Populus heterophylla L.
Salicaceae -- Willow family
R. L. Johnson
Swamp cottonwood (Populus heterophylla) is of secondary importance among bottom-land hardwoods. The species, sometimes referred to as black cottonwood, river cottonwood, downy poplar, or swamp poplar, may grow on sites that are too wet for other native poplars. It is a difficult species to grow from cuttings, a characteristic that limits its commercial value.
Swamp cottonwood inhabits the wet bottom lands and sloughs of the Coastal Plain from Connecticut and southeastern New York to Georgia and northwestern Florida, west to Louisiana. It grows north in the Mississippi Valley to southeastern Missouri, western Tennessee, Kentucky, southern Illinois, Indiana, Ohio, and southern Michigan (5).
-The native range of swamp cottonwood.
The climate is humid throughout the range of swamp cottonwood. Average annual rainfall varies from about 890 mm (35 in) in northern Indiana to 1500 mm (59 in) in southern Louisiana. Approximately 1140 to 1240 min (45 to 49 in) of rain falls annually along the Atlantic coast, nearly half of it from April through August. Yearly temperatures average 10° to 13° C (50° to 55° F) in the North to 18° C (65° F) along the south Atlantic coast and 21° C (70° F) on the gulf coast. Minimum annual temperatures range from -29° C (- 20° F) in the North to -1° C (30° F) in the South. Frost-free days range from 180 to 300.
Though most often found on heavy clays, swamp cottonwood also grows on the edges of, but not in, the muck swamps of the Southeast. Optimum growth and development is in the deep, moist soils of shallow swamps and low-lying areas near tidewater (4).
Sites that are too wet for eastern cottonwood (Populus deltoides) will support swamp cottonwood. Examples are shallow swamps, sloughs, and very wet river bottoms where the water table remains near the soil surface for all but 2 or 3 months in the summer and early fall. In the southern part of its range, low, wet flats provide the driest sites occupied by swamp cottonwood. However, in southern Illinois it is a dominant or codominant tree on soils with available moisture varying from 3 to 21 percent for the 61- to 76-cm (24- to 30-in) layer (4).
Swamp cottonwood grows naturally on at least eight major soil types common to the Midsouth: Alligator, Amagon, Arkabutla, Forestdale, Perry, Rosebloom, Sharkey, and Tensas. The soils represent several families and the orders Alfisols, Inceptisols, and Entisols. They range from 4.6 to 5.9 in pH and from 24 to 65 percent clay in the surface 0.3 m (1 ft) (1).
Swamp cottonwood is sparse throughout its range and is not a major species in any forest cover type. It is most often found in the following types (2): Baldcypress (Society of American Foresters Type 101), Baldcypress-Tupelo (Type 102), and Water Tupelo-Swamp Tupelo (Type 103).
Common tree associates include sandbar willow (Salix exigua), black willow (S. nigra), peachleaf willow (S. amygdaloides), green ash (Fraxinus pennsylvanica), water hickory (Carya aquatica), sycamore (Platanus occidentalis), sugarberry (Celtis laevigata), red maple (Acer rubrum), American elm (Ulmus americana), pumpkin ash (F profunda), Carolina ash (F. caroliniana), waterlocust (Gleditsia aquatica), persimmon (Diospyros virginiana), and overcup oak (Quercus lyrata).
Major small tree and shrub associates are waterelm (Planera aquatica), buttonbush (Cephalanthus occidentalis), swamp-privet (Forestiera acuminata), and possumhaw (Ilex decidua).
Flowering and Fruiting- Swamp cottonwood is dioecious. Flowers are proteranthous, appearing from March to May. Staminate catkins are rather stiffly pendant, oblong, cylindric, and 5 to 10 cm 2 to 4 in) long; pistillate catkins are 5 to 15 cm (2 to 6 in) long, pendulous, slender, and raceme-like. Pollination is by wind. Fruits ripen and the seeds fall from April through July.
