Tree-of-heaven (Ailanthus altissima), also known ailanthus, Chinese sumac, and stinking shumac, is a deciduous tree in the mostly tropical Simaroubaceae family. Mature trees can reach 80 feet in height. Ailanthus has smooth stems with pale gray bark and twigs which are light chestnut brown, especially in the dormant season. Its large compound leaves are 1-4 feet in length, alternate, and composed of 10-41 smaller leaflets. Each leaflet has one or more glandular teeth along the lower margin. The leaf margins are otherwise entire or lacking teeth. Ailanthus is a dioecious (“two houses”) plant meaning that male and female flowers occur on separate plants. Flowers occur in large terminal clusters and are small and pale yellow to greenish. Flat, twisted, winged fruits each containing a single central seed are produced on female trees in late summer to early fall and may remain on the trees for long periods of time. The wood of ailanthus is soft, weak, coarse-grained, and creamy white to light brown in color. All parts of the tree, especially the leaves and flowers, have a nutty or burned nut odor.
Tree-of-heaven is a fast-growing tree and a prolific seeder that can take over sites, replacing native plants and forming dense thickets. Ailanthus also produces chemicals that prevent the establishment of other plant species nearby. Its root system may be extensive and has been known to cause damage to sewers and foundations.
Tree-of-heaven is a common tree in disturbed urban areas, where it sprouts up just about anywhere, including alleys, sidewalks, parking lots, and streets. For example, the book “A Tree Grows in Brooklyn,” by Betty Smith, is based on the tree-of-heaven. Away from cities, ailanthus is commonly seen in fields, and along roadsides, fencerows, woodland edges and forest openings. It occurs as seedlings that pop up by the hundreds in recently planted fields and as persistent thickets in rocky, untillable areas. In the United States, ailanthus is recognized to be a serious agricultural pest.
NOTE: It is important not to confuse native shrubs and trees with ailanthus. Native sumacs (Rhus) and trees like ash (Fraxinus), hickory (Carya), black walnut, butternut and pecan (Juglans) can be distinguished from tree-of-heaven by having completely serrated (toothed) leaf margins.
Tree-of-heaven is mostly dioecious. Rarely, either bisexual trees or trees with both bisexual and unisexual flowers are found [71,104,148,211,328].
As with most species with wind-dispersed seed, tree-of-heaven appears to have a relatively uniform genetic system, with most diversity occurring among rather than within populations [94,249]. Because most North American tree-of-heaven populations originated from 3 introductions [71,139,288,312] (see General Distribution), they may be less genetically diverse than native Asian populations. A comparison of tree-of-heaven seedlings germinated from seed collections from 5 locations across the United States and 5 locations across China showed significant differences in height growth (P=0.01), root:shoot ratios (P=0.05), and leaf area (P=0.05) among seedlings from the United States and Chinese seedlings. Populations from the United States were taller, allocated relatively less biomass to roots than stems, and had greater leaf areas than Chinese populations [94]. In a common garden study comparing seedlings of populations within the United States, Feret and others [95] found California populations were significantly taller than eastern populations (P=0.01). Seed width and biomass were correlated with latitude (t=0.96), with northern populations having the widest, heaviest seeds. Feret [93] found some tree-of-heaven seed and seedling growth characteristics of provenances from the eastern United States and California appeared random, and they were not correlated (nor appeared best adapted) to site or geographic location. He reported significant differences between North American and Chinese tree-of-heaven provenances for seed and seedling characteristics. Contrary to expectations, there was no evidence of inbreeding depression in North American provenances compared to native Chinese provenances [93].
Botanical description: The following description of tree-of-heaven provides characteristics that may be relevant to fire ecology and is not meant for identification. Keys for identification are available (for example, [71,104,105,135,199,317,324]). Billings and others [28] provide a key for identifying tree-of-heaven and other eastern trees in winter.
Tree-of-heaven is deciduous. It may reach 60 to 70 feet (18-21 m) in height, 80 feet (24 m) in crown width, and 20 feet (6 m) in trunk diameter at maturity [8,71,105,308]. The champion tree as of 2010 was in Virginia; it reached 55 feet (17 m) in height, 48 feet (15 m) in spread, and 20 feet in diameter [8]. Tree-of-heaven may be shrubby when suppressed beneath the canopy or pruned regularly [104]. It has smooth, thin bark and a straight bole. Branches are brittle and self-pruning [71,104,105,308]. There are 2 branch types: long and short shoots. Long shoots are sterile and may extend 18 feet (5 m), while short shoots bear flowers and rarely reach more than 18 inches (46 cm) long [63]. The large, malodorous leaves are pinnately compound, with prominent glands on the back of each leaflet. Leaflets range from 15 to 41 in number, and total leaf length may reach 3 feet (1 m) [71,104,105,308]. Leaf stipules have nectaries that excrete sugars [30].
Most flowers are unisexual, but some trees may have perfect flowers [328]. The inflorescence is a 4- to 7-inch (10-20 cm) long panicle with 6- to 8-mm long flowers. Staminate flowers have a strong odor that is objectionable to humans. Fruits are one-seeded, dry schizocarps with wings. They are 1 to 2 inches (2.5-5 cm) long and propeller-shaped, resembling maple (Acer spp.) fruits [71,104,135,136,147,149,250,317]. The fruits grow in clusters; a fruit cluster may contain hundreds of seeds [163]. Seeds average 0.2 Ã 1.0 inch (0.6 Ã 0.25 cm) in area [9] and 27 mg in mass [201].
Roots are shallow and wide-spread [211]. Young trees have a taproot and several large lateral roots [230,248,249], although the taproot may diminish with age [211]. Ramets do not have taproots. In dry, rocky soil or beneath pavement, tree-of-heaven grows long, horizontal roots that do not branch until reaching a more favorable substrate [230,248,249]. Roots near the trunk thicken with age, serving as storage organs. Deep roots send out smaller roots that grow near the soil surface; stem shoots generally sprout from these shallow roots. Most roots occur in the upper 18 inches (46 cm) of soil [211].
Stand structure: Tree-of-heaven typically occurs in clumps, although it may form rows along streams, roads, and fences, and occasionally it grows as widely spaced, single stems. Clumping can result from an even-aged seedling establishment or from clonal expansion through root sprouting [249]. Open-grown colonies may eventually become dense by sprouting. Davis [61] observed a half-acre (0.2 ha) stand in Kentucky that had 32 stems. Stands subject to infrequent control measures may develop into even-aged thickets [77,151]. Untreated stands self-thin, so the stand tends to become even-aged over time. Two years after tree-of-heaven harvest in Pennsylvania, density of tree-of-heaven sprouts averaged 17,860 two-year-old sprouts/acre (mean height=9 feet (3 m)), with 10,019 one-year-old sprouts/acre (mean height=2 feet (0.6 m)). After 3 years many of the sprouts had died, so dead stems were more common than live stems [151]. Sprouts volunteering in closed-canopy understories remain suppressed and few in number. For example, Hunter [148] reported scattered, single-stemmed trees-of-heaven—rather than thickets—in the understory of a mixed-evergreen forest in northern California. On the Jefferson National Forest, Virginia, tree-of-heaven had a clumped distribution on low-leave shelterwood sites and a random distribution on clearcuts and high-leave shelterwood sites [47].
Figure 3—Over- and understory trees-of-heaven. Photo © John M. Randall, The Nature Conservancy.There are few clues as to tree-of-heaven's original growth habit in China, where it is native. As valued ornamentals, mature trees-of-heaven in China are often pruned to aesthetically pleasing, single-stemmed forms, with sprouts harvested for firewood and medicinal uses [140,249].
Life span: Tree-of-heaven is typically short-lived, with life spans ranging from 30 to 70 years [85,163,211]. Cloning from root sprouts can extend ramet life hundreds of years [148]. Sprouts from the first tree-of-heaven in North America, planted in Philadelphia's Bartram Botanical Garden 1784, still existed at the turn of the 21st century [45].
Physiology: Tree-of-heaven has several physiological adaptations that probably aid in its establishment and spread. It appears to be allelopathic [64,184,190,206,209,217]. Chemical extracts from the leaves, bark, roots, and seeds have inhibited germination and growth of other plant species in the laboratory [129,130,131,184]. Allelopathic chemicals (ailanthone and other compounds identified in these sources: [9,10,64,206]) are concentrated in roots and in young trees, with young ramets producing more toxins than older trees [129,184].
Open-grown trees-of-heaven are highly efficient at photosynthesis [30,33,34,122,194,207] and store large quantities of photosynthate in stems and roots [30,33,34,194,207]. Foliar nectaries excrete photosynthates during growth and flower initiation [31]. A study in a Mediterranean region of Spain found that even after 5 years of once- or twice-yearly cuttings, leaves from new tree-of-heaven sprouts showed higher rates of stomatal conductance in spring than leaves of uncut trees-of-heaven. The authors conjectured that regulating stomatal conductance helps sprouting trees-of-heaven grow quickly [54].
Tree-of-heaven is drought tolerant [87]. In Nava-Constan and others' [54] experiment in Spain, trees-of-heaven showed more positive leaf water potentials than native flowering ashes (Fraxinus ornus). Conflicting information states tree-of-heaven is intolerant [108] or tolerant [18,87] of flooding. Further observations and field studies are needed to resolve this conflict.
Tree-of-heaven is highly tolerant of most industrial pollutants [207,253], although it is sensitive to ozone pollution [111]. In a highly polluted area of Armenia, tree-of-heaven showed the least damage and best growth of 8 urban tree species (Derojan 1958 cited in [93]).
Tree-of-heaven is native to Taiwan and central China, where it occurs from 22o to 34o N in latitude [304]. It is nonnative in North America, where it is distributed from British Columbia, southern Ontario and Quebec, and Maine south to Florida, Texas, southern California, and Mexico [105,158,208,297,328]. Tree-of-heaven spread in North America apparently followed 3 introductions from China. It was 1st imported to Pennsylvania in 1784 as an ornamental [71,93,139,287,288,312]. A 2nd introduction occurred in New York in 1820, where tree-of-heaven was again planted as an ornamental [62]. Both eastern introductions were from English stock imported from China. Tree-of-heaven was commercially available in eastern nurseries by 1840. The 3rd introduction was in California during the Gold Rush of the mid-1800s. Chinese immigrating to work in the gold fields and in construction of the transcontinental railroad brought tree-of-heaven to California, probably because of the tree's medicinal and cultural importance in their homeland (see Other Uses) [139,288]. Tree-of-heaven was first planted in Hawaii in 1924 [321]. Plants database provides a map of tree-of-heaven's distribution in the United States.
A century after the North American introductions, tree-of-heaven is still most common in its initial centers of distribution: the Northeast and California. In the eastern United States, it is most invasive from New England south to the mid-Atlantic states [93,233]. Tree-of-heaven is frequently found in the upper Midwest. It is weakly invasive in the middle and southern Great Plains [114,258]. It is uncommon south of North Carolina in the Southeast and in the South [82,233,324], but it is spreading in the South. By a 2008 estimate, tree-of-heaven was present in over 214,000 acres (86,600 ha) of southern forests [208]. In the West, tree-of-heaven is common throughout much of California and is locally common in Oregon and Washington [93,137]. In California, it is invasive in the Bay Area, the Central Valley, and in foothill counties with a history of gold mining (figure 2) [139,153]. It grows along waterways in the Pacific Northwest, including banks of the Snake and Columbia rivers [137]. In the Southwest it invades riparian zones and mesic canyons [287].
Figure 2—Tree-of-heaven (just to left of fence) in Coumbia State Park, CA. USDA, Forest Service image by Janet Fryer.Tree-of-heaven has established in temperate climates throughout the world. Its earliest introductions may have been in Japan and Korea, where it is probably not native [191]. It was introduced in Europe in the 1700s and has become widespread there [86,163,288]. Using seed from European trees, introductions in Argentina, Australia, and Africa followed [139,140]. Kowarik [172] views human settlements as centers of distribution for tree-of-heaven, with roads providing the migration routes.
