Liquidambar styraciflua

An ancestor of Liquidambar styraciflua is known from Tertiary-aged fossils in Alaska, Greenland, and the mid-continental plateau of North America, much further north than Liquidambar now grows. A similar plant is also found in Miocene deposits of the Tertiary of Europe.[11]

Liquidambar styraciflua is a medium-sized to large tree, growing anywhere from 50–70 feet (15–21 m) in cultivation and up to 150 feet (46 m) in the wild,[12] with a trunk up 2–3 feet (0.61–0.91 m) in diameter, on average.[13] Trees may live to 400 years.[14] The tree is a symmetrical shape and crowns into an egg shape when the branches get too heavy after its first two years of cultivation.[7]

Another distinctive feature of the tree is the peculiar appearance of its small branches and twigs. The bark attaches itself to these in plates edgewise instead of laterally, and a piece of the leafless branch with the aid of a little imagination readily takes on a reptilian form; indeed, the tree is sometimes called "alligatorwood".[11] The bark is a light brown tinged with red and sometimes gray with dark streaks and weighs 37 lbs. per cubic foot.[13] It is deeply fissured with scaly ridges.[11] The branches carry layers of cork.[15] The branchlets are pithy, many-angled, winged, and at first covered with rusty hairs, finally becoming red brown, gray or dark brown.[11] As an ornamental tree, the species has a drawback—the branches may have ridges or "wings" that cause more surface area, increasing weight of snow and ice accumulation on the tree. However, the wood is heavy and hard with an interlocking grain,[7] but is difficult to season.[16]

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  Small branches with edgewise plates of bark
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  Deeply ridged bark

The leaves usually have five (but sometimes three or seven) sharply pointed palmate lobes.[12] They are 3–5 inches wide on average and have three distinct bundle scars.[17]

They are long and broad, with a 6–10 cm (2 ¹⁄₄–4 in) petiole. The rich dark green, smooth, shiny, star-shaped leaves generally turn brilliant yellow, orange, red, and purple colors in the autumn.[7] This autumnal coloring has been characterized as not simply a flame, but a conflagration. Its reds and yellows compare to that of the maples (Acer), and in addition it has the dark purples and smoky browns of the ash (Fraxinus).[11] However, in the northern part of its range, and where planted in yet colder areas, the leaves are often killed by frost while still green. On the other hand, in the extreme southern or tropical parts of its range, some trees are evergreen or semi-evergreen, with negligible fall color. The leaves are three to seven inches broad with glandular serrate teeth. The base is truncate or slightly heart-shaped. They come out of the bud plicate, downy, pale green, when full grown are bright green, smooth, shining above, paler beneath.[11] They contain tannin and when bruised give a resinous fragrance.[16]

While the starry five-pointed leaves of Liquidambar resemble those of some maples (Acer), Liquidambar is easily distinguished from Acer by its glossy, leathery leaves that are positioned singly (alternate), not in pairs (opposite) on the stems. Luna and Promethea moth caterpillars feed on the leaves.[18]

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  Summer foliage
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  Autumn foliage

Flower of sweetgum

The flowers typically appear in spring and persist into autumn/fall, sometimes persisting into winter. They are typically about 1–1.5 inches (25–38 mm) in diameter and are covered with rusty hairs.[13] The flowers are unisexual[18] and greenish in color. Staminate flowers in terminal racemes two to three inches long, the pistillate in a solitary head on a slender peduncle borne in the axil of an upper leaf. Staminate flowers destitute of calyx and corolla, but are surrounded by hairy bracts. Stamens indefinite; filaments short; anthers introrse. Pistillate flowers with a two-celled, two-beaked ovary, the carpels produced into a long, recurved, persistent style. The ovaries all more or less cohere and harden in fruit. There are many ovules but few mature.[11]

The distinctive compound fruit is hard, dry, and globose,1–1.5 inches (25–38 mm) in diameter, composed of numerous (40–60) capsules.[13] Each capsule, containing one to two small seeds, has a pair of terminal spikes (for a total of 80–120 spikes). When the fruit opens and the seeds are released, each capsule is associated with a small hole (40–60 of these) in the compound fruit.

Fallen, opened fruits are often abundant beneath the trees; these have been popularly nicknamed "burr (or bir) balls",[14] "gum balls",[19] "space bugs", "sticker balls",[20] or "spike balls".

