PIEDMONT BOTTOMLAND FOREST (TYPIC LOW SUBTYPE)
Concept: Piedmont Bottomland Forests are communities of the higher parts of large Piedmont floodplains, away from the river and naturally dominated by a mix of bottomland oaks, hickories, Liriodendron tulipifera, and Liquidambar styraciflua, rather than the characteristic Piedmont Levee Forest species. The Typic Low Subtype covers most examples on lower terraces, ridges, and flat floodplains, without an appreciable component of upland species, and lacking the distinctive composition of the Northern Low Subtype. Quercus lyrata may be present but is not dominant as it may be in Piedmont Swamp Forest.
Distinguishing Features: Piedmont Bottomland Forests are distinguished from Piedmont Levee Forests by lack or scarcity of the characteristic levee species. They are distinguished from Piedmont Swamp Forest by shorter flooding duration and by vegetation reflecting the drier conditions. In the most intact examples, oaks are characteristic, along with Liriodendron tulipifera, Liquidambar styraciflua, and Acer rubrum.
The Typic Low Subtype is distinguished from the Northern Low Subtype by the absence or near absence of Quercus palustris. It is distinguished from the High Subtype by the predominance of bottomland species without an appreciable component of most upland species. Plants that Matthews (2011) found to be more common in this subtype and less common or absent in the High Subtype include Saururus cernuus, Carex lupulina, Carex tribuloides, Elymus virginicus, Glyceria striata, Juncus effusus, Persicaria virginiana, Ulmus americana, and Quercus lyrata. However, a few characteristically upland species still occurred with some frequency in this type, including Quercus alba and Carya ovata. A number of more mesophytic species are largely absent in this subtype. In examples cleared in the past, Liquidambar or Liriodendron may strongly dominate, so that distinguishing the subtypes is difficult. However, in more intact examples, at least some bottomland oaks will be present.
Synonyms: Quercus pagoda – Quercus phellos – Quercus lyrata – Quercus michauxii / Chasmanthium latifolium Forest (CEGL007356). Ecological Systems: Southern Piedmont Large Floodplain Forest (CES202.324).
Sites: Piedmont Bottomland Forests occur on floodplain ridges and on irregular or flat terraces well above the river. They usually are behind a natural levee or riparian zone with Piedmont Levee Forest but may sometimes extend to the river channel. The Typic Low Subtype is most extensive and well-developed in Triassic basin floodplains but may potentially occur on any large floodplain.
Soils: Soils are coarse to fine-textured alluvial soils. Most are mapped as Chewacla (Fluvaquentic Dystrudept), less often as Riverview (Fluventic Dystrudept) or other alluvial soils.
Hydrology: The Typic Low Subtype is intermittently flooded for short periods, but somewhat longer and more frequent than the High Subtype. Wetness may vary with microtopography and soil texture. Though parts may be well-drained when the river is not in flood, local low areas or areas with clayey soils may be poorly drained.
Vegetation: The Typic Low Subtype is a forest consisting of a mix of bottomland and swamp species. In Matthews, et al. (2011), the most constant trees are Fraxinus pennsylvanica, Acer rubrum, Liquidambar styraciflua, Quercus phellos, and Ulmus americana, but Quercus michauxii, Quercus pagoda, Quercus lyrata, and Carya carolinae-septentrionalis are frequent and often dominant or codominant. Other trees that are fairly frequent, though not codominant, include Quercus nigra and Quercus alba. The understory is dominated by Carpinus caroliniana, with no other typical understory species frequent. The only highly constant shrub species is Ilex decidua. Viburnum prunifolium and Ligustrum sinense are fairly frequent, as is Rubus sp. Vines are a regular component. Bignonia capreolata, Parthenocissus quinquefolia, Smilax rotundifolia, Toxicodendron radicans, and Lonicera japonica are highly constant, and Thyrsanthella difforme and Vitis spp. are also fairly frequent. Herbs have relatively low cover. Boehmeria cylindrica, Arisaema triphyllum, Viola spp., and Microstegium vimineum are highly constant in plots. Other herb species that are at least fairly frequent include Asarum canadense, Polystichum acrostichoides, Carex spp. (intumescens, typhina, tribuloides, blanda, amphibola, caroliniana, debilis and others), Ranunculus abortivus, Saururus cernuus, Chasmanthium latifolium, Elymus virginicus, Festuca subverticillata, Galium tinctorium, Glyceria striata, Impatiens capensis, Juncus coriaceus, Persicaria virginiana, Poa cuspidata, Sanicula canadensis, Sceptridium biternatum/dissectum, Solidago spp., and Erechtites hieracifolia.