Due to its ability to sprout from the roots, fire is not recommended for tree-of-heaven control [147,163,275]. Limited fire studies [11,20,39,150,196,247] and anecdotal information [185] suggest that fire may increase tree-of-heaven density. Tree-of-heaven is capable of rapid growth even under adverse conditions, and it grows best with increased nutrient and open canopy conditions [14,34,124,163,166,173,228], which fire can create. Pomp [247] recommends removing tree-of-heaven from the understory prior to prescribed burning or logging.
Whenever there is a nearby tree-of-heaven seed source, disturbed sites require monitoring and follow-up treatments to prevent tree-of-heaven invasion and spread. Roadways, skid trails, and other disturbed grounds are corridors for tree-of-heaven invasion [40,46]. By disturbing soil, fire suppression efforts may lead to postfire establishment if there are trees-of-heaven nearby. Fire-fighting machinery can also disperse tree-of-heaven seed. Washing equipment before it enters fire suppression zones can help prevent tree-of-heaven invasion on burned sites. Postfire monitoring and treatment of skid trails is also advised [232].
Preventing invasive plants from establishing in weed-free burned areas is the most effective and least costly management method. This may be accomplished through early detection and eradication, careful monitoring and follow-up, and limiting dispersal of invasive plant propagules into burned areas. General recommendations for preventing postfire establishment and spread of invasive plants include:The fire ecology of tree-of-heaven in its native China was not available in English-language literature as of this writing (2010). Tree-of-heaven is common in long-settled, densely populated regions of China. Although its pharmacological use is mentioned in early Chinese writings, Hu [140] reported that Chinese-language references to its historical ecology were lacking as of 1979. Tree-of-heaven's ability to sprout from photosynthate-storing roots and establish from off-site, wind-dispersed seed; its extraordinarily rapid growth rate; early age of seed production; and its appearance in early successional plant communities in North America (see Botanical and Ecological Characteristics) all suggest that tree-of-heaven has been subject to evolutionary pressures of frequent, stand-replacement disturbances. Whatever FIRE REGIMES tree-of-heaven's native ecosystems were subject to in the past, tree-of-heaven's ability to establish on disturbed sites and persist into late succession suggests that it is well-adapted to survive under a wide range of FIRE REGIMES. See the Fire Regime Table for information on FIRE REGIMES of North American plant communities in which tree-of-heaven may occur. Find further fire regime information for the plant communities in which this species may occur by entering the species name in the FEIS home page under "Find FIRE REGIMES".
In North America, tree-of-heaven has invaded ecosystems where, for the most part, historic FIRE REGIMES are no longer functional (for example, see [12,239,313,322]). Dodge [76] speculates that fire exclusion in rare sugar maple-sweet birch communities of High Mountain Park Preserve, New Jersey, may have fostered establishment of tree-of-heaven. However, only a few fire studies have been conducted on tree-of-heaven to date (2010), so it is impossible to assess how—or whether—tree-of-heaven has altered historic FIRE REGIMES in North America.
Tree-of-heaven sprouts after top-kill by fire [103,274,299]. It sprouts from the roots, root crown, and/or bole after other aboveground damage (see Vegetative regeneration); so any of these sprouting strategies may occur after top-kill by fire, with root sprouting most likely. Anecdotal accounts suggest that tree-of-heaven is "able to sprout vigorously when...burnt" [185] and persists in some areas "despite...burning" [309]. The fire studies below provide other evidence of tree-of-heaven's ability to survive fire. It is likely—given its large, extended root system [211,230,248,249] and its ability to root sprout and show increased growth rates under fertilization [124]—that tree-of-heaven can flourish in postfire environments. As one of the fastest-growing trees in North America [166,244,259], tree-of-heaven may outcompete native woody species for the open spaces and flush of nutrients that often occur after fire. Further studies on the fire ecology of tree-of-heaven are needed.
Tree-of-heaven can also establish from seed after fire. Although this was documented in only 1 study as of 2010 [196], the potential for postfire tree-of-heaven seedling establishment seems high. Its seed disperses easily by wind [82,163,275], and tree-of-heaven is known to establish from seed in early-successional, disturbed environments other than burns [34,163,166,228] (see Successional Status).
Fire studies: Tree-of-heaven sprouted after the Chavez and Rio Grande Complex wildfires in Rio Grande cottonwood (Populus deltoides subsp. wislizeni) communities along the Middle Rio Grande, New Mexico [299]. It also sprouted after top-kill following 2 separate automobile-ignited fires at the same location in San Diego County, California. It was the only green vegetation in the burned area in postfire year 1 [103]. In mixed oak-pine forests of the Cooper State Forest and Wildlife Management Area, Tennessee, tree-of-heaven grew on sites burned under prescription [20].
Studies on the George Washington National Forest, West Virginia [196,247], and Tar Hollow State Forest, Ohio [11,39,150] show that prescribed burning or thinning and burning may increase tree-of-heaven abundance over prefire or pretreatment levels.
One year after late March prescribed fires on the George Washington National Forest, tree-of-heaven increased on 2 of 3 study sites compared to prefire abundance; these increases were not considered significant except on the upper portion of 1 site, where tree-of-heaven increased exponentially. All postfire tree-of-heaven regeneration was from seedling establishment. The community was a Table Mountain pine-pitch pine-chestnut oak-scarlet oak/mountain-laurel (Pinus pungens-P. rigida-Quercus prinus-Q. coccinea/Kalmia latifolia) forest [196] where fire had been excluded for 81 years. Elevation ranged from 1,880 to 2,782 feet (573-848 m) [247]. Plots on the 3 study sites (Heavener Mountain, Dunkle Knob, and Brushy Knob) were distinguished by aspect and elevation (above or below 2,400 feet (732 m)) [196]. Fire management objectives were to reduce fuel loads, reduce understory shrubs and trees, and increase wildlife forage in groundlayer vegetation. Fire conditions were [196]:
Fire weather conditions for prescribed March fires in a pine-oak forest on the George Washington National Forest [196] Heavener Mountain Dunkle Knob Date 25 March 2009 29 March 2009 Air temperature 18-27 °C 11-21 °C Relative humidity 32-50% 29-76% Wind direction mostly SW; SE in late afternoon NW and SE Windspeed 1.6-9 km/h 2-10 km/hTree-of-heaven was not present on Brushy Knob sites either before fire or in postfire year 1. It increased significantly in density (P=0.032) and importance value (P=0.007) on the upper-northeast section of Heavener Mountain compared to adjacent unburned areas. In postfire year 1, tree-of-heaven seedlings also established on unburned portions of Heavener Mountain and on burned, southwest aspects of Heavener Mountain and on Dunkle Knob. Seedling establishment in the upper-northeast section of Dunkle Knob was about 4.5 times greater than that of the other sites combined [196].
Tree-of-heaven abundance before and after the prescribed March fires [196] Site Density (stems/ha) Importance value* Basal area (m²/ha) prefire postfire year 1 prefire postfire year 1 prefire postfire year 1 Heavener Mountain, lower-northeast section, seedlings 0.00 0.00 0.00 0.00 Heavener Mountain, upper-northeast section, seedlings 0.00 277.78 0.00 0.93 Dunkle Knob, lower-southwest section, seedlings 0.00 277.78 0.00 0.56 Dunkle Knob, upper-southwest section, seedlings 0.00 833.33 0.00 2.04 Dunkle Knob, upper-southwest section, overstory 2.22 2.22 0.56 0.56 0.09 0.09 Dunkle Knob, upper-northeast section, seedlings 277.78 6,388.89** 0.23 14.78** *Importance value=(relative density + relative basal area)/2Since tree-of-heaven seedlings occurred on both burned and unburned areas of Heavener Mountain, Marsh [196] suggested that tree-of-heaven increases on burned sites were not due to fire alone. Increases were not correlated with aspect or elevation, so those factors apparently did not affect tree-of-heaven establishment. Tree-of-heaven was the only nonnative tree species on the site. Overall, the prescribed fires did not significantly increase abundance of nonnative invasive shrubs or herbs. Fuel loads were significantly less in postfire year 1 compared to prefire loads (P=0.0052). Shrub density was reduced 19% to 21%. Forbs and ferns were more abundant after than before fire on sites where fire behavior was "most severe" [196].
In a follow-up study on Dunkle Knob, Pomp [247] found that on most sites, tree-of heaven had returned to prefire levels by postfire year 3. Tree-of-heaven seedlings were producing root sprouts, suggesting that tree-of-heaven density would soon increase beyond prefire levels. Dunkle Knob is an "extremely dry" site. Tree-of-heaven abundance was positively correlated with either moist areas where fire severity was low, areas with a history of logging, or with an open canopy. Outside of riparian zones, tree-of-heaven abundance in postfire year 3 was not significantly associated with fire severity. In contrast to Marsh's [196] earlier findings, Pomp [247] found tree-of-heaven abundance was positively correlated with increasing elevation (R²=0.37).
In a mixed-oak forest in Tar Hollow State Forest, thin-and-burn treatments apparently promoted tree-of-heaven compared to thin-only and burn-only treatments, although the results could be due to pretreatment vegetation composition. Plots were thinned in fall and winter to a density of 19.5 m²/ha; burning was conducted the following March and April [39]. All plots were in white-tailed deer exclosures [11,39]. The following summer, tree-of-heaven was the 4th most common species among 26 woody plants in thin-and-burn treatments [11]. It did not occur on thin-only or burn-only plots [39]. By postfire year 3, tree-of-heaven was "widely distributed" on thin-and-burn plots, with mean density of 17.1 stems/100 m² compared to 5.8 stems/100 m² in thin-only plots and 0.6 stem/100 m² in burn-only plots. Seedling establishment was the assumed method of postfire establishment due to the scarcity of trees-of-heaven before fire [150]. Hutchinson and others [150] concluded that tree-of-heaven's high posttreatment occurrence on thin-and-burn plots was due to its higher pretreatment density on thin-and-burn units (18 trees/100 m²) compared to thin-only (8 trees/100 m²) and burn-only (1 tree/100 m²) units.
Few quantitative measurements of tree-of-heaven litter, aboveground biomass, or leaf area indices were available as of 2010, so the ability of tree-of-heaven to alter fuel loads of native ecosystems is unclear. Fuel studies are needed on tree-of-heaven.
Although tree-of-heaven has large, finely divided leaves [71,104,105,250,308], its leaves may not contribute more to total leaf litter load than the leaves of many associated tree species in deciduous forests. In Connecticut, mean depth of tree-of-heaven litter alone was similar to litter depth averages under most associated tree species in the mixed-deciduous forest (about 0.8 inch (2 cm)). Oaks contributed most biomass to the litter layer, which was sampled in July and August [167]. Tree-of-heaven litter may decay more rapidly than litter of many associated deciduous trees. In Spain, tree-of-heaven litter decayed faster than native English elm (Ulmus minor) (P=0.01) [50]. A study in Maryland found that in first-order streams, litter of native trees contained more lignin and decayed more than twice as slowly as tree-of-heaven litter (P<0.0001) [284].
Tree-of-heaven may contribute important amounts of woody debris to fuel loads in invaded areas. It frequently sheds broken branches in all size classes. The brittle branches break easily even when green, and branch die-back from drought or frost is common [148,151]. In riparian areas by the Middle Rio Grande in New Mexico, tree-of-heaven and other nonnative, invasive woody species were implicated as the main source of heavy fuel loads, with altered hydrologic regimes likely partially responsible for nonnative invasions. The author states "excess woody materials consisting of exotic species and dead and downed fuels of all species are the primary fuels of catastrophic fires" in riparian bosques on the Middle Rio Grande. The community was a Rio Grande cottonwood bosque before nonnative invasions. Contributions to total fuel load were not broken down by species, but saltcedar and Russian-olive (Elaeagnus angustifolia) apparently contributed more woody fuels than tree-of-heaven [152].
