Alluvial forest

Distinguishing Features

Alluvial forest in the piedmont and lower mountains is easily recognized by its location in floodplains and its characteristic species, such as sycamore, river birch, and box elder.


Alluvial forest occurs along streams and rivers that seasonally or intermittently flood their banks. Alluvial forests vary in size from broad river valleys to narrow strips of streamside vegetation. The relatively small floodplain forests of the piedmont are called alluvial forests for the nutrient rich "alluvium" that is deposited when rivers and streams overflow their banks. The brown color of the water is the result of silt and clay particles that erode from mountain and piedmont soils and are then carried downstream in the sediment. Moist, nutrient-rich soils, long growing seasons, and high summer temperatures provide favorable conditions for plant growth. Flooding benefits plants by continually replenishing soil nutrients and moisture, but long periods of standing water deplete soils of oxygen and can limit plant growth. Because plant species differ in their ability to cope with poorly aerated soils, the frequency and duration of flooding has a strong influence on the distribution and composition of plants within floodplain forests.


Alluvial forests in the piedmont and lower mountains have a well-developed canopy, an open to dense understory, and an herbaceous layer that is both dense and diverse. Common canopy trees include sycamore (Platanus occidentalis), river birch (Betula nigra), sweetgum (Liquidambar styraciflua), tulip tree (Liriodendron tulipifera), and green ash (Fraxinuspennsylvanica). Understory trees include box elder (Acer negundo), red maple (Acer rubrum), ironwood (Carpinus caroliniana), and American holly (Ilex opaca). A species-rich shrub layer includes spicebush (Lindera benzoin), pawpaw (Asimina triloba), strawberry bush (Euonymus ameri-canus), silky dogwood (Cornus amomum), painted buckeye (Aesculus sylvatica) and giant cane (Arundinaria gigantea). Vines are common and diverse and include species such as poison ivy (Toxicodendron radicans), Virginia creeper (Parthenocissus quinquefolia), and crossvine (Bignonia capreolata).

A dense herbaceous layer often develops beneath a fairly open canopy. Two tall herbs are giant ragweed (Ambrosia trifida) and New York ironweed (Vernonia nove-boracensis). Common ferns include Christmas fern (Polystichum acrostichoides), royal fern (Osmunda regalis), and resurrection fern (Pleopeltispolypodioides), a species that grows on the trunks or branches of trees, and on large rocks. Grasses and sedges are also common, including river oats (Chasmanthium latifolium), an ecologically important species that traps and stabilizes soils (thereby reducing erosion).

Natural levees sometimes occur near the water's edge. These slightly elevated areas form when silt and other particulates are deposited at the base of stream-side vegetation. Dominant species include sycamore, river birch, and box elder. In lower parts of the floodplain, where standing water persists for longer periods, swamp forests can occur. Species diversity is relatively low in swamp forests because only species tolerant of prolonged flooding can survive. Canopy dominants include sweet gum, willow oak (Quercusphellos), red maple, swamp cottonwood (Populus heterophylla), and green ash. Vines are common in swamp forests, but the herbaceous layer is generally sparse.

Seasonal Aspects

In early spring, woody trees such as swamp chestnut oak (Quercus michauxii), river birch, black walnut (Juglans nigra), and shagbark hickory (Carya ovata) release enormous amounts of pollen from conspicuous, drooping male catkins. Because their pollen is generally dispersed by wind prior to leaf expansion, pollination success is enhanced. Some insect-pollinated plants also flower before their leaves emerge, including pawpaw, with its nodding, purple, bell-shaped flowers, and spicebush, with its tiny yellow male and female flowers (on separate plants). One of the first woody plants to leaf out (and flower) in early spring is painted buckeye—its upright clusters of elongate flowers vary in color from yellowish green to pinkish red. Spring-flowering vines include the red tubular flowers of crossvine and the fragrant white (fading to yellow) flowers of Japanese honeysuckle (Lonicerajaponica). Poison ivy and Virginia creeper, in contrast, have clusters of tiny flowers that often go unnoticed.

Spring ephemerals such as dimpled trout lily (Erythronium umbilicatum) and spring beauty (Claytonia virginica) produce leaves and flowers in late winter and early spring, then die back and go dormant before the canopy trees leaf out. In summer, New York ironweed's purplish flower heads on tall stalks are popular food-gathering spots for butterflies, bees, and beetles.

