Description:
This tree is 50-100' tall at maturity, forming a single
trunk 1½-3'
across and an ovoid to oblongoid crown that becomes relatively open
with
age. Trunk bark on a mature tree is light to medium gray and coarsely
furrowed. The bark of branches and twigs is gray and more smooth, while
young shoots are light green and densely short-pubescent. Terminal buds
of winter branches are brown and slightly resinous. The alternate
deciduous leaves are 4-6" long and 3-4" across; they are oval-cordate
in shape and their margins are serrate-crenate. Leaf tips are blunt,
while leaf bases are rounded to cordate. New leaves are densely
short-pubescent on both sides. Mature leaves are medium to dark green
and glabrous on their upper surfaces, while their lower surfaces are
pale green and glabrous or nearly so. Petioles are 2-3½" long, whitish
green to pale yellow, short-pubescent to glabrous, and terete (not
flattened).
Swamp Cottonwood is dioecious, forming staminate (male) and
pistillate (female) catkins on separate trees. Staminate catkins are
narrowly cylindrical, yellowish, pendulous, and 2-4" long, consisting
of numerous staminate florets along a central axis. Each staminate
floret (about 1/8" or 3 mm. long) consists of 12-20 stamens and a
calyx-like
disk that is shaped like a shallow saucer. Adjacent to each of these
florets, there is a fringed brown bract that is typically glabrous on
one side and pubescent on the other. Pistillate catkins are narrowly
cylindrical, greenish, widely spreading to ascending, and 2-3" long
(becoming 3-6" long with the maturity of the seed capsules). Each
pistillate floret (about 1/8" or 3 mm. long) consists of an ovary with
2-4
stigmata and a calyx-like disk that is toothed. Each stigma has a
flattened fan-like surface with an undulate outer margin. Adjacent to
each of these florets, there is a fringed brown bract that is typically
glabrous on one side and pubescent on the other. Both staminate and
pistillate florets have pedicels about ¼" long (becoming ½" long with
the maturity of the seed capsules). The blooming period occurs during
mid-spring for about 1-2 weeks. The florets are cross-pollinated by the
wind. Afterwards, the pistillate florets are replaced by seed
capsules that turn brown at maturity, splitting open into 2-3 parts to
release their seeds during the late spring or early summer. These
capsules are up to ½" long, glabrous, and
ovoid in shape. The minute seeds are embedded in tufts of cottony
hairs; they are distributed by either wind or water. The woody root
system is shallow and spreading.
Cultivation: The preference is
full or partial sun, wet conditions, and an acidic soil containing clay
(less often sand or silt). This tree grows moderately fast, but it is
relatively short-lived (maximum age of about 100 years). Viability of
the seeds persists for only 1-2 weeks; they require sunlight and
moisture for germination. Individual trees can produce seeds in as
little as 10 years of age. Seasonal flooding of the ground is readily
tolerated.
Range
& Habitat: The native Swamp Cottonwood is
uncommon in southern Illinois and absent from the rest of the state
(see
Distribution
Map).
Illinois lies along the NW range-limit of this
species. Habitats include floodplain woodlands, swamps, and low areas
around lakes. Common canopy associates include
Taxodium distichum
(Bald
Cypress),
Nyssa aquatica
(Water Tupelo), and
Salix
nigra (Black
Willow). Swamp Cottonwood is a pioneer species that thrives in open
disturbed areas with abundant moisture. As compared to one of its main
competitors,
Populus
deltoides (Eastern Cottonwood), it is able to
tolerate locations that are more waterlogged.
Faunal
Associations: Swamp Cottonwood is a host tree to many of
the same
insects that feed on other
Populus
spp. These species include the
larvae of
Plectrodera
scalator (Cottonwood Borer) and other wood-boring
beetles,
Chrysomela
scripta (Cottonwood Leaf Beetle), various
aphids, and the caterpillars of many moths (see
Insect Table and
Moth
Table). Caterpillars of the butterflies
Limenitis archippus (Viceroy),
Limenitis arthemis
astyanax (Red-Spotted Purple),
Nymphalis antiopa
(Mourning Cloak), and
Papilio
glaucus (Tiger Swallowtail) occasionally feed on the
foliage of these trees, as do the caterpillars of the skipper
Erynnis icelus
(Dreamy Duskywing). Some mammals also use these trees
as sources of food. The Meadow
Vole feeds on the bark of saplings during the winter, while the
White-Tailed Deer occasionally feeds on the twigs and foliage. The
Beaver feeds on the bark and wood, and uses the branches in the
construction of its dams and lodges. In addition, such birds as the
Yellow Warbler, Northern Parula, Warbling Vireo, and Pileated
Woodpecker construct nests in wetland
Populus spp.
Photographic
Location: A low area near a lake in southern Illinois.
Comments:
Most people are unfamiliar with Swamp Cottonwood because it is uncommon
and rarely cultivated. It is distributed primarily along the lower
Mississippi River basin and southeastern Atlantic coastal areas,
although there are local distributions of this tree along the lower
Great Lakes in northern Indiana and northern Ohio. Swamp Cottonwood can
be distinguished from other
Populus
spp. by the shape of its relatively
large leaves (oval-cordate) and their blunt tips. Its leaves resemble
those of
Populus
balsamifera (Balsam Poplar), but this latter tree has
a more northern distribution and the terminal buds of its winter
branches are very resinous. Among native and naturalized
Populus spp.
in Illinois, Swamp Cottonwood has a unique technical characteristic:
Its pistillate (female) florets have discs with distinctly toothed
margins, instead of smooth (entire) or undulate margins. Similar to
Populus deltoides
(Eastern Cottonwood), the wood of Swamp Cottonwood is
light-weight and relatively weak; it has been used to make cardboard
boxes, wooden crates, interior parts of furniture, and high-quality
pulp paper. Other common names of
Populus
heterophylla include Black
Cottonwood, River Cottonwood, Downy Poplar, and Swamp Poplar.