Biodiversity of Swan Creek Watershed
By Milo Pyne
former Natural Heritage Botanist
Tennessee Ecological Services Division, Nashville
The Western Highland Rim of Tennessee is prominent on aerial photos or satellite images as being overwhelmingly forested. Within this region of the state, the Swan Creek watershed stands out for having a number of rare plants and invertebrates, as well as being connected to some key pieces of public land.
While some parts of the region have been converted to pine plantations, it is mostly allowed to regenerate following harvest of the hardwood timber. Aside from scattered pieces of state and federal public lands, including Wildlife Management Areas and State Parks, it is overwhelmingly private land, Parts are in small holdings and parts in larger tracts owned by forest industry.
The Tennessee Natural Heritage Program has been aware of the significance of the biological diversity of the Swan Creek watershed for several years, and has conducted several inventories on the area. The Heritage Program, part of the Ecological Services Division of the Tennessee Department of Environment and Conservation (TDEC/ESD), maintains a database of the locations of Tennessee’s rare plants and animals. The information in this database is used to identify lands in need of conservation, and is consulted by numerous public agencies who use it in planning projects in order to avoid impacts to natural resources and biodiversity.
A Natural Areas Registry provides a way to afford recognition and provide management recommendations to landowners and public agencies whose land contains habitat for rare plants and animals. In recent years, additional populations of several rare plants have been located in the Swan Creek watershed.
The most globally rare of these is the Federal Endangered Tennessee yellow-eyed-grass, Xyris tennesseensis. This small plant with grass-like leaves is not really a grass, but is a member of a small specialized family the Xyridaceae. Members of the genus Xyris have yellow flowers which emerge from a cone-like structure at the top of a leafless stem.
The Tennessee yellow-eyed-grass is found in Tennessee, Alabama, and Georgia; one of the largest known populations is in the watershed of Big Swan and Little Swan Creeks. This species in found only in a specialized habitat called a “seep-fen” where cold spring water flows across a shallow soil composed of limestone-like gravel mixed with organic muck derived from the breakdown of plant material.
One of the five Tennessee sites is on the lands of the Natchez Trace Parkway and is Registered as a State Natural Area with the Tennessee Natural Heritage Program.Others are on private land; one landowner agreed to restrict logging in the vicinity of the plants due to efforts of the Tennessee Field Office of the Nature Conservancy. More populations need to be located and protected for this species’ survival to be assured.
Several other plants which are listed as rare in Tennessee are found in this “seep-fen” habitat. There are similar habitats in Missouri and New York. The other state rare plants include the Large-leaved grass-of-Parnassus (Parnassia grandifolia), the Fen orchis (Liparis loeselii), and the Small-headed rush (Juncus brachycephalus). These are all plants which are more common in other parts of the United States, but which are rare in Tennessee.
The Grass-of-Parnassus is not a grass at all, but a plant with beautiful five-petaled white flowers and kidney-shaped leaves; the flowers open in the late fall, when not many plants are blooming. This seep-fen habitat is particularly rare in Tennessee, and occurs only in small patches which are isolated from one another. The physical characteristics of the small seeps provide the special conditions which the rare plants require in order to thrive. Frequently, if the seeps are large enough, there is not enough soil for trees to be rooted and grow. These open seeps receive more sunlight, and herbs or shrubs dominate the ground layer.
The Tennessee yellow-eyed grass is most often found in this type of site. The Grass-of-Parnassus can grow in even smaller seeps which are under the shade of forest trees. Some sites of this type are found on the upper reaches of Swan Creek; often native lilies occur here as well, but they do not bloom reliably in the shade.
Another interesting feature of limestone seeps is that they may provide habitat for dragonflies. The adults lay their eggs in the loose wet soil of the seeps. When the eggs hatch, the larvae are able to live in the clean, clear water which seeps out of the sloping surfaces. The sunny areas around the seeps provide areas where the adults can forage for prey during the summer. A new species of dragonfly, the Tennessee Snaketail (Ophiogomphus acuminatus) has recently been described from Lewis County, Tennessee.
One current project of collaboration between Tennessee Natural Heritage and TVA Entomologists is to determine if this rare dragonfly shares the same habitat as any of the rare pants of the Lewis County seeps or if any rare plants can be found a the sites where ihe dragonfly occurs.
Rare plant conservation is most valuable when efforts to conserve species result in better understandings of the functions and values of the systems which contain them. Rare species frequently serve as the visible indicators of systems which we may not fully understand.
Another globally rare plant which lives in a very different habitat is Eggert’s sunflower (Helianthus eggertii). Yellow sunflowers frequently confound the identification efforts of all but the most determined botanists. Actually, this species is relatively easy to identify if one knows exactly what to look for. It blooms in July and August, like most sunflowers; its flowers (actually composite heads of many small flowers) are relatively large (about 3.5 inches across), its stem is smooth and waxy, and the tapering leaves with rounded bases are smooth except for a scattered roughness on the upper surface. This plant is globally quite rare, and has been a Federal Category 2 (C2) Candidate for a number of years.The U.S. Fish and Wildlife Service is close to listing it as Threatened. It prefers a habitat type which was presumably more widespread when fire was a more common event in the landscape.
This grass and herb-dominated habitat type is called “barrens”, and is related to the prairies of the Midwest, both in structure, species composition, and ecology. Presumably, when fire occurred more frequently, and large grazing animals (such as bison) roamed free, there were large areas of parts of Tennessee and the Southeast which had relatively few trees, with abundant stands of native grasses and flowering herbs, like composites and legumes. Under present conditions, this community persists on roadsides and recently disturbed areas.
In Tennessee, Eggert’s sunflower is most frequent in the Lewis/Lawrence County area and in Coffee County on the Eastern Highland Rim. A large population is known from the lands of the Arnold Engineering Development Center (AEDC) near Tullahoma. Several stands are known from roadsides or clearings on the land of The Farm, near Summertown, and from nearby highway rights-of-way.
As the highway sites are vulnerable to destruction during road widening, the management of sites on large tracts of public land and on the lands of sympathetic private landowners is critical to the species’ survival.
The conservation of lands in the Swan Creek and Little Swan Creek watershed is important not only as part of a largely forested portion of the state which benefits many species, including neotropical migrant songbirds but some specific habitat types provide homes for several rare plants. In particular, the globally rare Tennessee yellow-eyed-grass and Eggerts sunflower have significant populations in the Swan Creek watersheds.