Tree of Heaven

Appearance

"A. altissima" is a medium-sized tree that reaches heights between 17 and 27 metres with a diameter at breast height of about 1 metre. 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 colour. They have lenticels as well as heart-shaped leaf scars with many bundle scars 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 colour, smooth, lustrous, and containing raised lenticels that become fissures with age. The ends of the branches become pendulous. All parts of the plant have a distinguishing strong odour that is often likened to peanuts, cashews, or rotting cashews.

The leaves are large, odd- or even-pinnately compound, and arranged alternately on the stem. They range in size from 30 to 90 cm 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 5 to 18 cm long and 2.5 to 5 cm 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 colour with light green veins, while the undersides are a more whitish green. The petioles are 5 to 12 mm long. The lobed bases and glands distinguish it from similar sumac species.

The flowers are small and appear in large panicles up to 50 cm in length at the end of new shoots. The individual flowers are yellowish green to reddish in colour, each with five petals and sepals. The sepals are cup-shaped, lobed and united while the petals are valvate, white and hairy towards the inside. They appear from mid-April in the south of its range to July in the north. "A. altissima" is 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 odour while flowering to attract pollinating insects. Female flowers contain ten sterile stamens with heart-shaped anthers. The pistil is made up of five free carpels, each containing a single ovule. Their styles are united and slender with star-shaped stigmas. The male flowers are similar in appearance, but they of course 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; a fruit cluster may contain hundreds of seeds. The seeds borne on the female trees are 5 mm in diameter and each is encapsulated in a samara that is 2.5 cm long and 1 cm broad, appearing July though 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. The females can produce huge amounts of seeds, normally around 30,000 per kilogram of tree, and fecundity can be estimated non-destructively through measurements of dbh.

Distribution

"A. altissima" is native to northern and central China, Taiwan and northern Korea. In Taiwan it is present as var. "takanai". In China it is native to every province except Gansu, Heilongjiang, Hainan, Jilin, Ningxia, Qinghai, Xinjiang, and Tibet.

The tree prefers moist and loamy soils, but is adaptable to a very wide range of soil conditions and pH values. It is drought-hardy, but not tolerant of flooding. It also does not tolerate deep shade. In China it is often found in limestone-rich areas. The tree of heaven is found within a wide range of climatic conditions. In its native range it is found at high altitudes in Taiwan as well as lower ones in mainland China. In the U.S. it is found in arid regions bordering the Great Plains, very wet regions in the southern Appalachians, cold areas of the lower Rocky Mountains and throughout much of the California Central Valley. Prolonged cold and snow cover cause dieback, though the trees re-sprout from the roots.

Habitat

"A. altissima" is native to northern and central China, Taiwan and northern Korea. In Taiwan it is present as var. "takanai". In China it is native to every province except Gansu, Heilongjiang, Hainan, Jilin, Ningxia, Qinghai, Xinjiang, and Tibet.

The tree prefers moist and loamy soils, but is adaptable to a very wide range of soil conditions and pH values. It is drought-hardy, but not tolerant of flooding. It also does not tolerate deep shade. In China it is often found in limestone-rich areas. The tree of heaven is found within a wide range of climatic conditions. In its native range it is found at high altitudes in Taiwan as well as lower ones in mainland China. In the U.S. it is found in arid regions bordering the Great Plains, very wet regions in the southern Appalachians, cold areas of the lower Rocky Mountains and throughout much of the California Central Valley. Prolonged cold and snow cover cause dieback, though the trees re-sprout from the roots.Ailanthus is an opportunistic plant that thrives in full sun and disturbed areas. It spreads aggressively both by seeds and vegetatively by root sprouts, re-sprouting rapidly after being cut. It is considered a shade-intolerant tree and cannot compete in low-light situations, though it is sometimes found competing with hardwoods, but such competition rather indicates it was present at the time the stand was established. On the other hand, a study in an old-growth hemlock-hardwood forest in New York found that Ailanthus was capable of competing successfully with native trees in canopy gaps where only 2 to 15% of full sun was available. The same study characterised the tree as using a "gap-obligate" strategy in order to reach the forest canopy, meaning it grows rapidly during a very short period rather than growing slowly over a long period. It is a short lived tree in any location and rarely lives more than 50 years. Ailanthus is among the most pollution-tolerant of tree species, including sulfur dioxide, which it absorbs in its leaves. It can withstand cement dust and fumes from coal tar operations, as well as resist ozone exposure relatively well. Furthermore, high concentrations of mercury have been found built up in tissues of the plant.