Seed Production and Dissemination- Trees start seed production at about 10 years. Reddish-brown obovoid seeds number about 330,700/kg (150,000/lb) (9). Seeds are very small, light in weight, and tufted with hairs, features that allow them to be blown over 100 in (330 ft) by wind and to float for a considerable distance in water. Water is an important transporting agent since the bottom lands normally flood during the seedfall period. Numerous seeds are produced annually, but under natural. field conditions they remain viable for no more than a week or two.
Seedling Development- Germination is epigeal. Best seedling establishment is from seeds that quickly settle on unshaded, moist mineral soil in shallow swamps, deep sloughs, and along often-flooded creeks or rivers. Seedlings require nearly full sunlight to survive and grow. They also need an abundance of moisture, especially during the early part of the growing season. Seedlings usually occur in groups but seldom cover a large area (4). Early growth is relatively rapid but will not match that of eastern cottonwood. On a well-drained soil in southern Illinois, swamp cottonwood seedlings grew at about the same rate as silver maple seedlings.
Vegetative Reproduction- Cuttings from juvenile plants will root but probably not as well as those of eastern cottonwood (4). Stumps less than 30 cm (12 in) in diameter are likely to produce sprouts.
Growth and Yield- The largest swamp cottonwood trees on record are 35 to 40 m (115 to 130 ft) tall and 165 to 190 cm (65 to 75 in) in d.b.h. (3,8). Trees are considered mature at about 30 in (100 ft) tall and 75 to 90 cm (30 to 36 in) d.b.h. Annual height growth on poorly drained sites is slightly faster over a 30-year period than that of green ash, about 0.8 in (2.5 ft). Diameter growth on a good site in the South may approach 20 cm (8 in) in 10 years (4). Swamp cottonwood grows little after 40 or 50 years and seldom remains sound after 80 years (4). Large trees often are crooked and short boled. Trees with three merchantable logs are exceptional.
Per-hectare volumes are unknown for this species since it is nearly always a single tree or grows in very small patches. Based on known volumes for eastern cottonwood (6), a pure stand of mature swamp cottonwood would likely yield 280.0 to 350.0 m³/ha (20,000 to 25,000 fbm/acre, Doyle log rule).
Rooting Habit- The root system of swamp cottonwood is probably shallow like that of eastern cottonwood and most other lowland species. Poplars in general have strong horizontal surface roots from which vertical plunging roots develop. Length of horizontal roots can be considerable. Plunging roots are limited in development by the water table or soil condition.
Reaction to Competition- Swamp cottonwood is classed as intolerant of shade, though probably less so than eastern cottonwood. Individuals may survive partial shade when they are young, but older trees require full sunlight.
Damaging Agents- There are no reported insect or disease problems associated specifically with swamp cottonwood. But the ones that attack eastern cottonwood probably also damage swamp cottonwood. Important insect enemies include the cottonwood leaf beetle (Chrysomela scripta), cottonwood twig borer (Gypsonoma haimbachiana), poplar borer (Saperda calcarata), and the cottonwood borer (Plectrodera scalator). Among the more important diseases are Melampsora leaf rust (Melampsora medusae) and a number of canker diseases, including Septoria, Cytospora, and Fusarium (7).
There is no market specifically for the small volume of swamp cottonwood harvested. The wood resembles that of eastern cottonwood and is generally sold as such. Among the uses for cottonwood lumber and veneer are boxes, crates, and interior parts for furniture. Pulpwood is used in high-grade book and magazine paper.
To date, few other uses have been found for the species. It is rarely cultivated for ornament, does not produce important wildlife food, and is important to flood or erosion control only in very small, localized areas. Instead of swamp cottonwood, a closely related species, eastern cottonwood, is chosen for planting because it outperforms swamp cottonwood on all except the wettest sites.
The species is rare in tree collections and has received little attention from geneticists. There are no reported races or hybrids.