Flammability of tree-of-heaven was not reported in the literature as of 2010. Its growth habit and stand structure suggest that once ignited, tree-of-heaven stands probably burn easily. The large, finely divided leaves provide a surface-to-volume ratio favorable for ignition and burning. Dibble and others [69] conducted laboratory tests comparing fuel characteristics of nonnative invasive species with those of cooccurring native species. They found no significant difference between tree-of-heaven and quaking aspen in either total heat release or effective heat of combustion of leaves and twigs [69].
Tree-of-heaven does not have exacting germination requirements, although germination may proceed slowly. Tree-of-heaven embryos are dormant, and stratification improves germination rates [19,112,148,211]. Seeds dispersed in the field likely overwinter before germinating. Hunter [148] reported a 30% germination rate for seeds that overwintered on parent trees and dispersed in spring. A study in Spain found tree-of-heaven schizocarp size was not correlated with rates of either germination or seedling establishment [66].
Substrate is seemingly not important for successful tree-of-heaven germination. Seed can germinate and establish in highly compacted soil [249] and in pavement cracks [291]. The seed is salt tolerant. Studies of several eastern hardwood species found roadside salt did not appreciably affect tree-of-heaven germination; native oak and birch seeds were far more adversely affected by road salt [27].
Immersion, light intensity, and presence of litter affect tree-of-heaven germination rates. Short immersion in water may enhance tree-of-heaven germination. A German study showed 87% mean germination for tree-of-heaven seed lots soaked for 3 days, 53% germination for unsoaked seed lots, and 32% germination for seed lots soaked for 20 days [174]. Tree-of-heaven may germinate in low light, but resulting emergents are unlikely to establish [85]. Litter has both negative and positive effects on germination. In eastern deciduous forests, oak (Quercus spp.) leaf litter delayed tree-of-heaven germination and increased seedling mortality, but it did not affect subsequent biomass of surviving tree-of-heaven seedlings [89]. Litter may have positive effects on tree-of-heaven germination and establishment by reducing interference from herbaceous species [90].
Although seed production is prolific, tree-of-heaven seedling establishment is infrequent on many sites [148,198,232]. Despite tree-of-heaven's large seed output in a Connecticut site (see Seed production) [198], seedling establishment was low. The authors concluded that tree-of-heaven required canopy gaps to establish in otherwise closed-canopy forests [198]. Dry climate may limit tree-of-heaven recruitment in the Great Plains and the western United States [93,232]. Even so, tree-of-heaven has successfully expanded its range through seed spread and seedling establishment [166,245,257,262], and establishment from seed appears more common than generally indicated in the literature [163,275]. In a root excavation study in New York, Knapp and Canham [166] found initial tree-of-heaven recruitment in gaps in an eastern hemlock (Tsuga canadensis) forest was from off-site seed, not root sprouts. In the Black Rock Forest of New York, tree-of-heaven likely established from seed after a blowdown and subsequent logging and herbicide spraying in the 1960s. By the early 2000s, an oak-hickory forest had redeveloped; tree-of-heaven was present but not reproducing under the canopy [17]. Tree-of-heaven also established from seed on harvested oak forests on the Jefferson National Forest, Virginia [47].
Kostel-Hughes and others [169] surmised that tree-of-heaven is best adapted to establishment in early succession, when litter layers are lacking or shallow. In the greenhouse, tree-of-heaven seeds showed no significant differences in germination rate when placed on top of the litter, buried shallowly (0.4-0.8 inch (1-2 cm)), or buried deeply (2 inches (5 cm)). However, seedling height (P<0.001), aboveground biomass (P<0.03), and root:shoot ratio (P<0.001) decreased with increasing burial depth of seeds. The authors noted that some oak seedlings lifted up and hence reduced litter as they emerged, and some tree-of-heaven seedlings emerged from those reduced-litter microsites. Based on studies by Facelli and Pickett [90], they conjectured that a portion of tree-of-heaven's invasive success may be due to its ability to allocate more biomass to shoots than roots when emerging in deep litter. Although less root biomass means less ability to absorb water and nutrients, tree-of-heaven may compensate by allocating more resources to roots later in the growing season [169].
Two studies show tree-of-heaven germinated more slowly but had greater total emergence than native tree species. In the greenhouse, tree-of-heaven germinated later than native sweetgum, American sycamore, and nonnative princesstree [215,216]. In West Virginia field experiments in mixed-hardwood communities, stratified tree-of-heaven seeds showed better emergence across several sites compared to yellow-poplar seeds (P<0.0001). Tree-of-heaven showed no preference for north- vs. south-facing slopes, while yellow-poplar establishment was better on north slopes. Comparing tree-of-heaven germination across sites, there was no significant difference in emergence on clearcut and selective-cut sites (~80%), but tree-of-heaven emergence was significantly lower in intact forest (<20%) compared to logged sites (P=0.001) [171].
In Seoul, South Korea, tree-of-heaven seedlings established south of parent plants due to seed dispersal by prevailing northwest winds. The tree is nonnative there and is considered an urban weed because of its prolific seedling establishment and spread [53].
The tree-of-heaven embryo is well equipped for rapid growth. Although it lacks an endosperm, it has 2 large cotyledons with stored oils that provide energy for emergence and early growth [104,175,197,211,328]. Whether initial regeneration is accomplished from seed or by cloning, tree-of-heaven usually grows quickly on favorable sites. It is among the fastest-growing trees in North America [166,244,259]. Both the common name (tree-of-heaven) and the scientific name (Ailanthus, sky-tree) refer to the species' ability to attain height quickly [71,308]. In the eastern United States, tree-of-heaven's annual growth rate averaged 6 feet (1.8 m) for bole sprouts, 2.7 feet (0.8 m) for root sprouts, and 1.3 feet (6.5 m) for seedlings [151]. Root sprouts in California may exceed 3.5 feet (1 m) in their 1st year [148]. Tree-of-heaven sprouts generally grow faster than seedlings, although seedlings often grow 3.3 to 6.6 feet (1-2 m) in their 1st year [211]. A fact sheet states that tree-of-heaven may reach 80 feet (20 m) tall and 6 feet (2 m) in diameter in 10 years [87]. Two years after planting in a New York City common garden, height growth of tree-of-heaven seedlings was at least 3 times that of native sweetgum (Liquidambar styraciflua) and nonnative Norway maple seedlings [230].
Mean seedling stem and root growth of 3 tree species after 3 years in a common garden [230] SpeciesStem length (cm)
Lateral root length (cm)
Mean Minimum/ maximum Mean Minimum/ maximum tree-of-heaven 82.2 31/ 172 114.4 53/ 200 sweetgum 51.0 12/ 77 23.8 2/ 46 Norway maple 36.1 23/ 49 33.2 14/ 66Even with its rapid height growth, tree-of-heaven may concentrate early growth in roots. A greenhouse study showed tree-of-heaven seedlings had higher root:shoot ratios than native sweetgum, American sycamore (Platanus occidentalis), and nonnative princesstree seedlings; this was true whether tree-of-heaven was grown in disturbed soils that lacked organic matter or in soils collected from mixed-hardwood forests left undisturbed for ≥50 years [215,216].
Tree-of-heaven saplings may average 3 feet (1 m) of height growth per year for at least 4 years [1]. Relatively rapid growth continued into the pole size class in New York: pole-sized trees-of-heaven growing in canopy gaps gained 2 to 4 mm of radial growth annually, the highest rate of 6 tree species measured (the other 5 species were native) [166]. In a New England survey, trees-of-heaven reached 33 to 49 feet (10-15 m) in height and 3.7 to 4.3 inches (9-11 cm) DBH in 30 years [1], and a 55-year-old tree on the George Washington National Forest, West Virginia, had a DBH of 15 inches (37 cm) [247]. In North America, growth is usually fastest for trees-of-heaven in California's mediterranean climate. Trees in the Central Valley have an 8-month growing season, so those trees may be 35 to 63 feet (10-20 m) tall by 12 to 20 years of age [149]. In Pennsylvania, growth slowed greatly after age 20 to 25, with height increases of 3 inches (7.6 cm) or less per year [151].
Once established, tree-of-heaven density increases by root sprouting. One ramet may occupy over 1 acre (0.4 ha). Sprout growth slows to several centimeters per year if sprouts become shaded [149]. In West Virginia, Kowarik [173] reported an average growth rate of 0.36 foot/year (0.11 m) for tree-of-heaven sprouts suppressed in the understory of an oak (Quercus spp.)-sugar maple forest.
Browsing and/or unfavorable site conditions can reduce tree-of-heaven growth. Cattle, deer, and small rodent browsing may slow tree-of-heaven establishment and growth [229]. Browsing effects may vary with animal density and by site. In a New York oak-hickory woodland, Forgione [99] found no significant differences in tree-of-heaven seedling establishment on open plots and plots with white-tailed deer exclosures. On Mediterranean islands of Spain, France, and Greece, tree-of-heaven clones on intermittent streams were significantly smaller than those on old fields and roadsides (P=0.022), and clones at relatively high elevations were larger than those at low elevations (P=0.004) [292].
Tree-of-heaven occurs on a variety of sites in North America, ranging from very poor to very productive. In Ithaca, New York, it was positively correlated with urban sites where rooting space was limited and other species could not establish (P=0.05) [230,248]. In contrast, soils in the Central Valley of California, where tree-of-heaven is also common, are nutrient-rich and productive [128]. Little information is available on tree-of-heaven's original habitats in China; it is common there as a cultivated tree [139,140,249].
Tree-of-heaven has been characterized as "the most adaptable and pollution tolerant tree available" for urban plantings [74]. Highly tolerant of industrial gases, dust, and smoke, it is common on disturbed urban sites, especially alleyways, roadsides, and fence rows [71,105,114,214,324]. It is generally more common in urban, suburban, and rural than wild environments [57,181]. In wildlands, tree-of-heaven occurs on floodplains and other disturbed sites, riparian areas, open woodlands and forests, and rock outcrops [41,114,137,164,280,312]. After Hurricane Camille, tree-of-heaven was positively associated with debris avalanche chutes in Virginia [145]. It was most frequent on roadsides in an oak-hickory forest in West Virginia [141]:
Tree-of-heaven frequency on different sites within a West Virginian oak-hickory forest [141] Habitat Frequency (%) roadsides 26 streams 19 mature forest 13 open forest 7 railroad rights-of-way 6 residential 6 trails 3 old fields 0 other 20Tree-of-heaven has invaded rare sugar maple-sweet birch (Betula lenta) rock outcrop communities in High Mountain Park Preserve, New Jersey [76]. In the Southwest, it invades canyons, arroyos, and riparian zones, including the banks of the Rio Grande [6,203].
Soils and topography: Tree-of-heaven tolerates a wide range of soil moisture conditions [82,211]. In oak-hickory woodland of Sussex County, New Jersey, it grows in permanently swampy, ridgebottom soils of an abandoned Boy Scout camp [18]. At the other extreme, tree-of-heaven tolerates dry, rocky soils and extended drought, aided by its large, water-storing roots. Even seedlings show drought tolerance, often volunteering in pavement cracks and other dry sites [113,293]. In Kansas, mature trees-of-heaven and eastern redcedars showed better survivorship during the "Dust Bowl" drought of 1934 than associated tree species [278].