Plants bearing conspicuous fruits in late summer and fall, whose seeds are dispersed by animals, include spicebush, with its bright red berry-like fruits; red mulberry (Morus rubra), with its elongate blackberry-like fruits; and silky dogwood, with its bluish white fruits. Seeds dispersed by wind include the winged seeds of box elder, the woolly seeds of black willow (Salix nigra), and the seeds within the ball-like fruiting structures of sweetgum and sycamore. Trees such as shagbark hickory, river birch, and sycamore are easily identified (even in winter) by their distinctive peeling bark.


Alluvial forest occurs throughout the piedmont and lower mountains. At higher elevations, alluvial forest is often dominated by montane species such as yellow birch and sweet birch (Betula alleghaniensis and B. lenta), with a dense understory of heath shrubs, including rosebay (Rhododendron maximum) and mountain dog-hobble (Leucothoe fontanesiana).


Flooding, deposition and loss of alluvial material, and windthrow result in a constantly changing environment. Floods disturb vegetation by washing away plants and soil. In more severe floods, small parts of the forest may be eroded or completely washed away. The tendency of streamside trees to lean toward the river has both positive and negative consequences. Leaning trees gain access to higher light levels that potentially increase growth rates, but because a greater part of their trunks are immersed in fast running water during heavy floods, they are particularly susceptible to being uprooted and washed away. Prolonged flooding can also stress or kill floodplain trees. Most floodplain trees can tolerate short periods of flooding, but few species can tolerate prolonged flooding during the growing season.

Floodplains are in a continually dynamic state with the building of new substrate and the loss of old substrate. Sandbars and mudflats form on the inside curves of rivers, and regular flooding deposits alluvial materials. Floodplain soils consist of alluvial material (alluvium) that can range from 15 to 250 ft. thick. Heavy mud deposition during the growing season kills herbaceous plants and the seedlings and saplings of woody plants. Degradation, or the loss of substrate, results from erosion during heavy flooding, a shift in climate, the construction of upstream dams, and other forms of human disturbance.

Storms, tornadoes, and occasional hurricanes can cause considerable wind damage. Shallow root systems allow floodplain trees to use the uppermost soil region, where anaerobic conditions are less likely, but make them more vulnerable to windthrow (in evidence of this, trunks of wind-toppled trees often lie scattered across floodplains). The openings created by downed trees increase the abundance and diversity of herbaceous plants and facilitate forest regeneration.

Beavers impact plants and other organisms. Beavers build dams on small floodplain streams with sticks and mud, flooding some of the surrounding alluvial forest. They adversely impact floodplain forests by felling trees for food and dam building. By feeding on the base of trees, beavers girdle and kill even large, established trees. Their greatest impact comes when trees die in flooded areas. Persistent flooding, and the resulting loss of trees and increased light, can change a floodplain forest into a marsh containing wetland shrubs and herbs. By building dams and creating ponds, beavers alter the vegetation and provide habitat for a number of animals, including fish, waterfowl, reptiles, and amphibians.

But beaver dams are also susceptible to various kinds of disturbance. For example, dams can break or deteriorate and temporarily or permanently drain their ponds. A free-flowing stream in a forested floodplain can take over 30 years to develop into a mature beaver pond. In the absence of major disturbance, beaver ponds slowly fill with sediment, trees invade, and eventually the pond returns to forest.

Conservation Aspects

Narrow bands of alluvial forest persist along the thousands of streams and creeks that flow through the piedmont and lower mountains. These wetland forests are important reservoirs of biodiversity as they are frequently the last contiguous woodland habitats for plants and animals in heavily populated piedmont areas. Also, because alluvial forest occurs where aquatic and terrestrial systems interface, species diversity and abundance can be high. Diversity in a particular area reflects forest age, the degree of flooding, soil composition, and disturbance history.

Streamside forests provide corridors (greenways) for the travel and migration of many animals. The forest canopy shades streams, plant roots stabilize stream-banks, and decomposing leaves add essential nutrients to soils, all of which contributes to a greater diversity of aquatic life. Forested floodplains are also a buffer against damaging floods, and they improve water quality by trapping and filtering pollutants washed from roads and urban areas, as well as the excess fertilizers and pesticides in farm run-off.

There is a long history of human impact on piedmont alluvial forests. Flood-plain forests were the first areas in the Southeast to be converted to croplands by Native Americans. By about 1,000 AD, Native Americans throughout the Southeast had developed a system of intensive agriculture based on corn, a crop that rapidly depletes the soil of nutrients. To avoid crop failure due to nutrient-depleted soils, corn was planted in floodplains where periodic flooding deposited nutrient-rich alluvium. Villages were frequently located along rivers and major streams that were also used for transportation. As Native American populations declined, Europeans continued to convert tracts of forested floodplains into cropland and pasture. By the early 1800s, cotton was widely planted in floodplains. Logging of the remaining forest began later that century. Today, piedmont floodplain forests exist in various stages of succession following tree removal. Only a few alluvial forest tracts of great age remain, and these are usually small, inaccessible patches on steep slopes that make farming or timbering difficult.

