Most natural grasslands of the world are maintained by fire, grazing or climatic factors (e.g. lack of rainfall, persistent cold/freezing in tundra), but in the depths of some of the deepest river gorges of Southern Appalachia are poorly known flood-maintained grasslands known to community ecologists as riverscours.
Riverscour communities occur on some of the larger streams of the Cumberland Plateau along high-gradient sections that have cut down deeply into the plateau surface. These streams are often popular for whitewater kayaking and often have Class I-IV rapids.
In Alabama, riverscour is associated with the Locust Fork of the Black Warrior River, the Little Cahaba River, and Little River, which has formed one of the deepest canyons in the eastern U.S. Given that the Plateau makes up such a small portion of Georgia, it is not surprising that riverscour grasslands are not well represented. One tiny patch occurs along Rock Creek downstream of Lula Falls just outside Chattanooga. In Kentucky, riverscour is found on the Big and Little South Forks of the Cumberland River as well as the Rockcastle River. Historically "scours" were also found on other Kentucky streams that have now been flooded by reservoirs. Tennessee is home to the largest and most diverse riverscour grasslands. The best developed ones are on the Big South Fork of the Cumberland River (the lower portion of which is in KY) and its tributaries, the New River and Clear Fork, as well as the Obed and Emory rivers (including their tributaries), and the Caney Fork River.
Outside of the Cumberland Plateau, riverscour communities can be found in a few other isolated places, such as the Ouachita Mountains of Arkansas and Oklahoma, the Allegheny Plateau of West Virginia and Pennsylvania, a few rivers in the Blue Ridge (e.g. Ocoee, Hiwasee) and Piedmont (Yadkin, Potomac), and the "shut-ins" of the Missouri Ozarks.
Riverscour vegetation varies greatly within the flood zone of a given stream. Imagine standing in the middle of the river in knee-deep water. Often one can find submerged aquatic species such as pondweeds (Potamogeton spp.) and riverweed (Podostemum ceratophyllum). Walking closer to shore in shallow water a few inches deep water-willow (Justicia americana) and golden-club (Orontium aquaticum) often occur, sometimes along with sedges. On the shore itself is a thin sandy shoreline only a couple of feet wide. Here one finds small annual herbs and grasses adapted to fluctuating water levels and moisture content of the sand, but sometimes this zone supports species more at home in the longleaf pine savannas of the Deep South and somewhat unexpected in mountainous terrain: dwarf sundew (Drosera brevifolia), yellow-eyed grasses (Xyris spp.), and bladderwort (Utricularia subulata).
Now, I know what you're thinking..."Dude, this ain't no stinking grassland!" Of course, you're right, but just be patient and let the intense richness and diversity of the narrow stream corridor blow your mind.
Stepping across the narrow sand shore and up onto the adjacent bank, you begin to encounter a variety of shrubs intermixed with a showy array of robust herbs. The variation is impressive and somewhat unusual in its mixture. Growing beside typical wetland shrubs like buttonbush (Cephalanthus occidentalis), indigo bush (Amorpha fruticosa), Virgina sweet-spires (Itea virginica), and hazel alder (Alnus serrulata), are species more typical of dry upland woodlands and glades such as ninebark (Physocarpus opulifolius), hawthorns (Crataegus), and shrubby St. Johnswort (Hypericum prolificum). Growing amongst these are cutleaf coneflower (Rudbeckia laciniata), cardinal flower (Lobelia cardinalis), wingstem (Verbesina alternifolia), ironweed (Vernonia gigantea), and other species typical of moist to occasionally wet soils. Again, not a grassland...I know.
The bank that you begin to ascend just two to three feet higher than the nearby stream is built on a deposit of sandstone boulders and cobblestones, with a healthy dose of sand filling the interstices. These cobble bars or boulder bars have been documented to be surprisingly stable over long periods of time, so once formed they are there for a while unlike gravel and sand bars which often shift after major floods events. Within this zone, the sand is kept moist much of the year and sometimes a few small trees might manage to grow up to 10 or 15 ft tall. They often have contorted trunks or in some cases may be bent over at the ground at a right angle but still remain alive. In most years the shrub and trees have old leaves, twigs, and other debris hanging from their branches from the season's floods as this lowest level nearest the stream regularly floods. Still, not a grassland...I get it.
At this point, turn around and look back at the stream. It's only about 6-20 feet behind you. Now, continue walking as you were. The elevation of the cobble bar increases a bit more as you walk farther from the centerline of the creek. One thing to notice as you walk is that the largest stones are found immediately in and on the bank of the stream. Some of them may be the size of automobiles. But as you walk farther away the stream center the rock sizes get smaller, grading from large boulder to small cobblestones. At the highest elevation on the bar, which may be anywhere from 5-15 or so feet above normal low water, is where the rock sizes are smallest. As flood waters spread out during floods, smaller particles as sand or gravel are carried farther away from midstream so it is at these highest elevations within the flood zones where we find prominent deposits of sand. Sometimes they occur in nearly pure beds and are reminiscent of sandhill communities or dunes near the coast.