Tree-of-heaven also tolerates a wide range of soil nutrient levels and other soil conditions. Best growth occurs on nutrient-rich, loamy soils, but tree-of-heaven establishes in nutrient-poor soils [93,163,211,328]. Tree-of-heaven tolerates all soil textures [216]. It often establishes on disturbed sites lacking topsoil [164]. In the Appalachians and the Northeast, the tree-of-heaven alliance occurs on limestone clifftops and on calcareous soils [223,237]. On reclamation sites, trees-of-heaven tolerated acid mine spoils better than calcareous spoils and grew on low-phosphorus soils [211]. Tree-of-heaven can grow on soils as low as 4.1 pH, in soluble salt concentrations of 0.25 mmhos/cm, and in soils with phosphorus levels as low as 1.8 ppm [245]. In a mixed-deciduous forest on Staten Island, New York, tree-of-heaven had the highest importance value and relative density of all tree species on neutral soils but was absent on acidic soils (pH ≤5.1) [186]. It tolerates compacted soils [230].
Topography on tree-of-heaven sites may be flat, rolling, or very steep, with tree-of-heaven potentially occurring on all aspects. Tree-of-heaven's spreading root system permits establishment and growth on steep inclines and cliff faces [7]. In Massachusetts, tree-of-heaven is reported on upland, interior wetland, and coastal areas [200]. On the floodplain of the Raitan River, New Jersey, tree-of-heaven was not important on low floodplains (<11 feet (3.3 m) above sea level), but it ranked in the top one-third of species' importance values on upper floodplains [101]. In a slippery elm-white ash woodlot in Ohio, tree-of-heaven presence on forest-roadside edges was similar on north- and south-facing exposures [85]. In Inwood Hill Park, a mixed-hardwood wildland site in Manhattan, tree-of-heaven was most common on west-facing ridges [98]:
Density of tree-of-heaven seedlings and saplings in a wildland park in New York City [98] Seedlings (<2 cm DBH) Saplings (2-10 cm DBH) Valley forest 55 9 East ridge 113 38 Ridgetops 0 50 West ridge 363 211Climate: Tree-of-heaven is most common in temperate climates, in both North America and its native China. It tolerates minimum temperatures of -38 °F (-39 °C) and maximum temperatures of 110 °F (45 °C). Mean annual precipitation ranges from 0.55 to 158 inches (14-4,010 mm) across tree-of-heaven's North American and Chinese distributions. Tree-of-heaven tolerates drought of several month's duration [3].
Climate within tree-of-heaven's North American distribution ranges from subtropical and wet in Florida; arid in the Great Plains and western United States; to cold and wet in the Northeast. It occurs in USDA hardiness zones 4 to 8 [144]. Annual mean maximum and minimum temperatures within its North American range are 15o F and 97 oF (-9 oC and 36 oC). It tolerates as much as 90 inches (2,290 mm) of mean annual precipitation in the Appalachian Mountains as little as 14 inches (360 mm) of annual precipitation and 8 months of drought in the western United States. Large, water-storing roots confer drought tolerance [140], although tree-of-heaven may not reach maximum growth on dry sites. On an "extremely dry" site on the George Washington National Forest, Pomp [247] observed that trees-of-heaven in an oak-pine forest only reached the canopy in riparian areas and on logged sites. Because seedlings are not cold resistant, extreme cold and prolonged snow cover restrict its occurrence to lower slopes in mountainous regions. Tree-of-heaven may be able to colonize in cold regions that experience several successive years of mild climate [211]. It is the only species in its genus that tolerates cold climates [140].
Elevation: Tree-of-heaven is reported from the following elevations in the western United States:
Elevational range of tree-of-heaven in 3 western states State Elevation California <6,600 feet [135,149] New Mexico 4,500-7,000 feet [199] Utah 790-5,900 feet [317]It grows from 4,900 to 5,900 feet (1,500-1,800 m) elevation in China [304].
Impacts: Tree-of-heaven is invasive in many regions of the United States. It can have detrimental effects on ecosystem processes, damage structures, and poses risk to human health.
Invasiveness: Tree-of-heaven's preference for disturbed, early-seral habitats, ability to spread from root sprouts, prodigious seed production, and rapid growth make it an "aggressive" invader [47]. Hammerlynck [122] speculated that tree-of-heaven invasiveness may be due in part to its unusually high capacity to photosynthesize and its high water-use efficiency. These abilities, coupled with pollution [207,253] and drought [87] tolerance, make tree-of-heaven especially successful in urban environments and may also help it invade wildlands [122]. Although it is more common in populated areas (see Site Characteristics), it may spread from developed areas to wildlands [146], especially those disturbed by logging [181] or other canopy-opening events. Tree-of-heaven is most invasive in eastern deciduous forests [34,161,163,166,173,228]. It has invaded the Wayne National Forest, Ohio [79], and is common in white oak-red oak-chestnut oak-Carolina basswood/eastern hophornbeam (Quercus alba-Q. rubra-Q. prinus-Tilia americana var. caroliniana/Ostrya virginiana) forest alliances of the Uwharrie National Forest, North Carolina [224]. In 2007, the Alabama Invasive Plant Council determined there were "extensive and dense" tree-of-heaven infestations in Alabama's managed forests and wildlands [2]. In California, tree-of-heaven is most invasive in riparian zones [44,81] but also invades oak woodlands and valley grasslands [44]. Tellman [287] states tree-of-heaven is "rapidly becoming a problem species" in the Southwest, where it invades riparian zones and other moist areas.
Tree-of-heaven has been rated a threat or potential threat in many areas of the United States. It is rated a potential invader in low-montane areas of the Cascade Range, the Sierra Nevada, and the Middle Rocky Mountains [231]; a high threat to northeastern deciduous and riparian forests and a potentially high threat to northeastern coniferous and mixed forests, grasslands, and fresh and tidal wetlands [70]; and a high threat to oak-hickory forests of the Southeast [279]. Other ratings of tree-of-heaven invasiveness as of 2010 include:
Ratings of tree-of-heaven's invasiveness by state Rating Area Affected ecosystems West Potentially high threat coastal southern Oregon and coastal California mixed-evergreen, coniferous, and riparian forests, grasslands, chaparral, and wetlands [165] very invasive, especially on disturbed sites Willamette Valley, Oregon Oregon prairie and white oak woodlands [222] Hawaii Large potential threat throughout native Hawaiian plant communities [272,321] Northeast 1 of the top 10 most invasive weeds Bronx River Parkway Reservation, New York riparian and urban forests [100] Appalachians Low threat Oak Ridge National Environmental Research Park, Tennessee oak-hickory [78] Southeast Highly invasive Savage Neck Dunes Natural Area Preserve on the Delmarva Peninsula, Virginia Chesapeake Bay beach, dune, and maritime forest communities [225] Highly invasive Petersburg National Battlefield and Colonial National Historical Park, Virginia oak-gum-cypress, loblolly-shortleaf pine, oak-hickory [237] and oak-hickory, oak-gum-cypress, maple-beech-birch, old-field, fresh wetland, and saltmarsh [236] communities, respectively Most invasive and rapidly spreading nonnative species on site Booker T. Washington National Monument, Virginia oak-hickory, maple-beech-birch, and old-field communities [235]There is some controversy about tree-of-heaven's ability to invade intact ecosystems. Some authors note that although tree-of-heaven established in North America over 100 years ago, it has not spread to many sites that appear to be good habitat for the species [219]. Hunter [148] considers it "only potentially invasive, and also potentially eradicable" in California. Huebner's [142] model predicts that in wildlands, tree-of-heaven will mostly spread along nonforested corridors and invade forests slowly. Tree-of-heaven may not invade closed-canopy forests [93,148]. It was ranked the least invasive of 18 nonnative, invasive species on the Oak Ridge National Environmental Research Park, Tennessee [78]. Long-distance seed dispersal, however, may enable tree-of-heaven to quickly invade disturbed openings in forest interiors [142]. Merriam [210] estimated a 4.76% increase in tree-of-heaven's rate of spread per year in North Carolina. Further information and research are needed to recognize undisturbed sites at risk for tree-of-heaven invasion [139].
Effects on ecosystem processes: As an early-seral species favoring disturbed sites (see Successional Status), tree-of-heaven grows extremely rapidly and interferes with growth of native species. The large, interconnected roots effectively occupy underground space and crowd out native species. Tree-of-heaven sometimes forms large thickets, displacing native vegetation [172,173]. It may affect natural successional trajectories, in part from competition for light and nutrients in early-successional environments, and possibly from allelopathy [129,130,131,140,184]. Tree-of-heaven and Amur honeysuckle (Lonicera maackii) were the most abundant and ubiquitous woody species in disturbed urban areas of Ohio. The authors speculate that these exotic species invasions have the potential to modify forest composition and ecological function of urban riparian systems [240].
Allelopathic species may alter stand structure and alter key ecosystem processes [314]. Tree-of-heaven's reputed allelopathy may slow succession in plant communities where it is invasive [129,163]. It is apparently toxic to other plants, rodents, and microbes. In an abstract of their laboratory work, Greer and Aldrich [116] report higher toxicity in leaves of young (≤2 years) trees-of-heaven than in leaves of older trees; they also found that minor injury to tree-of-heaven increases toxin production. Concentration of allelopathic chemicals is highest in young tree-of-heaven stands [45,184]; allelopathy may help tree-of-heaven seedlings establish [184] and clones to spread [209]. Seasonally, toxins are greatest in spring and decline as the growing season progresses. Allelopathic chemicals are present in all portions of the tree, but they are most concentrated in roots. The litter is also likely allelopathic. In the greenhouse, slash (Pinus elliottii) and Monterey pine (P. radiata) seed germination was inhibited by fresh tree-of-heaven litter. Allelopathy of tree-of-heaven litter was highest during pine's spring germination period and dwindled through the growing season [311]. In a red maple-sugar maple-northern red oak forest in northwestern Connecticut, tree-of-heaven did not inhibit emergence or survival of either red maple, sugar maple, or northern red oak seedlings beneath tree-of-heaven canopies, but tree-of-heaven significantly reduced seedling growth of red maple (R²=0.33), sugar maple (R²=0.31), and northern red oak (R²=0.49) compared to seedling growth on control sites. Activated carbon was added to the soil on control sites to neutralize the effects of tree-of-heaven's allelopathic chemicals. Allelopathic effects in the soil were positively related to nearness of tree-of-heaven trunks, and soils lost their allelopathic effects about 20 feet (5 m) from the trunks. Cumulative allelopathic effects were proportional to tree-of-heaven density, regardless of tree-of-heaven size [107].
Tree-of-heaven may alter litter layer depth and increase available soil nitrogen in native plant communities. In Spain, tree-of-heaven litter decayed faster than litter of native English elm (P=0.01), and nitrogen release was greater beneath trees-of-heaven than beneath English elms (P=0.005). Soils beneath trees-of-heaven were not higher in nitrogen, however; the authors speculated this may be due to quick nitrogen uptake by nearby plants [50]. Studies on Mediterranean islands of Spain, France, Italy, and Greece found tree-of-heaven presence significantly decreased soil carbon:nitrogen ratios and reduced diversity of native species compared to uninvaded plots (P<0.05 for both variables) [307]. In aquatic ecosystems, preference of invertebrate detritus-feeders for tree-of-heaven litter over litter of native trees (see Palatability) may alter decay rates of native species [284]. Relative palatability of tree-of-heaven litter may affect successional trajectories and increase invasibility of mixed-hardwood communities. In a greenhouse study, earthworms consumed or buried nearly 100% of tree-of-heaven litter; in turn, this increased establishment and growth of nonnative tall fescue (Schedonorus arundinaceus), which had been seeded onto mesocosms (soil-filled tubs) with litter, soil, and earthworms. Tall fescue recruitment was less in mesocosms with native American chestnut (Castanea dentata), northern red oak, or yellow-popular litter than in mesocosms with tree-of-heaven litter [22]. A study in a sugar maple-white ash-northern red oak forest in Connecticut found significantly greater total soil nitrogen, calcium, and nutrient- cycling rates on sites with tree-of-heaven compared to sites without it. This effect increased in soil samples nearer to tree-of-heaven and with increasing tree-of-heaven DBH [106].