Habitat loss due to cropland conversion, logging, urbanization, and industrial development continues to be a major threat to floodplain forests. The construction of dams on the upper portions of large rivers and the channelization of small streams change the dynamics of flooding and sediment deposition, which in turn severely alters the ecology and composition of floodplain communities.

Alluvial forests are particularly susceptible to invasion by exotic plants. Heavy infestations of nonnative species such as Chinese privet (Ligustrum sinense), Japanese honeysuckle (Lonicera japonica), and Japanese stilt grass (Microstegium vi-mineum) dramatically reduce the diversity and abundance of native plants.

Public concern over the loss of floodplain forests, along with a greater awareness of their many benefits, have increased in recent years, resulting in increased regulation and protection of forested wetlands by various state and federal agencies.


Blom, C. W. P. M., and L. A. C. J. Voesenek. 1996. Flooding: The Survival Strategies of

Plants. Trends in Ecology and Evolution 11:290-95. Brown, M. J. 1997. Distribution and Characterization of Forested Wetlands in the Carolinas and Virginia. Southern Journal of Applied Forestry 21:64-70. Sharitz, R., and W. J. Mitsch. 1993. Southern Floodplain forests. In Martin, W. H., S. G. Boyce, A. C. Echternacht, eds., Biodiversity of the Southeastern United States: Lowland Terrestrial Communities, 311-72. New York: Wiley and Sons.






Acer negundo

Box elder

pp. 220


Acer rubrum

Red maple

p. 222

Betula nigra

River birch

p. 228

Carpinus caroliniana

Ironwood, Musclewood

pp. 228


Liquidambar styraciflua


p. 242

Liriodendron tulipifera

Tulip tree, Yellow poplar

pp. 242


Platanus occidentalis


p. 253


Carya ovata

Shagbark hickory

p. 230

Celtis laevigata

Hackberry, Sugarberry

pp. 232-


'Fraxinus pennsylvanica

Green ash

Ilex opaca

American holly

pp. 239


Juglans nigra

Black walnut

pp. 240


Morus rubra

Red mulberry

p. 245

fQuercus michauxii

Swamp chestnut oak

Ulmus alata

Winged elm

p. 266

tUlmus americana

American elm

Shrubs and Woody Vines


Aesculus sylvatica

Painted buckeye

p. 269

Arundinaria gigantea

Giant cane, River cane

pp. 273-


Asimina triloba


p. 274


Bignonia capreolata


p. 275


Cornus amomum

Silky dogwood

p. 280


Euonymus americanus

Strawberrry bush

pp. 283-84

c O

Parthenocissus quinquefolia

Virginia creeper

pp. 296-97


Salix nigra

Black willow

pp. 311-12


Toxicodendron radicans

Poison ivy

pp. 315-16

c Z



Alnus serrulata

Tag alder

pp. 269-70

p to

Itea virginica

Virginia willow

pp. 289-90


Leucothoe fontanesiana

Mountain doghobble

p. 292


Lindera benzoin


pp. 293-94


Sambucus canadensis

Common elderberry

pp. 312-13


Viburnum acerifolium

Mapleleaf viburnum

pp. 319-20


Xanthorhiza simplicissima


pp. 322-23



m d



Arisaema triphyllum


p. 335


Chasmanthium latifolium

River oats

p. 347


Claytonia virginica

Spring beauty

pp. 351-52

Erythronium umbilicatum

Dimpled trout lily

pp. 370-71


Galium aparine

Catchweed bedstraw

p. 376


Osmunda cinnamomea

Cinnamon fern

pp. 412-13

Pleopeltis polypodioides

Resurrection fern

pp. 425-26


Polystichum acrostichoides

Christmas fern

p. 429


Stellaria pubera

Giant chickweed

p. 447


Vernonia noveboracensis

New York ironweed

p. 465

Zephyranthes atamasca

Atamasco lily

pp. 469-70

Invasive Exotic Plants

Ligustrum sinense

Chinese privet

pp. 292-93

Lonicera japonica

Japanese honeysuckle

pp. 294-95

Microstegium vimineum

Japanese stiltgrass

pp. 403-4

= plant not included in the species profiles (Part IV)

= plant not included in the species profiles (Part IV)

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  • Kian Fraser
    What are alluvial ferns?
    4 years ago

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