Home » Biologists in the Field » Saving Riparian Forests

Saving Riparian Forests

Our Plummer’s Hollow stream is a faint, unnamed blue line on Highbee’s stream map of Pennsylvania. Although it is only a mile and a half long and its streambed is less than ten feet wide, it greatly influences the streamside or riparian forest through which it flows. The riparian forest, in turn, is essential to a healthy stream.

That is why, when we were faced with our neighbor’s attempt to log down the steep mountainside, across our private access road we shared with him and then down to the edge of the stream, we fought hard to keep him as far above the road and away from the stream as possible. We knew, instinctively, that the forest cover was necessary protection for the stream. Furthermore, our son Mark had compiled a biological inventory of our property and discovered that the center of biological diversity is the riparian forest.

Our 100-year-old riparian forest includes such deciduous trees as cucumber magnolia, American basswood, American beech, tulip-tree, red and white oaks, white ash, red maple, and black birch. The smaller, coniferous component consists of many large eastern hemlocks and a smattering of mature white pines. Spicebush, rhododendron, witch hazel, mountain laurel and wild hydrangea, as well as a diverse selection of small saplings, make up the understory, and beneath them grow at least 12 species of ferns.

In spring the streambanks are blanketed with hundreds of purple trillium and foamflower, but six species of violets, round-lobed hepatica, sessile and perfoliated bellworts, Canada mayflower, wood betony, trailing arbutus, rue anemone, jack-in-the-pulpit, yellow mandarin, mitrewort, Indian cucumber-root, white clintonia, Solomon’s seal and Solomon’s plume are just a sampling of spring wildflowers that grace our riparian forest. In early summer Indian pipes and black cohosh light up the dark forest. By the end of August, horsebalm, white wood asters, blue-stemmed goldenrod, orange jewelweed, turtlehead, and white snakeroot bloom along the stream and road banks.

Some of our most exciting sightings of birds, amphibians, reptiles and mammals have occurred in the streamside forest. A northern saw-whet owl perched in a sapling near the stream one October night as we drove home from an Audubon meeting. On an April spring night, my husband Bruce and I had our only sighting, so far, of a least weasel as it raced along the streambank side of our road. A silver-haired bat flew in front of our car on the evening of September 1, 1994. Nineteen days we saw the first eastern coyote on our mountain as it leaped across in front of the car. In January of 1982 our hemlocks hosted our only flock of white-winged crossbills. Common snapping turtles have wandered up from the river several times, and in April of 1997 a mink picked through streamside debris in midday.

The stream, itself, is too small and shallow to harbor fish, but it does harbor a diversity of other water-dependent creatures as our son Dave discovered when, as part of a science project in high school, he censused their numbers and species. Using a potato rake to rake the stream bottom, he then trapped the small invertebrates in screening he had stretched across the stream. He also turned over rocks. In addition to a bevy of salamanders and crayfish, he found a bewildering assortment of tiny creatures and their homes including three kinds of caddisfly cases, four types of stonefly nymphs, and three species of mayfly nymphs.

All of these so-called stream macroinvertebrates are “shredders,” “collectors,” or “scrapers,” and make their living on some form of woody debris. “Shredders,” which include certain species of craneflies, beetles, flies, caddisfly larvae, and stonefly nymphs, feed on riparian litter (deciduous leaves, coniferous needles, twigs, branches, bark, nuts, and flowers) trapped in stream channels by fallen trees and debris. But first the litter must be changed by microbes and made digestible for the shredders, a process that takes weeks or months, depending on the species and water temperature as well as the kinds of leaf litter and the season of the year.

Ecologists have found that permanent streams in temperate zones usually have two different sets of shredders–one for autumn and winter and another for spring and summer. The kind of leaf litter is also important. Litter that rots fast comes from basswood, alder, and most herbaceous species. Medium litter is shed by maples and birches, and slow litter by oaks, rhododendron, beech, conifers and most ferns.

By feeding on leaf litter, the shredders change larger pieces of litter (known in scientific circles as “coarse particulate organic matter”) into smaller pieces mixed in with their own feces (“fine particulate organic matter”). The fine particulate organic matter, in turn, provides food for the “collectors”–mayfly nymphs, black fly and midge larvae. The “scrapers,” most notably some caddisfly larvae, feed on algae that grows on fine particulate organic matter found in rapidly flowing water. All three kinds of stream invertebrates provide food for “predators” such as fishfly and midge larvae and sculpin and trout.

