The Alluvial Forest occurs in the narrow floodplains of small streams or on large rivers where the floodplain is narrowed by bedrock. These floodplains are too small to differentiate communities by depositional landform. Flooding occurs for shorter periods and is more variable than on the larger floodplains of bottomland forests. The canopy is mixed, including floodplain species, widespread species, and upland species but with widespread species dominating.
Most of the canopy is widespread species such as sweet gum and tulip-tree but characteristic alluvial species such as sycamore, river birch or Southern hackberry are common.
Bottomland Forests that are located on the higher parts of large floodplains are flooded for brief to moderate periods during the growing season. These sites occur on terraces, on the higher parts of depositional ridge and swale systems, and on some wide flat floodplains. Natural vegetation is a mix of bottomland oaks, tulip-tree, sweet gum, and sometimes American beech mixed with upland oaks and hickories. Bottomland Forests that are located on lower terraces, ridges, and flat floodplains lack an appreciable number of upland species.
Bottomland Forests vary in composition depending on whether they are close to the river or higher up the floodplain. Oaks are usually a prominent feature and may include willow oak, basket oak, cherrybark oak, overcup oak, water oak, and sometimes white oak.
The Low Elevation Seep community occurs in seepages and springs at the base of slopes or the edges of bottomlands. Well-developed seeps contrast sharply with adjacent communities in their abundant wetland vegetation and permanently saturated, mucky soils. Seeps are fairly common and tend to be small, often shaded by the canopy of trees rooted in adjacent communities. Despite a lack of much standing water, seeps are important breeding and foraging sites for amphibians.
Low elevation seeps are distinguished by wetland vegetation including Southern wild raisin, Northern wild raisin, orange jewelweed, cinnamon fern, royal fern, netted chain fern, and sedge.
Because Orange County is a headwater region, large streams are lacking. Only at the southwestern corner, where the Haw River forms about two miles of the county border, is there a large enough river system to deposit the sediment volume needed for the development of a Levee Forest community.
Dominated by sycamore, river birch, box elder, and southern hackberry, this community occurs only along a narrow portion of the Haw River.
On sites where the soil is rocky and nutrient poor, the Monadnock Forest occurs. This rather rare natural community is found in several upland sites in Orange County, typically on isolated erosional remnant hills, but occasionally occurs on bluffs. Not all isolated hills support this community type. Abundance of rock and soil chemistry may be factors in determining occurrence. Extreme soil acidity, accompanied by aluminum toxicity, has been suggested as important.
Dominated by chestnut oak, species diversity is usually low, with an understory strongly dominated by sourwood, and red maple and having a patchy shrub and sparse herb layer.
In positions where steep, nutrient poor slopes are facing north and are thus relatively cool and moist, the uncommon Heath Bluff community can occur. This community is distinguished by a variable, usually open, tree canopy and a closed, often dense, shrub layer. Catawba rhododendron, mostly found only in the higher parts of the mountains, occurs at several sites around Chapel Hill and Durham.
The shrub layer is dominated by mountain laurel or Catawba rhododendron. The species diversity is generally very low.
The Swamp Forest is found mainly in the Triassic Basins. Usually located in the wetter parts of large floodplains, prolonged flooding is common and the community is dominated by flood-tolerant species, with a relatively sparse herb layer. While overcup oak is found here, the Swamp Forest is too wet for most other oak species.
The canopy is usually dominated by green ash, American elm, red maple or overcup oak, while the herb layer is generally dominated by sedges, lizard’s tail or false nettle.
The uncommon Upland Depression Swamp Forest is an isolated forested wetland found in poorly drained depressions on upland ridges and flats. It occurs on unusually flat areas with hardpan soils derived from mafic rocks or slates. Habitats include shallow, seasonally rain-flooded upland basins where water stands for part of the year but is not great enough to prevent a closed tree canopy from forming. Sites which hold enough standing water can be important breeding sites for amphibians.
Upland depressions vary in size and are usually dominated by willow oak, or sometimes overcup oak, swamp white oak, basket oak or sweet gum. The shrub and herb layers are sparse but typically include greenbrier and sedges.
Where circumneutral clay soils develop an impermeable hardpan, the Xeric Hardpan Forest may occur. Quite rare and restricted to the Piedmont, Xeric Hardpan Forest communities have a stunted, open tree canopy. This community is found on broad upland flats in environments that are xeric because of restricted rooting depth caused by dense or shrink-swell clay, sometimes in combination with rock. Canopy density is less than in dry forests and depends more on fire and disturbance history.
The canopy is xerophytic, dominated by post oak, with or without blackjack oak, Carolina shagbark hickory or pines. Base-loving flora such as white ash, winged elm, eastern redbud, viburnums, fragrant sumac, coralberry, and curlyheads are usually common.
Our terrain consists mainly of broad upland ridges on gently rolling hills, with low-energy streams cutting narrow floodplains. Soils are typically acidic, but rare areas of more neutral soil are found at scattered locations.
Soil moisture can be used to group our natural communities into four broad themes:
There are several types of floodplain communities in Orange County. The most common, the Alluvial Forest and the Bottomland Forest, are found in stream valleys of various sizes. Standing water is absent most of the time, but regular seasonal inundation and deposition of sediments are characteristic. They usually are lushly vegetated, with an abundance and high diversity of vines, shrubs, and herbs beneath a varied canopy.
Non-alluvial Forests are isolated wetlands surrounded by terrestrial communities. They are not associated with river floodplains though they may share some wetland species.
Mesic, or moist, hardwood forests occur on sheltered ravines and bluffs that face north or east. Locations on lower slopes catch water seeping in from above and, receiving less intense sunlight, allow moisture to last longer in the soil. Common in the piedmont, these communities usually occur a few acres at a time rather than in large expanses.
Dry-mesic to dry forests and woodlands generally occur in drier, well-drained habitats, but can also be found on relatively moist lower slopes or upland flats. Most commonly associated with acidic soils, these forests infrequently occur on soils that are more basic. Differences among sites in their soil types, moisture levels, and disturbance histories result in vegetation that is both complex and variable.
