Ailanthus is a genus of deciduous trees in the family Simaroubaceae, placed in the order Sapindales. The genus is native to East Asia, ranging south to northern Australasia, with the number of accepted living species disputed — some authorities recognize up to ten species, while others accept six or fewer. The genus also has a rich fossil record spanning from the Paleocene through the Tertiary, with fossil species documented across Europe, Asia, and North America.
The most widely known member is Ailanthus altissima (Mill.) Swingle, commonly called the tree of heaven, native to northern and central mainland China and Taiwan. It has spread far beyond its native range and is considered invasive across North America, Europe, Britain, and Australia, where it colonizes disturbed soils, roadsides, and urban environments with exceptional vigor. Other notable members include Ailanthus excelsa Roxb. of India and Sri Lanka, Ailanthus triphysa (Dennst.) Alston (white siris) of India, South-east Asia, and Australia, Ailanthus fordii Noot. of China, and Ailanthus integrifolia Lam. of New Guinea and Queensland.
The genus name derives from "ailanto," an Ambonese word probably meaning "tree of the gods" or "tree of heaven." Ailanthus altissima became a cultural touchstone as the central metaphor in Betty Smith's novel A Tree Grows in Brooklyn, symbolizing resilience in harsh urban conditions.
Etymology
The name Ailanthus is derived from "ailanto," an Ambonese word (from the Maluku Islands of Indonesia) probably meaning "tree of the gods" or "tree of heaven." This name was applied to the genus in recognition of the impressive stature of its trees.
Distribution
The genus is native from East Asia south to northern Australasia. Ailanthus altissima, the most widespread species, originates from northern and central mainland China and Taiwan but has become broadly naturalized and invasive across North America, Europe, Britain, and Australia. Other species are distributed across India, Sri Lanka, South-east Asia, Vietnam, New Guinea, and Queensland.
Ecology
Ailanthus altissima is regarded as one of the world's most problematic invasive trees, thriving in disturbed habitats, urban environments, roadsides, and waste ground. Its rapid growth, prolific seed production, and tolerance of poor soils and pollution allow it to outcompete native vegetation. The genus as a whole is associated with secondary and disturbed habitats throughout its native Asian range.
History
Ailanthus has a long geological history, with fossil species documented from the Paleocene through the Tertiary across Europe, Asia, and North America. Ailanthus confucii Unger is known from the Tertiary of Europe, Asia, and North America, while Ailanthus pythii Unger has been recorded from Miocene deposits in Iceland, Austria, and Greece, indicating the genus was once far more widely distributed. Ailanthus altissima was introduced to the West in the eighteenth century and gained fame as the resilient urban tree celebrated in Betty Smith's 1943 novel A Tree Grows in Brooklyn.