Socioeconomic factors: In developed areas, tree-of-heaven roots can damage buildings, foundations, and water facilities [140]. The rapidly growing, extensive root system also allows the tree to establish on steep to vertical structures including roofs and cracked walls, which are damaged if seedlings are not promptly killed [7,262]. Tree-of-heaven has damaged archeological sites in Europe. A study in Mediterranean Italy found it was the only nonnative tree species establishing in and damaging archeological remains [51]. In Portugal, tree-of-heaven roots damaged the walls and roof of the 800-year-old Sé Velha of Coimbra Cathedral after establishing in the wall mortar and clay roof tiles [7]. The water-seeking roots can stop up sewer lines and invade wells and cisterns, damaging infrastructures and giving potable water an unpleasant taste [62,140].
Tree-of-heaven poses human health risks. The pollen can cause an allergic reaction [68]. The sap causes a dermatitis reaction in some people [68,319], and prolonged exposure of broken skin to sap can have serious consequences. For at least one tree remover, exposure to tree-of-heaven sap on rope burns resulted in an elevated heart rate and chest pain that was severe enough to require hospitalization for several days [29].
Control: In all cases where invasive species are targeted for control, the potential for other invasive species to fill their void must be considered no matter what method is employed [38]. Control of biotic invasions is most effective when it employs a long-term, ecosystem-wide strategy rather than a tactical approach focused on battling individual invaders [192].
Prevention: It is commonly argued that the most cost-efficient and effective method of managing invasive species is to prevent their establishment and spread by maintaining "healthy" natural communities [192,270] (e.g., avoid road building in wildlands [296]) and by monitoring several times each year [155]. Managing to maintain the integrity of the native plant community and mitigate the factors enhancing ecosystem invasibility is likely to be more effective than managing solely to control the invader [138]. Weed prevention and control can be incorporated into many types of management plans, including those for logging and site preparation, grazing allotments, recreation management, research projects, road building and maintenance, and fire management [301]. See the Guide to noxious weed prevention practices [301] for specific guidelines in preventing the spread of weed seeds and propagules under different management conditions. The Center for Invasive Plant Management provides an online guide to noxious weed prevention practices. Nursery businesses can reduce tree-of-heaven introductions by not stocking tree-of-heaven and discouraging its use [126]. Gonzalez and Christoffersen [108] give suggestions for alternative landscaping species that are native in the Southeast and Southwest.
Posttreatment monitoring and retreatment are essential for this root-sprouting, rapidly growing species (see Regeneration Processes). Monitoring efforts are best concentrated on the most disturbed areas in a site, particularly along potential pathways for tree-of-heaven invasion: roadsides, trails, parking lots, fencelines, trails, and waterways [40,46,141]. In a West Virginia study, tree-of-heaven occurrence was heaviest along interstate freeways I-68 and I-79 [141]. Fortunately, tree-of-heaven's pattern of roadside and trail invasion makes accessibility, early detection, and treatment relatively easy on many sites [40]. Treated areas need checking once or twice a year, with any new sprouts or seedlings retreated as soon as possible so that plants do not have time to build up carbohydrate reserves and grow larger [139]. Whenever there is a nearby tree-of-heaven seed source, disturbed sites require monitoring and follow-up treatments to prevent tree-of-heaven invasion. Initial summer treatment impacts trees when their root reserves are low. Targeting female trees-of-heaven for control helps slow seed dissemination [139]. Monitoring is most effective when continued for at least a year after tree-of-heaven sprouting appears controlled [149]. Tree-of-heaven may show up on sites where treatments for other invasive weeds have created open, disturbed conditions. For example, tree-of-heaven and Norway maple seedlings invaded a New Jersey site after Norway maple removal treatments (cutting mature trees and hand-pulling Norway maple seedlings). Tree-of-heaven was not present on study plots prior to Norway maple removal [315].
Fire: Prescribed fire is not recommended to control tree-of-heaven due to tree-of-heaven's ability to establish from root sprouts and seed after disturbance [87,149] (see Fire Management Considerations). Nava-Constan and others' [54] experiment in Spain (see Physical and/or mechanical treatment) demonstrates tree-of-heaven's ability to increase by sprouting after top-kill. However, fire may be used as the initial top-kill treatment for tree-of-heaven control or for spot treatment. Instead of using mechanical or chemical methods to top-kill stems, a flame thrower or weed burner can heat-girdle tree-of-heaven boles [56]. As with all top-kill methods, tree-of-heaven sprouts after heat girdling [56,87], so follow-up treatments are needed to control sprouts.
Cultural: Maintaining a healthy overstory can help minimize invasive potential for tree-of-heaven [270]. Establishing a thick cover of native trees or grass helps shade out tree-of-heaven and discourages tree-of-heaven regrowth [139]. Broadcasting seed of native tree species may help slow tree-of-heaven establishment on some disturbed sites. In West Virginia, seedling establishment experiments in a harvested mixed-hardwood community showed yellow-poplar seedling survivorship 2 years after sowing was higher than that of tree-of-heaven seedlings (80% vs. 28%, P<0.01) on selection cuts. However, tree-of-heaven survivorship was higher than that of yellow-poplar on clearcut and unharvested control sites [171]. Moore and Lacey [216] found sweetgum and American sycamore germinated and established more quickly in the greenhouse than nonnative trees, including tree-of-heaven (P<0.001); they suggested establishing native tree seedlings on disturbed sites to help reduce invasion of nonnative tree species such as tree-of-heaven [216]. If artificial regeneration of native trees is indicated, it is important to establish the native seedlings quickly. With one of the fastest growth rates of any tree in North America [166], young trees-of-heaven may grow faster than and overtop young native tree species [149,166,173].
Physical and/or mechanical: Mechanical treatments, including cutting or girdling, are a good first step in controlling tree-of-heaven. Mechanical treatment alone encourages both stump and root sprouts, so follow-up treatments are required [149,316]. Cutting stems before flowering prevents seed spread, and cutting at ground level prevents bole sprouts [149]. Root and root crown sprouts can be controlled by further cutting treatments or herbicides [139,149,178], although herbicides are more effective (see Chemical control). Unless the treatment area is heavily shaded, it usually takes at least 5 years of follow-up mechanical treatment to control sprouts [58,139]. Removing the roots eliminates sprouting [58,87] but is impractical on most sites. Cutting without further treatment is not recommended because it promotes tree-of-heaven sprouting [58] and may increase tree-of-heaven abundance. A study in a Mediterranean ecosystem of Spain found that after tree-of-heaven was cut twice a year for 5 years, its density and leaf area index were greater on cut than on uncut plots (P<0.05) [54].
In small areas, seedlings can be controlled by hand pulling. Seedlings quickly develop extensive root systems, so the entire root needs to be removed to prevent sprouting [178]. Seedling and root sprout top-growth looks similar, but root sprouts are connected by large lateral roots. Tree-of-heaven seedlings can sometimes be compared to the variable number of leaflets and thicker stems of sprouts [149]. Grubbing roots may be effective for saplings [294]. Except for small infestations, grubbing for mature trees or well-established tree-of-heaven colonies. The root systems are extensive and nearly impossible to entirely remove, and even a small root fragment can produce root sprouts [139,149].
Biological: Biological control of invasive species has a long history that indicates many factors must be considered before using biological controls. Refer to these sources: [305,320] and the Weed control methods handbook [294] for background information and important considerations for developing and implementing biological control programs.
As of this writing (2010), there were no biological control agents approved for tree-of-heaven [134,260,294]. Eucryptorrhynchus brandti and E. chinensis, weevils native to China, are apparently host-specific tree-of-heaven feeders [72,134,260]. As of 2009, they were being tested as possible control agents for tree-of-heaven [134,260]. A wilt fungus (Verticullum albo-atrum) is also being tested for tree-of-heaven control [265]. Ding and others [73] provide a review of these and other biological control agents being tested for tree-of-heaven as of 2006.
Chemical: Herbicides are effective in gaining initial control of a new invasion or a severe infestation, but they are rarely a complete or long-term solution to weed management [42]. Herbicides are most large infestations when incorporated into long-term management plans that include replacement of weeds with desirable species, careful land use management, and prevention of new infestations. Control with herbicides is temporary, as it does not change conditions that allow infestations to occur in the first place (for example, [326]). See the Weed control methods handbook [294] for considerations on the use of herbicides in natural areas and detailed information on specific chemicals.
Systemic herbicides that kill roots (for example, triclopyr and glyphosate) currently provide the best chemical control for tree-of-heaven. Dicamba, imazapyr, and methsulfuron methyl have also provided control [83,84,139,178,275]. See The Nature Conservancy's Weed control methods handbook for considerations on the use of herbicides in Natural Areas and detailed information on specific chemicals. See these sources for information pertaining to chemical of tree-of-heaven in particular: [83,139,145,178,199,212,275,276,294,294,304,311,312,317].
Several studies indicate that herbicides control tree-of-heaven better than cutting [40,54,208], and probably better than prescribed fire (see Fire studies). In oak-hickory communities of Shenandoah National Park, low-volume basal application of herbicides (triclopyr, picloram, imazapyr, and combinations) gave better tree-of-heaven control than cutting. At posttreatment year 2, a shift toward native herbs had occurred on plots where tree-of-heaven was controlled, while nonnative herbs including garlic mustard (Alliaria petiolata) and burdock (Arctium minus) were more prevalent on plots where tree-of-heaven density remained high [40].
Mean tree-of-heaven density 2 years after control treatments in Shenandoah National Park [40] Treatment Stems/acre cutting 5,645 imazapyr 363 picloram + triclopyr* 81 *Several herbicide combinations and application rates were used. See Burch and Zedaker [40] for details.Herbicides meant for tree-of-heaven may kill nontarget trees, even when injected into tree-of-heaven stems. Spraying may be indicated when there are large thickets without nontarget species or as a follow-up treatment to other control methods, but in mixed stands herbicides will probably have a greater impact on nontarget species that lack tree-of-heaven's ability to root sprout than on tree-of-heaven [139,178,275,294]. Tree-of-heaven roots may exude herbicides, so caution is recommended when treating trees-of-heaven growing next to high-value trees [83]. In southeastern Ohio, 17.5% of native hardwoods within 10 feet (3 m) of trees-of-heaven injected with imazapyr were also killed. The authors surmised that root grafts and/or shared mycorrhizae translocated imazapyr from tree-of-heaven to native hardwood roots [187].
Chemical control programs targeting herbaceous species may unintentionally increase tree-of-heaven, depending upon the herbicides used. In West Virginia, diuron, simazine, and terbacil treatments for rough pigweed (Amaranthus retroflexus), barnyard grass (Echinochloa crus-galli), and other herbaceous weeds in a commercial apple (Malus sylvestris) orchard successfully reduced most herbaceous weeds; however, tree-of-heaven and nonnative tall fescue (Schedonorus arundinaceus) dominated the ground layer of plots treated with diuron and simazine. Terbacil gave best control of tree-of-heaven [295].