Ailanthus has been used to re-vegetate areas where acid mine drainage has occurred and it has been shown to tolerate pH levels as low as 4.1. It can withstand very low phosphorus levels and high salinity levels. The drought-tolerance of the tree is strong due to its ability to effectively store water in its root system. It is frequently found in areas where few trees can survive. The roots are also aggressive enough to cause damage to subterranean sewers and pipes. Along highways it often forms dense thickets in which few other tree species are present, largely due to the toxins it produces to prevent competition.

Ailanthus produces an allelopathic chemical called ailanthone, which inhibits the growth of other plants. The inhibitors are strongest in the bark and roots, but are also present in the leaves, wood and seeds of the plant. One study showed that a crude extract of the root bark inhibited 50% of a sample of garden cress seeds from germinating. The same study tested the extract as an herbicide on garden cress, redroot pigweed, velvetleaf, yellow bristlegrass, barnyard grass, pea and maize. It proved able to kill nearly 100% of seedlings with the exception of velvetleaf, which showed some resistance. Another experiment showed a water extract of the chemical was either lethal or highly damaging to 11 North American hardwoods and 34 conifers, with the white ash being the only plant not adversely affected. The chemical does not, however, affect the tree of heaven's own seedlings, indicating that "A. altissima" has a defence mechanism to prevent autotoxicity. Resistance in various plant species has been shown to increase with exposure. Populations without prior exposure to the chemicals are most susceptible to them. Seeds produced from exposed plants have also been shown to be more resistant than their unexposed counterparts.

The tree of heaven is a very rapidly growing tree, possibly the fastest growing tree in North America. Growth of one to two metres per year for the first four years is considered normal. Shade considerably hampers growth rates. Older trees, while growing much slower, still do so faster than other trees. Studies found that Californian trees grew faster than their East Coast counterparts, and American trees in general grew faster than Chinese ones.

In northern Europe the tree of heaven was not considered naturalised in cities until after the Second World War. This has been attributed to the tree's ability to colonise areas of rubble of destroyed buildings where most other plants would not grow. In addition, the warmer microclimate in cities offers a more suitable habitat than the surrounding rural areas. For example, one study in Germany found the tree of heaven growing in 92% of densely populated areas of Berlin, 25% of its suburbs and only 3% of areas outside the city altogether. In other areas of Europe this is not the case as climates are mild enough for the tree to flourish. It has colonised natural areas in Hungary, for example, and is considered a threat to biodiversity at that country's Aggtelek National Park.

Several species of Lepidoptera utilise the leaves of ailanthus as food, including the Indian moon moth and the common grass yellow. In North America the tree is the host plant for the ailanthus webworm, though this ermine moth is native to Central and South America and originally used other members of the mostly tropical Simaroubaceae as its hosts. In its native range "A. altissima" is associated with at least 32 species of arthropods and 13 species of fungi.

Due to the tree of heaven's weedy habit, landowners and other organisations often resort to various methods of control in order to keep its populations in check. For example, the city of Basel in Switzerland has an eradication program for the tree. It can be very difficult to eradicate, however. Means of eradication can be physical, thermal, managerial, biological or chemical. A combination of several of these can be most effective, though they must of course be compatible. All have some positive and negative aspects, but the most effective regimen is generally a mixture of chemical and physical control. It involves the application of foliar or basal herbicides in order to kill existing trees, while either hand pulling or mowing seedlings in order to prevent new growth.

Uses

In addition to its use as an ornamental plant, the tree of heaven is also used for its wood, medicinal properties, and as a host plant to feed silkworms of the moth "Samia cynthia", which produces silk that is stronger and cheaper than mulberry silk, although with inferior gloss and texture. It is also unable to take dye. This type of silk is known under various names: "pongee", "eri silk" and "Shantung silk", the last name being derived from Shandong Province in China where this silk is often produced. Its production is particularly well known in the Yantai region of that province. The moth has also been introduced in the United States.

The pale yellow, close-grained and satiny wood of ailanthus has been used in cabinet work. It is flexible and well suited to the manufacture of kitchen steamers, which are important in Chinese cuisine for cooking mantou, pastries and rice. Zhejiang Province in eastern China is most famous for producing these steamers. It is also considered a good source of firewood across much of its range as it moderately hard and heavy, yet readily available. The wood is also used to make charcoal for culinary purposes. There are problems with using the wood as lumber, however. Because the trees exhibit rapid growth for the first few years, the trunk has uneven texture between the inner and outer wood, which can cause the wood to twist or crack during drying. Techniques have been developed for drying the wood so as to prevent this cracking, allowing it to be commercially harvested. Although the live tree tends to have very flexible wood, the wood is quite hard once properly dried.