The fruit is a multicapsular spherical head and hangs on the branches during the winter. The woody capsules are mostly filled with abortive seeds resembling sawdust.[11] The seeds are about one-quarter of an inch thick, winged, and wind-dispersed. Goldfinches, purple finches, squirrels, and chipmunks eat the seeds of the tree.[18] The seeds stratify within 30–90 days at 33°–41 °F or soaked in water for 15–20 days.[17] The long-stemmed fruit balls of Liquidambar resemble those of the American sycamore or buttonwood (Platanus occidentalis), but are spiny and remain intact after their seeds are dispersed; the softer fruits of Platanus disintegrate upon seed dispersal. The long-persisting fallen spiked fruits can be unpleasant to walk on; sweet gum is banned in some places for this reason.[14] In abundance, they can leave a lawn lumpy. The winter buds are yellow brown, one-fourth of an inch long, acute. The inner scales enlarge with the growing shoot, becoming half an inch long, green tipped with red.[11]

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  Immature fruit
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  Mature fruits and seeds of sweetgum
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  American sweetgum tree balls (spiny seed pods) -- Liquidambar styraciflua
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  Leaf buds and Fruit.

Lumber

Harvesting red gum in Richland County, South Carolina, 1904

Sweetgum (Liquidambar styraciflua) is one of the most important commercial hardwoods in the Southeastern United States.[23] Its wood is bright reddish brown (with the sapwood nearly white) and may have black grain in the heartwood; it is heavy, straight, satiny, and close-grained, but not strong. It takes a beautiful polish, but warps badly in drying. The wood has a specific gravity of 0.5910. It is too liable to decay for outdoor use.[11]
In the carpentry industry, the timber is referred to as satin walnut[12] and is one of the most important materials for plywood manufacturers. It is used for furniture, interior trim, railroad ties, cigar boxes, crates, flooring, barrels, woodenware, and wood pulp.[24] It is also used for veneer for plywood. The wood is very compact and fine-grained, the heartwood being reddish, and, when cut into planks, marked transversely with blackish belts. Sweetgum is used principally for lumber, veneer, plywood, slack cooperage, fuel, and pulpwood. The lumber is made into boxes and crates, furniture, cabinets for radios, televisions, and phonographs, interior trim, and millwork. The veneer and plywood, (typically backed with some other kind of wood which shrinks and warps less) are used for boxes, pallets, crates, baskets, and interior woodwork. It was formerly used in the interior finish of railroad sleeping cars. Being readily dyed black, it is sometimes substituted for ebony for such uses as inexpensive picture frames.[11][25] The wood is also used to make chopsticks for the East Asian market, as it has a naturally light color with appeal to certain segments of the Asian market.[26]

The tree's gum resin, for which the tree is named, exudes from the bark of the tree when wounded.[18] It has many names, including liquid amber or copalm balsam. It is a kind of native balsam, or resin, resembling turpentine. It may be clear, reddish, or yellow, with a pleasant smell like ambergris. As the resin ages, it solidifies, the form in which it was historically exported in barrels. The resin is produced by stripping, boiling, and pressing the tree's bark.[14] The gum was used both medicinally and to make chewing gum.[27]

L. styraciflua seeds may be a renewable source of shikimic acid.[28]

The imperfect fungus Dicarpella dryina Sutton is a leaf parasite reported to occur on a wide range of host plants, including species of sweet gum (Liquidambar styraciflua L.). Limber and Cash reported that leaf spots produced by this pathogen on several different genera of forest trees were 2–5 mm diameter with regular margins. During the summer of 1994 in the Nacogdoches County area of Texas, a prominent leaf spot on sweet gum was widespread. Infected leaves had numerous necrotic lesions, each surrounded by a reddish halo. The lesions tended to merge resulting in large areas of dead tissue. Infection and fungal development of D. dryina were investigated on leaves of sweet gum using a combination of microscopic techniques. D. dryina infection on sweet gum has been associated with the disease red leaf spot. Results of this investigation indicate that D. dryina can penetrate leaf tissue directly, thus having the ability to initiate infection on both upper and lower leaf surfaces. In other regions of the U.S., sweet gum populations may not be as susceptible to local populations of this fungus.

Environmental stress factors may also be involved, as reports have indicated that herbicide application and chlorosis caused by iron deficiency may increase susceptibility of D. dryina. Tannins (a type of biomolecule found in trees to protect it from fire, insects, and bacteria) have been reported to occur in healthy tissue of a variety of plants including sweet gum. They may prevent pathogen invasion by inhibiting fungal enzyme activity. Although cells of healthy sweet gum tissue appear rich in tannins, these materials apparently were not effective in preventing fungal colonization by D. dryina.[34]