Southern Piedmont Small Stream Floodplain Forest

This ecological system consists of vegetated communities along streams and small rivers in the Piedmont of the southeastern United States where flooding and flood-related environmental factors affect vegetation composition and dynamics. The vegetation includes both non-forested bar and scour communities, as well as more extensive forested floodplain communities. The forests of these smaller floodplains and bottomlands are not differentiated by depositional landforms such as levees, sloughs, ridges, terraces, and abandoned channel segments, because these features are small and flooding regimes are variable. The system is affected by flooding through wetness, scouring, deposition of material, and input of nutrients. Piedmont floodplain systems are generally quite distinct from Coastal Plain ones, with steeper river gradients, harder rocks and more limited floodplain development. The near absence of Taxodium distichum, Nyssa spp., and other species of the Coastal Plain corresponds well to the geologic boundary in most places.

Almost all of the extent of the system is naturally forested. The forest canopy is usually a mix of mesophytic and widespread species such as Liriodendron tulipifera, Liquidambar styraciflua, and Acer rubrum, along with characteristic alluvial and bottomland species such as Platanus occidentalis, Betula nigra, Acer negundo, Celtis laevigata, Fraxinus pennsylvanica, Liquidambar styraciflua, Quercus michauxii, and Quercus pagoda. Fagus grandifolia may be present in drier portions, mixed with the other species. Successional areas are often strongly dominated by Pinus taeda, Pinus virginiana, Liquidambar styraciflua, or Liriodendron tulipifera. Lower strata in the forests may be either primarily of mesophytic species shared with moist uplands systems, or a mix of mesophytic and bottomland species. Non-forested vegetation is generally limited to very small patches or bands along the channel, and seldom forms distinct communities.

Examples occur on moderately to very high-gradient streams over a wide range of elevations, near streams and small rivers, on floodplains and terraces affected by river flooding and includes emergent bars and banks within channels. Depositional landforms, including levees, sloughs, ridges, terraces, and abandoned channel segments may be present, but occur less frequently and are smaller than the scale of the communities of the floodplain. Fine-scale alluvial floodplain features are abundant. The substrate is primarily alluvium. Soils are usually sandy to loamy, but include local clayey and gravelly areas. Soils are generally fertile, among the most nutrient-rich in the Piedmont region. Alluvial soils may be as important a factor as ongoing flooding in differentiating these systems from adjacent uplands. Emergent and vegetated bars of gravel to cobbles occur occasionally but are generally not extensive or as distinctive as they are on larger rivers. Floods are generally of short duration, and wetness is a major influence only within channels and where water is ponded in local depressions. The geologic substrate may be of any kind, but areas on Triassic sediments tend to have large floodplain systems even on fairly small streams.

The distinctive dynamics of stream flooding are presumably the primary reason for the distinctive vegetation of this system, though not all of the factors are well known. Small rivers and streams with small watersheds have more variable flooding regimes that larger rivers. Floods tend to be of short duration and unpredictably variable as to season and depth. Flood waters may have significant energy in higher gradient systems, but scouring and reworking of sediment rarely affect more than small patches. They are important in maintaining the small non-forested patches. In the forested floodplains, flood disturbances that kill established woody plants are rare, and canopy population dynamics are dominated by windthrow. In addition to disturbance, floods bring nutrient input, deposit sediment, and disperse plant seeds.

In pre-European settlement forests, community diversity in these streamside systems was much more complex than in the modified landscapes of today. Fire, beaver activity, and flooding of varied intensity and frequency created a mosaic whose elements included canebrake, grass and young Betula-Platanus beds on reworked gravel or sand bars, beaver ponds, and grass-sedge meadows in abandoned beaver clearings, as well as the streamside zones and mixed hardwood and/or pine forests that make up more than 95% of the land cover that exists today.

Flooding is the major disturbance process affecting the vegetation, with the substrate more rapidly drained than in flat floodplain areas. The higher gradients of most of these streams and rivers limit floods to fairly short duration. Flooding is most common in the winter, but may occur in other seasons particularly in association with hurricanes, tornados, or microbursts from thunderstorms. Flood waters may have significant energy in higher gradient systems, but scouring and reworking of sediment are important in maintaining the small non-forested patches of the bar and bank communities. Flooding can act as a replacement disturbance in areas where beavers impounded a channel or in rare years with severe prolonged flood events. There are two general types of floods: occasional catastrophic, prolonged floods (due to beaver activity or other severe event); and more frequent repeated minor flooding (i.e., several minor floods within a 10-year period).

The wind disturbance associated with flooding is very significant along small streams because of wet and less dense soils and shallow-rooted trees. Canopy tree mortality from more common windstorms would have resulted in tree-by-tree or small group replacement. Windthrow is the primary cause of mortality in bottomlands. Major storms or hurricanes occurring at approximately 20-year intervals would have impacted whole stands. Tornado tracks can be found passing across uplands and bottomlands [see one such indicated on a map of Umstead State Park, Raleigh, NC], leaving narrow swaths of felled trees (Landfire 2007a). The majority of windthrow in the Piedmont seems to have been the result of hurricanes and tornadoes spawned by them. Even though the Piedmont is removed from the coast by 25 to over 100 miles, extensive windthrow occurred in middle-aged and old-growth trees in Piedmont bottomlands following Hurricane Fran in 1996 (Xi et al. 2008). Bottomland Quercus species, even though seemingly in more sheltered positions, were much more heavily affected than hardwoods on adjacent uplands. Gaps as large as one hectare were seen intermixed in areas with extensive single-tree windthrow. Windthrow may also occur because of thunderstorm microbursts or tornados. In addition, ice damage is an infrequent but potentially catastrophic disturbance.

Fire does not appear to be a dominant factor, and most floodplain vegetation is not very flammable. However, historical references to canebrakes dominated by Arundinaria gigantea suggest that fire may have once been more possible and more important in at least some portions.

These systems are less commonly subject to alteration of flood regimes by upstream impoundments than are large rivers, but these alterations have extensively altered flood frequency and duration in some areas. Extensive erosion of uplands, caused by poor agricultural practices dating back to colonial times, transported large amounts of sediment into floodplains. Glenn (1911) cited numerous examples of extensive bank erosion in streams of the upper Piedmont of North Carolina. It is conceivable that all of the streams of the Piedmont have undergone such extensive bank erosion and channel downcutting that they have all been fundamentally altered since European settlement. In addition, a number of exotic plant species have invaded floodplains, more than in any other Piedmont system. Ireland et al. (1939) discuss how erosion in the Piedmont leads to massive head-cutting and gully formation, a process that continues to the present day.

The widespread introduction of Ligustrum sinense, Microstegium vimineum, and other exotic invasives has dramatically reduced native diversity in the understory.

Fluctuations in rainfall amount and frequency would be expected to affect the abundance and distribution of early- and later-successional vegetation types within the floodplain. Different climate change scenarios would be assumed to promote different outcomes; without better historical baseline data, it is hard to know how these different possible futures relate to past trends. Undoubtedly, the greatest historic stressors have been the conversion to intensive agriculture in the 1800-1950 period (with subsequent abandonment and re-establishment of forest vegetation) and the construction of dams for mills, hydropower, and water supply during the same period (Wharton 1978). The most significant potential climate change effects over the next 50 years include alteration of waterflow, most likely periods of drought alternating with more intense storms.

This system is widespread in the Piedmont, from Alabama to southern Virginia. The northern boundary in Virginia is roughly the watershed of the James River.