The soils of these elevated sandy zones drain very rapidly, which means that plants growing in this zone need to be drought adapted. Just beneath the surface of the sand is also a jumbled mass of cobble and boulder which makes it difficult for bulky-rooted plants to survive. These factors along with the intense periodic flash floods that strike multiple times per year (some of which have been documented as having flows of up to 150,000 cubic feet per second) combine to make this zone particularly well-suited for perennial bunchgrasses, herbs, and small shrubs. Finally, this is where we find our grasslands!!!
But before we discuss this unique riparian grassland community, what lies just beyond if you were to keep walking away from stream center? The grasslands tend to occupy narrow zones but can be anywhere from 10-100ft or more wide. Once you cross this narrow swath, two things generally are to be expected. First, you might continue increasing elevation slightly (could vary from several inches to a few feet) until you reach a fairly dense shrub/stunted tree zone that then grades into the forested mountain slope above the river. In other cases, the grassland might occur in the center of a large cobble bar that has a small channel on its back side or on an island where you are again met with alder, buttonbush, indigo bush, river birch, etc.
Tell us more about this so-called riverbank grassland...
The combination of flooding and rocky, shallow, excessively-drained soils in full sun, makes this zone perfect for grassland vegetation. Anyone who has spent time in prairies of the Midwest would have little trouble in identifying many of the plants in these little riverside prairies. Unlike the vast Midwestern grasslands, however, the largest known riverscour grassland is about one-quarter the size of a football field and most are smaller than the footprint of the average house.
Take one look around and right away many of the familiar prairie grasses are present: big bluestem (Andropogon gerardii), Indian grass (Sorghastrum nutans), little bluestem (Schizachyrium scoparium), switchgrass (Panicum virgatum), and even gamagrass (Tripsacum dactyloides). Dropseed (Sporobolus spp.), numerous species of rosette panic grass (Dichanthelium spp.), muhly grass (Muhlenbergia sylvatica), and river oats (Chasmanthium latifolium) are frequent. Even prairie cordgrass (Spartina pectinata) occurs on some. Additionally, other expected "prairie" species are here, such as blazingstars (Liatris spp.), asters (Symphyotrichum and Eurybia), goldenrods (Solidago), goat's rue (Tephrosia virginiana), and phloxes (Phlox spp.). My favorite herb, not unexpected to my friends I'm sure, is Sundrops (Oenothera fruticosa). Like many grasslands around the world, these often contain a diverse mixture of shrubs and stunted trees, and even a few species of woody vines, but the grasses form the dominant groundcover.
Species Found Nowhere Else in the World
Several of the plants in riverscour look "different" from non-riparian populations. In the late 1990s, Dr. Brian Keener, then a graduate student at the University of Alabama and now a professor at the University of West Alabama, co-discovered a new species of goldenrod in riverscour habitat along the Locust Fork of the Black Warrior River north of Birmingham. This goldenrod, which he named Solidago arenicola [meaning inhabiting sand in Latin], at first glance resembles the common slender goldenrod (Solidago erecta) which is common in dry woodlands and savannas of the uplands of the Southeast, but differs in certain technical features.
There are numerous other species in riverscour that researchers have realized are different and represent previously undescribed species. For example, the dominant grass in most of the riverscours of Tennessee and Kentucky is a type of big bluestem. Whether it is a new species, a new variety, or merely an ecotype remains to be determined but these riparian plants are unquestionably "different" from non-riparian populations. They are smaller, often topping out at 4-5 ft, have consistently narrower leaves and sheaths that are smooth, have generally fewer spikes in the inflorescence, and the parts of the spikelet (the glumes, lemmas, etc.) have different patterns of venation and hairiness. This riparian big bluestem could be more attractive as a landscape plant due its shorter stature which is not as rank and "floppy" as material in cultivation currently and is probably much more suitable to cultivation in an urban setting, where it is usually avoided. This highlights how there is still much to be offered by studying and exploring wild habitats in the South, particularly grasslands, and how some of these discoveries are relevant to real-world issues.
Right now, researchers are aware of numerous undescribed species in the riverscours of the Cumberland Plateau. On the scourse, they are often very common. For example, a new white-flowered aster similar to the bushy aster (Symphyotrichum dumosum) is abundant on many sites. Canadian licorice-root (Ligusticum canadense) is a member of the carrot family common in eastern woodlands and forests but on the cobble bars there is a clearly different type with strikingly different leaves that represents a new species. It has been observed for decades but is still waiting to be named.
While the new aster and licorice root are common on a few different rivers of Tennessee and Kentucky, other new species are highly localized. A putative new species of flowering spurge (Euphorbia) occurs along the Obed River and its tributary, Daddy's Creek, but is not found outside this watershed.