Integrated management: A combination of complementary control methods may be helpful for rapid and effective control of tree-of-heaven. Integrated management includes not only killing the target plant, but establishing desirable species and discouraging nonnative, invasive species over the long term. In a black oak community in Rondeau Provincial Park, Ontario, cutting and applying glyphosate to tree-of-heaven stumps best controlled trees-of-heaven for 2 years, while cutting alone increased tree-of-heaven density over pretreatment levels. Hand-pulling and mulching was done on seedlings that had not yet developed taproots and on juveniles (<20 inches (60 cm) tall)); juveniles were extracted by the roots to prevent sprouting [208]:
Tree-of-heaven density (shoots/m²) after control treatments in Rondeau Provincial Park [208] Juveniles Control Mature trees Treatment Hand-pulling & mulch Cut stump & glyphosate Cut stump Untreated Glyphosate-injected Pretreatment 12.7a 11.8a 12.8a 13.4a 14.2a Posttreatment year 1 1.7b 0.8b 9.2a 10.6a 9.8a Posttreatment year 2 5.8b 1.0c 20.9d 10.9a 8.9a Within rows, means followed by different letters are significantly different (P=0.05). Means of juvenile trees on experimental plots are compared only with means of juvenile trees on control plots, and means of mature trees on experimental plots are compared only with means of mature trees on control plots.There are few reports of either wildlife or domestic animal use of tree-of-heaven. White-tailed deer and domestic goats browse the foliage [35,147,308], but tree-of-heaven browse is apparently not preferred. In mixed-hardwood stands of Virginia, white-tailed deer browsed 0.4% of available tree-of-heaven seedlings and 12.6% of tree-of-heaven saplings in recently logged forests but preferred black tupelo (Nyssa sylvatica). Browsing pressure on tree-of-heaven was greater in mature stands—where other browse was less available—with white-tailed deer utilizing 33.3% of tree-of-heaven seedlings and 41.7% of saplings. Across tree species, browsing pressure was significantly greater in mature than logged stands (P=0.02) [49]. Meadow voles also browse tree-of-heaven [43]. An old-field study in New York showed meadow voles preferred tree-of-heaven seedlings over eastern white pine seedlings, but white-footed mice preferred eastern white pine, sugar maple, and white ash seedlings over tree-of-heaven seedlings [229].
Wildlife consumption of tree-of-heaven seeds is apparently light. A few birds, including pine grosbeak and crossbills, eat the seeds [147]. A New Jersey study found granivorous rodents ignored tree-of-heaven seeds in an old field [193].
Invertebrates use tree-of-heaven; the few studies available as of 2010 suggested tree-of-heaven is important in the diet of some invertebrate species. In a mixed-hardwood forest in New York, invertebrates browsed tree-of-heaven seedlings preferentially [43]. Tree-of-heaven nectaries attract ants, which may defend trees-of-heaven against other insects, including potential pollinators. Ants sometimes colonize hollow boles of fungus-infested trees-of-heaven [5]. A study in Maryland found detritus-feeding, aquatic isopods and caddisflies preferred tree-of-heaven litter to litter of 6 native species, possibly because tree-of-heaven litter decayed more rapidly than that of native species [284]. A greenhouse study found nonnative earthworms (Lumbricus terrestris) preferred tree-of-heaven litter to that of native yellow-poplar [22].
Palatability and nutritional value: Tree-of-heaven is unpalatable to ungulates [99]. The bark and leaves contain saponins, quassinoids, and other bitter compounds that discourage consumption [10,131,190,308].
Tree-of-heaven is a fairly good source of protein, especially early in the growing season [15].
Nutritional content (%) of tree-of-heaven browse in Pakistan [15] Digestible matter Crude protein Neutral- detergent fiber Acid- detergent fiber Hemi-The seeds are a good source of fatty oils [32,156]. Seed collected in the United States had 56% fatty oil and 23% protein content [10].
Cover value: No information is available on this topic.
Tree-of-heaven is most common in urban areas [141]. It varies from a minor to important component of wildland vegetation in its North American range. Because of its scattered and disjunct distribution in North America, tree-of-heaven occurrence is not well documented for all plant communities where
it may occur [211]. The following descriptions are not restrictive, but they include plant communities where tree-of-heaven is a known invader.
In wildlands of the East and Midwest, tree-of-heaven is a common component in oak-hickory (Quercus-Carya spp.) and maple-birch-beech (Acer-Betula-Fagus spp.) forests [286]. It has infested hundreds of acres of oak-hickory forest in Shenandoah National Park, Virginia [195,303]. In eastern oak-hickory forests, tree-of-heaven is frequently associated with native black locust (Robinia pseudoacacia), an early-successional species, and nonnative Norway maple (Acer platanoides) and princesstree (Paulownia tomentosa) [45,46,117,227,249,314]. Other early-seral associates are black cherry (Prunus serotina), gray birch (Betula populifolia), sweetgum (Liquidambar styraciflua), and eastern redcedar (Juniperus virginiana) [117]. In oak-hickory woodlands of Gettysburg National Military Park, Pennsylvania, tree-of-heaven is associated with overstory with black oak (Q. velutina), red oak (Q. rubra), scarlet oak (Q. coccinea), mockernut hickory (C. tomentosa), and bitternut hickory (C. cordiformis). Nonnative invasives other than tree-of-heaven include the understory species Japanese barberry (Berberis thunbergii) and multiflora rose (Rosa multiflora). Late-successional native understory species include white ash (Fraxinus americana), black cherry, sassafras (Sassafras albidum), and boxelder (Acer negundo) [91]. In silver maple (Acer saccharinum)-white oak-red oak forests of Ohio, tree-of-heaven occurs with American elm (Ulmus americana), hickory, black locust, black walnut (Juglans nigra), and black cherry [67]. Tree-of-heaven stands have been observed in estuaries and freshwater marshes on the Atlantic seaboard. Tree-of-heaven dominated the overstory of the estuary at Jug Bay Wetlands Sanctuary, Maryland [164].
On the Georgia piedmont, tree-of-heaven is an important species in early-successional loblolly pine (Pinus taeda)-black oak-white oak forests. Native flowering dogwood (Cornus florida) and yellow-poplar (Liriodendron tulipifera) and nonnative Chinese privet (Ligustrum sinense) and winged elm
(U. alata) are common associated species [55].
Tree-of-heaven is most common in riparian, valley, and foothill communities of California. In riparian areas of southern California, tree-of-heaven frequently associates with Fremont cottonwood (Populus fremontii), California sycamore (Platanus racemosa), mule-fat (Baccharis
salicifolia), and poison-oak (Toxicodendron diversilobum). A 1990 to 1992 survey in Chino Hills State Park showed 15% frequency for tree-of-heaven, with
its geographic range and "vigor" increasing over the study period [162].
Tree-of-heaven is invasive in riparian zones of the Southwest [135,266,273,281]. In the middle Rio Grande Basin of New Mexico, it occurs in Fremont cottonwood-Rio Grande cottonwood-sandbar willow (P. deltoides var. wislizenii-Salix exigua) bosques along with nonnative, invasive Siberian elm (U. pumila) and white mulberry (Morus alba) [97]. Tree-of-heaven has invaded arroyos of the Sandia National Laboratory, New Mexico; nonnative Siberian elm and saltcedar (Tamarix ramosissima) and native Apache plume (Fallugia paradoxa) and fourwing saltbush (Atriplex canescens) are common associated species. On upland sites it occurs in Colorado pinyon-oneseed juniper (Pinus edulis-Juniperus monosperma) stands with Gambel oak (Quercus gambelii) and Siberian elm [203].
The following vegetation classifications describe plant communities in which tree-of-heaven is a dominant or indicator species:
Tree-of-heaven has been widely planted as an ornamental because it grows quickly, can be trained into an attractive shape, and has attractive foliage and fruits. It was once widely cultivated in North America [312,324] and is still available as a horticultural plant [126]. It is planted less frequently now [312,324] because of its disagreeable odor and strong tendency to spread into areas where it is not wanted. It is occasionally used for shelterbelts and urban plantings [211,328].
Tree-of-heaven's tolerance to pollution has management uses. It is a bioindicator of ozone pollution, to which it is sensitive. When subjected to heavy ozone concentrations, the leaves show spotting damage and drop off [111]. Conversely, tree-of-heaven tolerates high levels of sulfur and mercury pollution, concentrating mercury in its leaves (review by [175]). It has been used to rehabilitate mine spoils in the eastern United States. Tolerant of saline soils and low pH, tree-of-heaven shows better growth on mine spoils than many native species [245]. Due to its tendency to spread on disturbed sites, however, it is not generally recommended for rehabilitation [147,178].
Tree-of-heaven provides shade, medicine, wood, clothing, and food for humans. The species has a long history of folk medicine and cultural use in Asia [140]. It is used as an astringent, antispasmodic, anthelmintic, and parasiticide. Fresh stem bark is used to treat diarrhea and dysentery; root bark is used for heat ailments, epilepsy, and asthma. The fruits are used to regulate menstruation and treat ophthalmic diseases. Leaves are an astringent and used in lotions for seborrhea and scabies [9,140]. Laboratory studies show tree-of-heaven has a potential role in modern medicine. Tree-of-heaven extracts have antibacterial, antioxidant [251], and antiinflammatory [154] properties. Pharmacological research is focusing on possible use of tree-of-heaven extracts for treating cancer, malaria, and HIV-1 infection [9,36,52,226]. In China, tree-of-heaven is grown commercially as a host for Attacus cynthia, a silkworm that produces coarse, durable silk [140,308]. Tree-of-heaven is a food for honeybees worldwide. Initially bad-tasting, tree-of-heaven honey ages to a high-quality, flavorful product [59,211].
Putative allelopathic tree-of-heaven extracts have potential use as herbicides [64,131,190].
Wood Products: Tree-of-heaven wood resembles ash (Fraxinus spp.) wood in appearance and quality. It is easily worked with tools and glue, and takes a finish well. Alden [4] and Moslemi and Bhagwat [219] summarize manufacturing properties of tree-of-heaven wood. Berchem and others [23] and Adamik and Brauns [1] provide information on properties and potential uses of tree-of-heaven wood fiber.
Tree-of-heaven is an important timber and fuelwood tree in China, and is planted for timber and afforestation in New Zealand, the Middle East, eastern Europe, and South America [14,140,268,304,328]. Zasada and Little [328] provide information on tree-of-heaven cultivation.
Leaf expansion typically begins in early spring, with trees-of-heaven flowering and dispersing pollen in late spring [148,211]. Tree-of-heaven's seasonal development may sometimes be slower than associated woody species. On Nantucket Island, Massachusetts, tree-of-heaven was noted in a 1912 publication as "nearly naked" in early June, when other tree species had already leafed out [26]. Seed ripening begins in late summer and continues through fall. Schizocarps usually persist on female trees through winter [198,211], but they may disperse anytime from October through the next spring [211]. In New York, schizocarp clusters usually broke off in fall, while individual schizocarps often persisted until spring [230].
Seasonally, tree-of-heaven's allelopathic toxins are greatest in spring and decline as the growing season progresses [311]. See Impacts and Control for more information on allelopathy.
The winged schizocarps are easily and widely dispersed by wind [53,82,163,170,180,181,261]. Entire schizocarp clusters may break off and disperse as a unit. In Ithaca, New York, tree-of-heaven schizocarp clusters often fell and dispersed in clumps in fall, resulting in patches of closely related seedlings. Over the winter, seeds dispersed individually as the fruit clusters disintegrated [230]. A New Jersey study found large, heavy tree-of-heaven seeds traveled as far as light tree-of-heaven seeds [181]. In Seoul, South Korea, tree-of-heaven seed traveled a maximum of 7.5 times the width of the parent crown [53]. On Staten Island, New York, tree-of-heaven seedlings volunteered on a restored landfill site planted to native woody species. A year after restoration plantings, tree-of-heaven count on fifty 10 × 30 meter plots totaled 65 seedlings, the 6th highest among 32 taxa that regenerated on the site. Distance to the nearest seed-bearing tree-of-heaven was 299 feet (70 m) [257]. In West Virginia, seeds from trees on steep slopes dispersed farther downhill than seeds on gentle or flat sites (P157]. Matlack [201] reported the following dispersal patterns for tree-of-heaven schizocarps collected in Delaware:
Dispersal characteristics for tree-of-heaven schizocarps in the laboratory. Data are means (SD) [201]. Rate of descent Lateral movement in still air Lateral distance in a 10 km/hr breeze (estimated) 0.56 m/sec (0.09) 0.87 m (0.10) 111.6 mDistance traveled was significantly greater than that of fruits or seeds of 37 other wind-dispersed species (P<0.05) [201].
