave Welsch of the U.S. Forest Service began to compile a document to advocate for riparian forests. Many of the experts he consulted stressed the forest’s role in pollution control. But at the Stroud Center, he got a unique perspective on the issue. From their experiments along White Clay Creek, Robin Vannote and Bern Sweeney had determined that forest buffers are much more than filters for pollution — they are an integral and essential part of the stream ecosystem.

In a series of experiments that began as part of the river continuum, geothermal, and microbial and molecular studies, Stroud scientists brought their interdisciplinary approach to the issue of riparian buffers.

Their reforestation projects demonstrated that trees were critical to maintaining the natural width, depth and sediments of a stream. Feeding experiments showed that aquatic insects, such as mayflies, thrive on leaf material from native trees but fare poorly on exotic invasives. Natural temperatures, which are maintained by forest shading, were found to be key factors in aquatic insects’ life histories and successful reproduction. Finally, the scientists documented the importance of woody debris in creating stream habitat and of forest shading in providing a balanced food base.

Armed with these ideas, Welsch reoriented his document. In "Riparian Forest Buffers," a work that has gained wide recognition throughout the country, he proposed streamside buffers, not only as a means of cleaning the water but also as an essential component of a natural stream habitat.

The Stroud Center continues to do wide-ranging research on riparian forests. The project at the Stroud Preserve, which is probing the capacity of tree buffers to filter agricultural run-off. In addition, a study sponsored jointly by the Environmental Protection Agency and the National Science Foundation seeks to assess the influence of the forest on the entire range of a stream life — the structure of its channel, its temperature and water quality, the cycling of nutrients, the production of algae, the activity of bacteria, and the diversity and production of aquatic insects and fish. The project involves every senior scientist at the Center and covers 15 streams in the White Clay, Brandywine and Elk watersheds.