One of the most remarkable endemic species in riverscour is a small shrub about a foot tall of the mint family known as the Cumberland Rosemary (Conradina verticillata). This shrub occurs in clonal patches in the sandiest zones. Oddly enough, all other members of the genus Conradina occur hundreds of miles to the south in the sandhills and dunes along the Gulf Coast, especially in Florida. This is the only mountain species in the genus. It begs the question of just how the Cumberland Rosemary evolved?
Out of Place
Within 100 meters of a given riverscour grassland, one can sometimes find species that are far removed from their main range. For example, in the depths of the Obed River Gorge in Morgan Co., Tennessee, one can find red elderberry (Sambucus pubens), white pine (Pinus strobus), and eastern hemlock (Tsuga canadensis)--all species characteristic of the Northeast and southeastern Canada--growing within close proximity to Deep South species such as Pineland Dropseed (Sporobolus junceus) and Southern Jointweed (Polygonum americanum) which are disjunct by at least 100 miles from their nearest occurrence in north Alabama.
Cumberland Sandgrass (Calamovilfa arcuata) was discovered as a new species in the late 1960s on the Tennessee portion of the plateau. This riverscour endemic, which can be mistaken for switchgrass (Panicum virgatum), has since been found in highly isolated occurrences far removed from its original site of discovery, including southeast Kentucky, north-central Alabama, extreme northwestern Georgia, and then of all places, west-central Arkansas and southeastern Oklahoma.
Refugia for Rare Species
Riverscour grasslands provide refuge for many rare plant species. In the collage below, pictured from left to right, then top to bottom are: Cumberland Rosemary (Conradina verticillata), Rockcastle Aster (Eurybia saxicastellii), Cumberland Sandgrass (Calamovilfa arcuata), Witch Alder (Fothergilla major), Black Warrior Goldenrod (Solidago arenicola), Large-flowered Barbara's Buttons (Marshallia grandiflora), Rock Grape (Viitis rupestris), Old-pasture Bluegrass (Poa saltuensis), Southern Jointweed (Polygonum americanum), Coastal Fetterbush (Euobotrys racemosa), Sweetfern (Comptonia peregrina), Pineland Dropseed (Sporobolus junceus), Virginia Spiraea (Spiraea virginiana), and Shortleaf Sneezeweed (Helenium brevifolium). These are just a few of the rare species known from riverscour grasslands in Tennessee and Kentucky.
Farther south in Alabama, riverscour habitats support some of the same rare species listed above but have a more pronounced influence of species or groups often associated with pine savannas. The Green Pitcher Plant (Sarracenia oreophila), one of the few "mountain-lovers" in the genus Sarracenia, would seem to most people to be out of place in a rocky mountain gorge given that most of the species in the genus are found in the Deep South pinelands. But in Little River Canyon National Preserve in northeast Alabama, Green Pitcher Plant can be found growing directly in riverscour habitats in the depths of the gorge in and among saturated sands and cobblestones. Here it grows with other plants out of place such as sundews (Drosera spp.), five species of yellow-eyed grass (Xyris spp.), pipwort (Eriocaulon), and whitehead bogbutton (Lachnocaulon anceps).
The South's Last Frontier
Riverscour habitats represent one of the last frontiers of the eastern American botany. Just getting to them and studying them has proven to be very challenging. There are many dangers in surveying Appalachia's understudied canyons. In one hike along a 3-mile stretch of the Obed River Gorge in 2007, 11 copperheads were observed. The countless rocks, logs, and steep slopes present an constant risk of breaking bones but fortunately we've always gotten out unscathed. It took us quite a while to master how to get down gorges safely. Riverscours never cease to surprise us. One particularly deep pool in the Obed River that we knew was deep turned out to be 64 feet deep. Currents and flash floods are always a concern given the remoteness of these gorges and the fact that once you are in the depths of a gorge if something goes wrong it can be very difficult to get out quickly. Planning is definitely something we've gotten good at.
It is no wonder that the grasslands of these remote river canyons are among the least understood in eastern North America. My students have found that every Cumberland Plateau river with rivescour vegetation is really quite unique. Grasslands on different rivers separated by just a few miles are often very different floristically and structurally. One thing they all share in common though is that they are packed with species. A single 10 x 10 meter plot of open grassland can have more than 50 species of flowering plants. Adjacent flood-scoured shrublands and woodlands with open grassy and herb-rich ground layers can harbor as many as 80 species in the same area.
Recently my graduate students and I have wrapped up surveys of four rivers: Big South Fork River of KY and TN (Chris Mausert-Mooney), Daddy's Creek of TN (Devin Rodgers), Caney Fork River of TN (Mason Brock), and the Locust Fork of the Warrior River of AL (Kelly Anderson). Recently, my student Zach Irick has begun surveying Little River in northeast AL and in 2018 we will be surveying the Clear Fork of the Big South Fork of the Cumberland River, helping to add to the already excellent work carried out by Claude Bailey Jr. several years ago.
Stay tuned for more highlights from riverscour grasslands...
Contributed by: Dwayne Estes