Water [174,232,261] and machinery [232] also disperse tree-of-heaven schizocarp clusters and seeds. A study in Germany found that for cities with rivers, the rivers were a secondary dispersal agent that moved wind-dispersed tree-of-heaven seed that landed in rivers from urban to distant rural areas [261]. Tree-of-heaven seeds dispersed in and along the Monogahela River of West Virginia showed germination rates similar to those of seeds on land (94% germination, P=0.006). Seeds from trees growing near water were most likely to land in and be transported directly by water; water-borne seeds stayed buoyant about 1.5 days [157]. In a pine-oak community in West Virginia, Marsh [196] observed that tree-of-heaven established near roads and on tree harvest sites; machinery may have helped disperse seeds onto these sites.
Tree-of-heaven produces many small, light seeds [77,92]. In a mixed-hardwood forest in Connecticut, tree-of-heaven averaged 4.84 inches (12.3 cm) DBH in size at first seed production. It had greatest average seed production of 10 overstory trees across 2 years; its seed production was 40 times that of the next highest-producing species. For parent trees of 12 inches (30 cm) DBH, tree-of-heaven produced means of 400,000 seeds in 1994 and 2 million seeds in 1995. Tree-of-heaven and white ash had the longest seed dispersal distances of the 10 trees, but most tree-of-heaven seeds fell within 20 feet (5 m) of the parent trunk. Tree-of-heaven seed rain spiked at 2,500 seeds/m²/12-inch DBH parent tree, showing greatest seed output of all 10 tree species. The authors concluded that tree-of-heaven is "exceptionally fecund even in competitive, closed-canopy forest stands" [198].
Flower, fruit, and seed production begins early in development. Six-week-old seedlings have flowered in the greenhouse [92], and 1-year-old seedlings and 2-year-old root sprouts have been observed in the field with fruit [148,163,275]. Trees in California produce viable seed by 10 years of age [148]. Heaviest seed production is from 12 to 20 years of age [211]. In France, which has a climate similar to California, 1.6- to 3.3-foot long (0.5-1 m) root sprouts produced seeds [30].
Mature female trees may produce several hundred flower clusters in a year. Hunter [148] reports that over 5 years, production of a female tree in Martinez, California, averaged 150, 183, 219, 439, and 56 clusters/year. An individual flower contains hundreds of seeds, so individual trees may produce >325,000 seeds/year [30,148]. Illick and Brouse [151] estimated that a small, 12-inch-diameter (30 cm) tree in Pennsylvania produced over a million seeds in 1 year. Most seeds are viable, even those that overwinter on the tree and disperse in spring [148,328]. Repeated top-kill reduces seed production [211].
Tree-of-heaven is mostly an early-successional species in forest ecosystems [257], and it is most common on disturbed sites throughout its North American range. Starting with a few stems along roadsides or woodland edges, tree-of-heaven may encroach into meadows, woodlands, and open forests [163,250,275]. In invades open eastern hardwood forests, sometimes sharing the canopy with native hardwoods into late succession [280]. In 1986, tree-of-heaven and princesstree were classified as the 2 most successful nonnative trees invading hardwood forests in the Northeast [227], where they often established after tree harvest or other disturbances [211,257]. Huebner [141] found tree-of-heaven was positively associated with either highly disturbed or urban counties in West Virginia (P<0.05). In mixed oak-pine forests on the Cumberland Plateau of Tennessee, tree-of-heaven occurred on early-seral sites disturbed by tornados or construction, on prescribed burns, and along roads, pipelines, and ditches [20]. It is an early-seral, subcanopy or canopy species characteristic of the black cherry-yellow-poplar-red maple-white ash forest association in the Delaware Water Gap National Recreation Area, Pennsylvania [242]. It sometimes codominates in successional black cherry-yellow-poplar-red maple-American ash forests in Valley Forge National Historical Park, North Carolina [246]. Tree-of-heaven is associated with eastern white pine/rhododendron (Rhododendron spp.) second growth on the Carl Sandberg Home National Historic Site, North Carolina [318].
Tree-of-heaven often establishes after logging [247]. It was most common on logged sites in oak-hickory forests on the Jefferson National Forest, especially on skid trails [47]. In another study in oak-hickory on the Jefferson National Forest, tree-of-heaven was most common on clearcut plots; less common on low-leave shelterwood plots; and least common on high-leave shelterwood plots, respectively [46]. On the George Washington National Forest, 71% of the sites where tree-of-heaven was found had previously been logged; the other sites were along roads [247]. Tree-of-heaven density was much greater in recently logged mixed-hardwood stands than mature forest stands in south-central Virginia; only yellow-poplar was more abundant on logged sites [49].
Tree-of-heaven density (stems/ha) in mixed-hardwood in Virginia [49] Recently logged stands Mature stands Seedlings 10,138 (6,692)* 320 (147) Saplings 318 (112)* 244 (212) *Significant difference between logged and mature stands (P≤0.05)).Tree-of-heaven may also establish in early-seral shrubfields [238] and old fields. In Rock Creek, Washington, DC, tree-of-heaven is a common component in an Allegheny blackberry-multiflora rose (Rubus allegheniensis-Rose multiflora) shrubland [289]. In Maryland, it grew with black locust and princesstree on a farm abandoned 14 years previously [21]. A review of studies conducted in the Hutcheson Memorial Forest Center, New Jersey—which has the largest set of old-field permanent plots established for the longest time in the United States—determined that tree-of-heaven typically established 2 to 4 years after field abandonment [221]. Tree-of-heaven may also occur in middle old-field succession. It was not reported in initial old-field succession in Pennsylvania, but was found in old fields that were abandoned approximately 20 years before. In year 20, hardwoods were forming a 23- to 39-foot (7-12 m) canopy over the herbs. Tree-of-heaven was common in these young hardwood stands [161].
Capacity for rapid growth, mycorrhizal associations [143], and—in eastern hardwood forests with dense white-tailed deer populations—relative unpalatability compared to associated hardwood species [99,166] may confer advantages to tree-of-heaven during early succession. A West Virginia study found mycorrhizal colonization of tree-of-heaven seedling roots was significantly greater on a steep, barren slope (62%) compared to a forest site adjacent to a stream (38%) (P<0.002). The authors suggested that tree-of-heaven established more successfully on open sites due to this trend [143].
Tree-of-heaven is moderately shade tolerant [16,47,108,196] and may persist into late forest succession, typically at low levels in the subcanopy until a canopy gap allows further invasion or expansion [47,166,173,328]. In urban oak-hickory woodlots of central Massachusetts, tree-of-heaven was present only on disturbed sites and did not invade intact woodlands [25]. Gaps created by storms [145,325], hemlock woolly adelgid [228] or gypsy moth defoliation, windstorms [145], or possibly fire [20] may facilitate tree-of-heaven invasion. In Nelson County, Virginia, tree-of-heaven occurred in 2 of 4 avalanche debris chutes surveyed 10 years after Hurricane Camille, but it did not occur in adjacent, undisturbed hardwood forest [145]. Xi and others [325] found tree-of-heaven was invasive following hurricanes in mixed oak-sweetgum piedmont forests of Duke Forest, North Carolina.
Once established in a gap, tree-of-heaven may grow into the forest canopy [166]. It occurs in mature upland hardwood forests of northern New Jersey (review by [60]). It also occurs in upland, mature mixed oak-pignut hickory (Carya glabra) forest in the Black Rock Forest of New York. However, it is most common on disturbed sites, especially near reservoirs [17]. On the Cumberland Plateau of Tennessee, tree-of-heaven was a minor species in gaps within a sugar maple-red maple-yellow-poplar forest. Tree-of-heaven density averaged 4.7 saplings/ha, which represented <0.1 % relative density of tree species in forest gaps [127]. In Ohio, tree-of-heaven apparently invaded canopy gaps in an old-growth slippery elm-white ash woodlot from adjacent secondary stands. Its density was similar in secondary and old-growth stands, although density decreased with distance from roads. In a 2002 survey, tree-of-heaven had increased in all size classes compared to a 1980 survey. Density of trees-of-heaven in the canopy had increased threefold, but subcanopy and smaller trees had the largest increases. Survivorship of sprouts from 2001 to 2002 averaged 42% [85].
Tree-of-heaven does not regenerate from seed under its own canopy [17,85,173]. The seedlings are intolerant of deep shade [34,99,118,119,173], and tree-of-heaven does not photosynthesize efficiently in shade [33,34] at any age. A West Virginia survey found 100% mortality of seedlings in the understory of an oak (Quercus spp.)-sugar maple forest [170]. Seed plantings in New Jersey showed best tree-of-heaven establishment in an open-grown herbaceous community, intermediate establishment in an eastern redcedar/little bluestem (Schizachyrium scoparium) woodland, and least establishment in a closed-canopy oak-hickory forest. Mortality rate was over 90% for tree-of-heaven seed planted under a closed-canopy oak-hickory forest [99]. In the Black Rock Forest of New York, tree-of-heaven likely established from seed after a blowdown and subsequent logging and herbicide spraying in the 1960s. By the early 2000s, an oak-hickory forest had redeveloped; tree-of-heaven was present but not reproducing under the canopy [17].
Tree-of-heaven is generally uncommon in closed-canopy, late-successional hardwood forests lacking gaps [25,34,328]. However, trees-of-heaven on forest edges may spread into the surrounding understory by root sprouts, which may grow slowly but persist with shade [132,148,173]. Without canopy-opening disturbance, under-canopy sprouts remain suppressed and grow slowly [118,148,173]. Tree-of-heaven was scarce in a midsuccessional, mixed oak-hickory forest in southern Illinois, with 2.5 stems/ha, relative dominance of 0.1%, and a low importance value of 0.4% [188].
Little information was available on successional patterns of tree-of-heaven where it is native. A study on coal mine spoils in Shanxi, China, found tree-of-heaven dominated late stages of spoil recovery, after the grassland and shrubland stages had succeeded to a tree-of-heaven woodland [189].Tree-of-heaven sprouts from the roots, root crown, and bole [93,145,163,185,211,275]. Although reproduction from seed is not rare, sprouting is its most common method of regeneration [163]. In Ithaca, New York, 58% of 1-year-old, excavated tree-of-heaven stems were root sprouts and 42% were seedlings [230]. Young trees that are cut to the root crown before bark becomes thick and corky often sprout from both the root crown and roots [163,275]. Bole damage promotes root, root crown, and bole sprouting [148,163]. Death or injury of the main stem usually results in prolific root sprouting [211]. Top-growth damage is not necessary for root sprouting to occur, however. Even as seedlings, trees-of-heaven produce horizontal roots capable of sprouting [45]. Except in the rose (Rosacea) [96,269] and willow (Salicaceae) (review by [327]) families, root sprouting without top damage is uncommon in woody species (reviews by [65,120]), but it is a powerful regeneration strategy for species employing it. Roots have more nutrient- and photosynthate-storing capacity than rhizomes, conferring better protection from aboveground disturbances such as fire [159,160] and show a stronger sprouting response after top-kill [65,120]. Tree-of-heaven sprouts are more likely to persist in low-light conditions, such as within a subcanopy, than are seedlings [85].
With tree-of-heaven's spreading root system, root sprouts may appear as far as 50 to 90 feet (15-27 m) from the parent stem [151,163,275]. During drought, tree-of-heaven translocates stem water into roots and begins stem die-back. Die-back may be extensive during extended droughts, but tree-of-heaven typically survives drought by sprouting from the roots when there is sufficient water to support new growth. Sprouting after frost die-back is common in tree-of-heaven's northern limits [93].
