Genus Ailanthus in Family Simaroubaceae

Genus Description

Ailanthus (family Simaroubaceae) is a small genus of roughly five species of dioecious trees and shrubs, native across temperate to tropical Asia and northern Australia, with broad distributions in secondary forests, roadsides and open sites, and from low elevations to montane zones. Ailanthus altissima is commonly treated as the type species and is the most widespread (POWO, 2024; WFO, 2024). The genus is readily recognized by large, alternate, imparipinnate leaves with 9–27 (–35) leaflets, each leaflet bearing conspicuous marginal glands or “pearl” glands at the lobe teeth, giving the margin a distinctly pitted appearance; young shoots are often red or pubescent and exude a bitter, sometimes foetid sap when cut. Inflorescences are large, terminal or axillary panicles bearing numerous small, apetalous to rudimentary-petaled flowers; male flowers have 5–10 exserted stamens and a reduced or absent ovary, while female flowers have a superior, usually 2–5-carpellary ovary with 2–5 distinct styles and a single ovule per carpel. Fruits are flattened, winged samaras arranged in dense clusters, each seed filling only the central cavity of the wing.

Diversity and range are concentrated in China, with additional taxa in the Himalayas, Indochina and northern Australia; A. altissima is native from China and Taiwan through Korea to Japan and Vietnam, and A. triphysa occurs from India through Southeast Asia to northern Australia. The center of diversity lies in southeastern and southwestern China, with several narrowly endemic species (Zhang et al., 2007). Typical habitats include disturbed forests, forest margins and secondary woodlands, and the group tolerates a range of soils, including urban sites.

Pollination is by wind in females and by small insects (notably thrips) in males, a rare dimorphism within Sapindales (Thompson et al., 2004). Dispersal is anemochorous by the samaras, but vegetative spread by root suckers and stump sprouts is locally dominant. Chromosome counts suggest a base number around x = 20 (Saxena, 1977). Vegetative regeneration and suckering confer high persistence in disturbed landscapes.

Taxonomically, Ailanthus has been treated consistently within Simaroubaceae in recent floristic treatments; most authors recognize about five species, although synonymization of A. giraldii with A. altissima has been proposed and remains unresolved (Zhang et al., 2007). Subtribal placement is stable within the family, but familial circumscription has been subject to broader recircumscription of Simaroubaceae in molecular systematic revisions (Clayton et al., 2007), with Ailanthus consistently resolved in Simaroubaceae.

Ailanthus altissima is widely cultivated as an urban ornamental and roadside tree due to fast growth, drought tolerance and soil stabilization, yet it is a significant invasive weed in North America, Europe and parts of South America, forming monospecific stands and suppressing native vegetation (Kowarik, 1996). Its wood has limited timber use; A. triphysa is a local source of lightweight timber. Ailanthus is not a major crop genus.

Conservation status and outlook: most taxa are widespread and common, although invasive A. altissima exerts ecological impacts beyond its native range. Research gaps include resolution of species boundaries, demographic pathways of invasion and targeted management in invaded landscapes.

• Ailanthus altissima ((Mill.) Swingle)
• Ailanthus excelsus (Roxb.)
• Ailanthus fordii (Noot.)
• Ailanthus integrifolia (Lam.)
• Ailanthus triphysa ((Dennst.) Alston)
• Ailanthus vietnamensis (H.V.Sam & Noot.)