There are currently no active references in this article.
Liquidambar comprises four species with a disjunct distribution, in North America (L. styraciflua L.) and eastern and southwestern Asia (L. acalycina, L. formosana and L. orientalis). Sweet gums are medium to large deciduous trees with a conical or spreading crown. The bark is thick and the branches often have distinctive corky wings. The leaves are palmate with three lobes (Chinese species) or five (to seven) lobes (American species and L. orientalis), and can produce a spectacular display of autumn colour. They are alternate and have serrated margins. Stipules are typically present and may be partially fused to the petiole; they are deciduous and leave a distinctive scar. Liquidambar is monoecious, with unisexual flowers on separate inflorescences. The individual flowers are rather inconspicuous, as petals and sepals are absent. The male inflorescence is a spike or a globose head, and several are gathered together in a raceme. Each inflorescence is subtended by a pubescent, foliose bract. The male flowers have multiple stamens, which conceal the individual flowers. Female inflorescences are globose, pedunculate and subtended by a bract. The female flowers have a curved or hooked style, but are otherwise inconspicuous. The fruit is a woody capsule with two valves and is surrounded by scales formed from persistent staminodes. The style is persistent and conspicuous in the fruit. Seeds are produced in abundance, though many are sterile. They have a membranous wing (Zhang et al. 2003b).
Liquidambar is often considered a member of the Hamamelidaceae, as in the Flora of China treatment (Zhang et al. 2003b). Within that family it was placed in subfamily Altingioideae, together with Altingia Noronha and Semiliquidambar H.T. Chang. The recognition of the Altingiaceae as a separate family was suggested as early as 1828, and its independent status has since been confirmed by DNA-based studies (APG 2003). Within the Altingiaceae, generic delimitation is problematic. Liquidambar and Altingia are easily separated, but species with intermediate morphology were placed in the genus Semiliquidambar. The work of Shi et al. (2001), however, using DNA data, has shown that none of the three genera can be maintained in their present formats, and suggests that all species should be merged into Liquidambar.
The horticultural merits and requirements of the genus Liquidambar have recently been reviewed comprehensively by Eric Hsu and Susyn Andrews (2005). Dirr (1998) provides further information on the cultivars and cultural requirements of L. styraciflua, which is by far the most important species with its superb autumn colour and range of cultivars, even if its prickly fruit clusters are not always welcome to the tidy-minded. Although it is still sometimes seen thus-labelled, the Mexican and Central American taxon L. macrophylla Oerst. is now regarded as synonymous with L. styraciflua, forming a disjunct population in the Central American mountains south to Nicaragua, like many such principally North American species. In cultivation it leafs out earlier and retains its leaves longer, and is slightly less hardy than trees from more northerly provenances (Hsu & Andrews 2005). From a strictly botanical viewpoint, the Chinese L. formosana var. monticola Rehder & E.H. Wilson should be regarded as indistinguishable from and synonymous with L. formosana (originally described from Taiwan), as their characters overlap. To distinguish the two provenances and indicate (for garden use) the possibly greater hardiness of the mainland material, these trees should be labelled L. formosana Monticola Group (Hsu & Andrews 2005). In general Liquidambar species appreciate a hot summer, ample soil moisture and fertility, and preferably acidic or neutral conditions.
A small genus of trees with a remarkably scattered distribution in nature; one species being found in Asia Minor, one in eastern N. America, and one or more in China and Formosa. In general appearance they bear most resemblance to the maples (Acer), but are easily distinguished by their alternate, not opposite leaves. The flowers have no beauty, being greenish or yellowish, and borne in small globose heads. Male and female flowers are in separate heads, the male flower-heads in short racemes; the female heads solitary. The male flowers consist of stamens only, the females of calyx and carpel only. The best known and most useful of liquidambars is L. styraciflua, which, like the rest, should, if possible, be raised from imported seeds. These frequently do not germinate until the second year. Failing them, layering must be resorted to. Young plants are apt to be injured by late spring frosts.
This genus is placed by some authorities in the family Altingiaceae, which is intermediate between the Hamamelidaceae and Platanaceae. The only other member of this family is the tropical and subtropical genus Altingia of south-east Asia.