All of this and more I learned at the Third Annual Coldwater Conference in February 1999 which brought together a wide diversity of people from the public and private sectors interested in restoring riparian buffer zones along the streams and rivers of the commonwealth. To Albert Todd, United States Forest Service liason to the Chesapeake Bay Program, riparian habitat is “a linear system that forms the circulatory system for life.”

Pennsylvania has over 50,000 miles of streams, he told us, but 14,000 are in need of restoration including many sections that have been cleared or straightened by engineers or farmers. Probably his most profound statement, and one that humans have the most trouble accepting where nature is concerned, is “Messy is good.” Instead of clearing out the trees and branches that fall into streams, they should be allowed to remain there because they anchor streambed sediment that not only feeds invertebrates, but provides hiding places for them. They, in turn, feed fish. “It is a little known fact,” he concluded, “that fish do grow on trees.”

I was reminded of the ice storm that brought down over 100 trees across our stream several winters ago. Many visitors to our property were visibly disturbed because we did not have them harvested by a logger and accused us of “wasting good wood.” We, on the other hand, were interested in watching the natural processes of an undisturbed forest.

The following spring we were rewarded by the singing and subsequent nesting of winter wrens, birds that had previously only been autumn migrants and occasionally winter inhabitants of the riparian habitat. It took an abundance of fallen trees to provide the upturned roots in which they build their cavity nests.

In addition to feeding stream dwellers and providing nesting habitat for winter wrens, riparian forests perform many other ecological functions according to Chuck Williams of Clarion University. He has been studying riparian forests in the Allegheny National Forest and has found that they buffer streams from sedimentation and the runoff of excess nutrients and toxins.

Like us, he also discovered that riparian forests are “centers of diversity. Plants [especially] are very diverse. Our field work on the ANF has found that riparian systems support perhaps the greatest herbaceous plant species’ diversity of forests of the region.” That is because these areas contain a variety of soil types, moisture, and light conditions and are subject to frequent flooding which create a mixture of small habitats that are able to support a wider variety of plant species in a single area. As we have found, both wetland and upland plant species live in a riparian forest.

Williams also emphasized the importance of coarse woody debris in creating plant diversity and mentioned that more mature forests produce much more coarse woody debris. In addition, riparian forests and their accompanying streams are essential to a wide range of amphibians.

“At least 15 species of salamanders are associated with riparian forests,” he told us.

Ornithologist Margaret Brittingham of Penn State University has studied steamside buffer zones and has discovered that the wider the buffer zone, the higher the number and abundance of breeding bird species. Four-hundred-foot buffers produced almost a full complement of bird species, except for the Louisiana waterthrush, which requires 700 feet. Along four herbaceous and six woody, half-mile-long corridors no more than 20 feet wide in Lancaster County, 71 bird species that are tolerant of human disturbance bred. In other words, birds that are already common such as song sparrows, American robins, Canada geese, mallards, red-winged blackbirds and tree swallows, preferred to breed in more human-impacted areas.

But Brittingham’s four forested sites with a half-mile buffer zone produced an entirely different array of birds whose numbers are dwindling, in part, because they are losing undisturbed habitat–Acadian flycatchers, wood thrushes, ovenbirds, veeries, scarlet tanagers and Louisiana waterthrushes, as well as eastern towhees. Although towhees like early successional and woody corridors, they don’t like human disturbance, hence their surprising preference for riparian forests. With the exception of veeries, those species are most abundant in our Plummer’s Hollow riparian forest.

Dr. Bernard Sweeney of the Stroud Water Research Center mentioned still more benefits of riparian forests to aquatic habitat. For instance, the large roots of trees growing along streambanks provide homes for macroinvertebrates. Those same trees also produce flowers, which some aquatic insects feed on when they emerge. The twigs trees shed into the stream are often used as shelter by caddisfly larvae and dobsonflies lay their eggs on fallen logs in or above the stream.

Sadly, though, despite the benefits of riparian forest to the natural world, few landowners understand their importance. Dan Dutcher of Penn State University’s School of Forest Resources has been talking to landowners about their streams. He has found little interest in taking care of riparian zones. Most landowners believe that, “If you don’t keep the woods back, they will encroach on the fields.” There is an overwhelming tradition of fighting back nature and keeping their landscape neat. Mown or grazed grass, even along streambanks, is seen as both practical and aesthetically pleasing to the human senses.

Many of these same folks enjoy fishing and watching birds and animals. Somehow we must convey Todd’s message that “messy is good” and “fish [as well as birds, mammals, wildflowers, amphibians, and reptiles] do grow on trees.”

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