Ailanthus altissima ( ), commonly known as tree of heaven or ailanthus tree, is a deciduous tree in the quassia family. It is native to northeast, central China, and Taiwan. Unlike other members of the genus Ailanthus, it is found in temperate climates rather than the tropics.
The tree grows rapidly, and is capable of reaching heights of in 25 years. While the species rarely lives more than 50 years, some specimens exceed 100 years of age. It is considered a noxious weed and vigorous invasive species, and one of the worst invasive plant species in Europe and North America. In 21st-century North America, the invasiveness of the species has been compounded by its role in the life cycle of the also destructive and invasive spotted lanternfly.
Description
Ailanthus altissima is a medium-sized tree that reaches heights between with a diameter at breast height of about . The bark is smooth and light grey, often becoming somewhat rougher with light tan fissures as the tree ages. The twigs are stout, smooth to lightly pubescent, and reddish or chestnut in color. They have lenticels and heart-shaped leaf scars (i.e., a scar left on the twig after a leaf falls) with many bundle scars (i.e., small marks where the veins of the leaf once connected to the tree) around the edges.
The buds are finely pubescent, dome-shaped, and partially hidden behind the petiole, though they are completely visible in the dormant season at the sinuses of the leaf scars. The branches are light to dark gray in color, smooth, lustrous, and contain raised lenticels that become fissures with age. The ends of the branches become pendulous. All parts of the plant have a distinguishing strong odor that is often likened to peanuts, cashews, or rotting cashews.
The leaves are large, odd- or even-pinnately compound on the stem. They range in size from in length and contain 10–41 leaflets organised in pairs, with the largest leaves found on vigorous young sprouts. When they emerge in the spring, the leaves are bronze, then quickly turn from medium to dark green as they grow. The rachis is light to reddish-green with a swollen base. The leaflets are ovate-lanceolate with entire margins, somewhat asymmetric and occasionally not directly opposite to each other. Each leaflet is long and wide. They have a long, tapering end, while the bases have two to four teeth, each containing one or more glands at the tip. The leaflets' upper sides are dark green in color with light green veins, while the undersides are a more whitish green. The petioles are long. The lobed bases and glands distinguish it from similar sumac species.
The flowers are small and appear in large panicles up to in length at the end of new shoots. The individual flowers are yellowish green to reddish in color, each with five petals and sepals. The sepals are cup-shaped, lobed and united while the petals are valvate (i.e., they meet at the edges without overlapping), white and hairy towards the inside. They appear from mid-April in the south of its range to July in the north. Ailanthus trees are dioecious, with male and female flowers being borne on different individuals. Male trees produce three to four times as many flowers as the females, making the male flowers more conspicuous. Furthermore, the male plants emit a foul-smelling odor while flowering to attract pollinating insects. Female flowers contain 10 (or rarely five through abortion) sterile stamens (stamenoids) with heart-shaped anthers. The pistil is made up of five free carpels (i.e., they are not fused), each containing a single ovule. Their styles are united and slender with star-shaped stigmata. The male flowers are similar in appearance, but they lack a pistil and the stamens do function, each being topped with a globular anther and a glandular green disc. The fruits grow in clusters; similar to the ash tree (Fraxinus excelsior), the fruits ripen to a bright reddish-brown color in September. A fruit cluster may contain hundreds of seeds.
The seeds borne on the female trees are in diameter and each is encapsulated in a samara that is long and broad, appearing July through August, but can persist on the tree until the next spring. The samara is large and twisted at the tips, making it spin as it falls, assisting wind dispersal, and aiding buoyancy for long-distance dispersal through hydrochory. Primary wind dispersal and secondary water dispersal are usually positively correlated in A. altissima, since most morphological characteristics of samaras affect both dispersal modes in the same way – except for the width of the samaras, which in contrast affects both types of dispersal in opposing ways, allowing differentiation in the dispersal strategies of this tree. The females can produce huge numbers of seeds, normally around 30,000 per kg, and their number can be estimated by measurement of the tree's diameter at chest height.