Range and Abundance: Ranked G2? but perhaps G3. This subtype is scattered in the eastern and central Piedmont. The largest examples are in Triassic basins, but examples in other areas are equally abundant. Good examples appear more numerous than for the High Subtype, as might be expected for wetter, less easily farmed sites, but extensive remnants are still few.
Associations and Patterns: The Typic Low Subtype occurs naturally as a large patch community, occurring either as large expanses or as part of a mosaic with large aggregate area, but it is now often reduced to small remnants. Few occurrences are large. It is usually associated with Piedmont Levee Forest, sometimes with Piedmont Swamp Forest or with the High Subtype, but sometimes occupies all or most of a floodplain’s width. It may contain embedded Floodplain Pools. On the edge of the floodplain, it will grade to various mesic or dry-mesic upland forests, most commonly Mesic Mixed Hardwood Forest.
Variation: Variation within this subtype is not well known. Matthews, et al. (2011) had only a single finer grouping in their classification that appears to correspond to this subtype.
Dynamics: As a community at intermediate elevation above the river, the Typic Low Subtype floods at higher frequency and duration than the High Subtype but lower than Piedmont Swamp Forest. Flood currents likely are slow when they reach this community, so that scouring and movement of litter and debris probably are minor, but deposition of nutrients in sediment probably is important for fertility. It is unclear how the altered hydrological regimes caused by land clearing in the watersheds has affected them.
As this subtype is often locally the highest part of its floodplain, beaver ponds probably are relatively unlikely to flood much of it, but ponds may affect it near sloughs and tributary streams.
Comments: As noted in the comments on the Piedmont and Mountain Floodplains theme, the classification of bottomland and swamp forest has been particularly confused, and for that reason the central concepts and circumscription of the 3rd Approximation communities have been changed more than others in the 4th Approximation. The distinction between the Typic Low Subtype and Piedmont Swamp Forest remains among the more uncertain. As a community dry enough to support a substantial presence of a number of oak species, it is recognized as a subtype of Piedmont Bottomland Forest in the 4th Approximation, with Piedmont Swamp Forest narrowed from its 3rd Approximation concept to include only communities wet enough to largely exclude most of the oak species. However, the analysis by Matthews, et al. (2011) did tie the equivalent community to their swamp and bottomlands group, where it seems to represent the bottomlands. They distinguished them from the higher oak-hickory flats, but it is not entirely clear why. The Typic Low Subtype does share a high abundance of the species most often dominant in swamps. However, it also contains a large number of species shared with the oak-hickory flats and absent in the swamps forests it is grouped with. Further investigation may be needed.
The Matthews, et al. (2011) community that corresponds closely to this subtype is called Quercus (phellos, pagoda, michauxii) – Ulmus americana / Ilex decidua / Arisaema triphyllum. They noted that it generally occurred on wide, flat Triassic Basin floodplains. Quercus lyrata occurs in it, but only in wet inclusions. However, Natural Heritage Program records include a number of occurrences outside of Triassic basins that appear to represent this community. The reason for this discrepancy needs further investigation.
Rare species: Vascular plants: Carex decomposita, Eurybia mirabilis, and Urtica chamaedryoides. Invertebrate animals: Gomphus abbreviatus.
References: Matthews, E.M., R.K. Peet and A.S. Weakley. 2011. Classification and description of alluvial plant communities of the Piedmont region, North Carolina, U.S.A. Applied Vegetation Science 14: 485-505.