History The tree of heaven is mentioned in the oldest extant Chinese dictionary and listed in many Chinese medical texts for its purported curative ability. The roots, leaves, and bark are used in traditional Chinese medicine, primarily as an astringent. The tree has been grown extensively both in China and abroad as a host plant for the ailanthus silkmoth, a moth involved in silk production. Ailanthus has become a part of Western culture, as well, with the tree serving as the central metaphor and subject matter of the best-selling American novel A Tree Grows in Brooklyn by Betty Smith.
The tree was first brought from China to Europe in the 1740s, and to the United States in 1784. It was one of the first trees brought to the West during a time when chinoiserie was dominating European arts, and was initially hailed as a beautiful garden specimen. However, enthusiasm soon waned after gardeners became familiar with its suckering habits and its foul odor. Despite this, it was used extensively as a street tree during much of the 19th century.
Outside Europe and the United States, the plant has been spread to many other areas beyond its native range, and is regarded internationally as a noxious weed. In many countries, it is an invasive species due to its ability both to colonise disturbed areas quickly and to suppress competition with allelopathic chemicals. The tree also resprouts vigorously when cut, making its eradication difficult and time-consuming. This has led to its being called "tree of hell" among gardeners and conservationists.
Taxonomy The first Western scientific descriptions of the tree of heaven were made shortly after it was introduced to Europe by Pierre Nicolas d'Incarville, a French Jesuit, who had sent seeds from Peking via Siberia to his botanist friend Bernard de Jussieu in the 1740s. The seeds sent by d'Incarville were thought to be from the economically important and similar-looking Chinese varnish tree (Toxicodendron vernicifluum), which he had observed in the lower Yangtze region, rather than the tree of heaven. D'Incarville attached a note indicating this, which caused much taxonomic confusion over the next few decades. In 1751, Jussieu planted a few seeds in France and sent others on to Philip Miller, the superintendent at the Chelsea Physic Garden, and to Philip C. Webb, the owner of an exotic plant garden in Busbridge, England.
Confusion in naming began when the tree was described by all three men with three different names. In Paris, Linnaeus gave the plant the name Rhus succedanea, while it was known commonly as grand vernis du Japon. In London, the specimens were named by Miller as Toxicodendron altissima, and in Busbridge, it was dubbed in the old classification system as Rhus Sinese foliis alatis. Records exist from the 1750s of disputes over the proper name between Philip Miller and John Ellis, curator of Webb's garden in Busbridge. Rather than the issue being resolved, more names soon appeared for the plant: Jakob Friedrich Ehrhart observed a specimen in Utrecht in 1782 and named it Rhus cacodendron.
Light was shed on the taxonomic status of Ailanthus in 1788 when René Louiche Desfontaines observed the samaras of the Paris specimens, which were still labelled Rhus succedanea, and came to the conclusion that the plant was not a sumac. He published an article with an illustrated description and gave it the name Ailanthus glandulosa, placing it in the same genus as the tropical species then known as A. integrifolia (white siris, A. triphysa). The specific glandulosa, referring to the glands on the leaves, persisted until as late as 1957, but it was ultimately made invalid as a later homonym at the species level. The current species name comes from Walter T. Swingle, who was employed by the United States Department of Plant Industry. He decided to transfer Miller's older specific name into the genus of Desfontaines, resulting in the accepted name Ailanthus altissima.
Ailanthus altissima is classified in Ailanthus alongside five other east Asian and Australian species in the family Simaroubaceae.
Varieties The three varieties of A. altissima are:
Ailanthus altissima var. altissima The autonymic variety which is native to mainland China.
Ailanthus altissima var. sutchuenensis This variety is native to southeast and south central China. It differs from the other varieties in having red branchlets.
Ailanthus altissima var. tanakae Variety tanakae is endemic to northern Taiwan. It differs from the type in having yellowish bark, odd-pinnate leaves that are also shorter on average at long with only 13–25 scythe-like leaflets. It is listed as endangered in the IUCN Red List of threatened species due to loss of habitat for building and industrial plantations.
Synonyms Ailanthus altissima has synonyms of the species or one of its varieties.