On the Himalayan foothills of India, trees-of-heaven with root crown girths between 12 and 16 inches (30 and 40 cm) showed greatest root sprout production following road construction. Trees in the largest-diameter class did not produce sprouts [179].
Ailanthus altissima /eɪˈlænθəs ælˈtɪsɪmə/,[3] commonly known as tree of heaven, ailanthus, varnish tree, or in Chinese as chouchun (Chinese: 臭椿; pinyin: chòuchūn), is a deciduous tree in the family Simaroubaceae.[1] It is native to northeast and 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 15 metres (49 ft) in 25 years. While the species rarely lives more than 50 years, some specimens exceed 100 years of age.[4] Its suckering ability allows this tree to clone itself indefinitely.[5] It is considered a noxious weed and vigorous invasive species,[1] and one of the worst invasive plant species in Europe and North America.[6] In 21st-century North America, the invasiveness of the species has been compounded by its harboring of the also destructive and invasive spotted lanternfly.[7][8]
Ailanthus altissima is a medium-sized tree that reaches heights between 17 and 27 m (56 and 89 ft) with a diameter at breast height of about 1 m (3 ft 3 in).[9] 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.[10] 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,[11] or rotting cashews.[12]
The leaves are large, odd- or even-pinnately compound on the stem. They range in size from 30 to 90 centimetres (1 to 3 ft) 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.[13] 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 5–18 cm (2–7 in) long and 2.5–5 cm (1–2 in) wide. They have a long, tapering end, while the bases have two to four teeth, each containing one or more glands at the tip.[10] The leaflets' upper sides are dark green in color with light green veins, while the undersides are a more whitish green. The petioles are 5–12 millimetres (0.2–0.5 in) long.[11] The lobed bases and glands distinguish it from similar sumac species.
The flowers are small and appear in large panicles up to 50 cm (20 in) in length at the end of new shoots. The individual flowers are yellowish green to reddish in color, each with five petals and sepals.[9][11] 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.[10][14][15] 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 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.[10][14] 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.[10] The fruits grow in clusters; similar to the ash tree (Fraxinus excelsior), the fruits ripen to a bright reddish-brown color in September.[16] A fruit cluster may contain hundreds of seeds.[6] The seeds borne on the female trees are 5 mm (0.2 in) in diameter and each is encapsulated in a samara that is 2.5 cm (1 in) long and 1 cm (0.4 in) 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,[9][11] and aiding buoyancy for long-distance dispersal through hydrochory.[17] 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.[18] The females can produce huge numbers of seeds, normally around 30,000 per kg,[9] and fecundity can be estimated nondestructively through measurements of diameter at chest height.[17]
In China, the tree of heaven has a long and rich history. It was 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.[1] 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 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 considered internationally as a noxious weed.[1] In many countries, it is an invasive species due to its ability both to colonize disturbed areas quickly and to suppress competition with allelopathic chemicals.[1] The tree also resprouts vigorously when cut, making its eradication difficult and time-consuming. This has led to the tree being called "tree of hell" among gardeners and conservationists.[19]
The first scientific descriptions of the tree of heaven were made shortly after it was introduced to Europe by French Jesuit Pierre Nicholas d'Incarville, 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.[10]
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.[10]
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, now A. triphysa). The name is derived from the Ambonese word ailanto, meaning "heaven-tree" or "tree reaching for the sky".[10][20] 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.[10] 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.[21] Altissima is Latin for "tallest",[22] and refers to the sizes the tree can reach. The plant is sometimes incorrectly cited with the specific epithet in the masculine (glandulosus or altissimus), which is incorrect since botanical, like Classical Latin, treats most tree names as feminine.
The three varieties of A. altissima are:
Ailanthus altissima is native to northern and central China,[1] Taiwan[27] and northern Korea.[28] It was historically widely distributed, and the fossil record indicates clearly that it was present in North America as recently as the middle Miocene.[29] In Taiwan it is present as var. takanai.[26] In China it is native to every province except Gansu, Heilongjiang, Hainan, Jilin, Ningxia, Qinghai, Xinjiang. It is also not found in Tibet.[23] It has been introduced in many regions across the world, and is now found on every continent except Antarctica.[1][19]
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.[9] In China, it is often found in limestone-rich areas.[24] The tree of heaven is found within a wide range of climatic conditions.[9] In its native range, it is found at high altitudes in Taiwan[26] and lower ones in mainland China.[10] These are virtually found anywhere in the U.S., but especially 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, forming dense thickets that displace native plants.[1] Prolonged cold and snow cover cause dieback, although the trees resprout from the roots.[9]
The earliest introductions of A. altissima to countries outside of its native range were to the southern areas of Korea and to Japan. The tree may be native to these areas, but the tree is generally agreed to be a very early introduction.[30] Within China, it has also been naturalized beyond its native range in areas such as Qinghai, Ningxia, and Xinjiang.[24]
In 1784, not long after Jussieu had sent seeds to England, some were forwarded to the United States by William Hamilton, a gardener in Philadelphia. In both Europe and America, it quickly became a favoured ornamental, especially as a street tree, and by 1840, it was available in most nurseries.[10][20] The tree was separately brought to California in the 1890s by Chinese immigrants who came during the California Gold Rush. It has escaped cultivation in all areas where it was introduced, but most extensively in the United States.[27] It has naturalized across much of Europe, including Germany,[31] Austria, Switzerland, the Czech Republic, the Pannonian region (i.e. southeastern Central Europe around the Danube River basin from Austria, Slovakia, and Hungary south to the Balkan ranges) and most countries of the Mediterranean Basin.[28] In Montenegro[32] and Albania[33][34] A. altissima is widespread in both rural and urban areas, and while in the first it was introduced as an ornamental plant, it very soon invaded native ecosystems with disastrous results and became an invasive species.[32] Ailanthus has also been introduced to Argentina,[27] Australia (where it is a declared weed in New South Wales and Victoria),[35] New Zealand (where it is listed under the National Pest Plant Accord and is classed an "unwanted organism"),[36] the Middle East, and in some countries in South Asia such as Pakistan.[37] In South Africa, it is listed as an invasive species that must be controlled, or removed and destroyed.[38]
In North America, A. altissima is present from Massachusetts in the east, west to southern Ontario, southwest to Iowa, south to Texas, and east to the north of Florida. In the west, it is found from New Mexico west to California and north to Washington.[9][27] In the east of its range, it grows most extensively in disturbed areas of cities, where it was long ago present as a planted street tree.[10][27] It also grows along roads and railways. For example, a 2003 study in North Carolina found the tree of heaven was present on 1.7% of all highway and railroad edges in the state, and had been expanding its range at the rate of 4.76% counties per year.[39] Similarly, another study conducted in southwestern Virginia determined that the tree of heaven is thriving along roughly 30% of the state's interstate highway system length or mileage.[40] It sometimes enters undisturbed areas, as well, and competes with native plants.[27] In western North America, it is most common in mountainous areas around old dwellings and abandoned mining operations.[41][42] It is classified as a noxious or invasive plant on National Forest System lands and in many states[43] because its prolific seed production, high germination rate, and capacity to regrow from roots and root fragments enable A. altissima[44] to out-compete native species. For this reason, control measures on public lands[45] and private property[46] are advised where A. altissima has naturalized.
The tree of heaven 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.[1][9] It is considered a shade-intolerant tree and cannot compete in low-light situations,[47] though it is sometimes found competing with hardwoods, but such competition rather indicates it was present at the time the stand was established.[9] 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 2-15% of full sun was available. The same study characterised the tree as using a "gap-obligate" strategy to reach the forest canopy, meaning it grows rapidly during a very short period rather than growing slowly over a long period.[48] It is a short lived tree in any location and rarely lives more than 50 years.[9] 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.[27]
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 (approximately that of tomato juice). 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.[27] 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.[10] 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.[27] The roots are poisonous to people.[49]
Ailanthus produces an allelopathic chemical called ailanthone, which inhibits the growth of other plants.[50] 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 (Lepidium sativum) seeds from germinating. The same study tested the extract as an herbicide on garden cress, redroot pigweed (Amaranthus retroflexus), velvetleaf (Abutilon theophrasti), yellow bristlegrass (Setaria pumila), barnyard grass (Echinochloa crusgalli), pea (Pisum sativum cv. Sugar Snap), and maize (Zea mays cv. Silver Queen). It proved able to kill nearly 100% of seedlings with the exception of velvetleaf, which showed some resistance.[51] 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 (Fraxinus americana) being the only plant not adversely affected.[52] The chemical does not, however, affect the tree of heaven's own seedlings, indicating that A. altissima has a defence mechanism to prevent autotoxicity.[50] 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.[53]
The tree of heaven is a very rapidly growing tree, possibly the fastest growing tree in North America.[43] Growth of one to two metres (3 ft 3 in to 6 ft 7 in) 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.[43]
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 colonize areas of rubble of destroyed buildings where most other plants would not grow.[28] In addition, the warmer microclimate in cities offers a more suitable habitat than the surrounding rural areas; it is thought that the tree requires a mean annual temperature of 8 °C (46 °F) to grow well, which limits its spread in more northern and higher altitude 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.[28] In other areas of Europe this is not the case as climates are mild enough for the tree to flourish. It has colonized natural areas in Hungary, for example, and is considered a threat to biodiversity at that country's Aggtelek National Park.[28]
Several species of Lepidoptera use the leaves of Ailanthus as food, including the Indian moon moth (Actias selene) and the common grass yellow (Eurema hecabe). In North America the tree is the host plant for the ailanthus webworm (Atteva aurea), though this ermine moth is native to Central and South America and originally used other members of the mostly tropical Simaroubaceae as its hosts.[54] In the US, it has been found to host the brown marmorated stink bug and the Asiatic shot-hole borer.[19] The spotted lanternfly (L. delicatula), relies on the metabolites of A. altissima for the completion of its life cycle and the pervasiveness of A. altissima is seen as a driving factor in L. delicatula's invasive spread outside of China.[55][56] In its native range A. altissima is associated with at least 32 species of arthropods and 13 species of fungi.[24]
In North America, the leaves of ailanthus are sometimes attacked by Aculops ailanthii, a mite in the family Eriophyidae. Leaves infested by the mite begin to curl and become glossy, reducing their ability to function. Therefore, this species has been proposed as a possible biocontrol for ailanthus in the Americas.[57] Research from September 2020 indicates a verticillium wilt, caused by Verticillium nonalfalfae, may function as a biological control for A. altissima,[58] with the weevil Eucryptorrhynchus brandti serving as a vector.[59]
Due to the tree of heaven's weedy habit, landowners and other organisations often resort to various methods of control to keep its populations in check. For example, the city of Basel in Switzerland has an eradication program for the tree.[28] 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 to kill existing trees, while either hand pulling or mowing seedlings to prevent new growth.[60][note 1]
In addition to its use as an ornamental plant, the tree of heaven is also used for its wood 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.[1] 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 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.[10]
The pale yellow, close-grained, and satiny wood of ailanthus has been used in cabinet work.[1][61] 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 in eastern China is most famous for producing these steamers.[10] The plant is also considered a good source of firewood across much of its range, as it is moderately hard and heavy, yet readily available.[62] The wood is also used to make charcoal for culinary purposes.[63] However, there are problems with using the wood as lumber; 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.[64]
Tree of heaven is a popular ornamental tree in China and valued for its tolerance of difficult growing conditions.[24] It was once very popular in cultivation in both Europe and North America, but this popularity dropped, especially in the United States, due to the disagreeable odor of its blossoms and the weediness of its habit. The problem of odor was previously avoided by only selling pistillate plants since only males produce the smell, but a higher seed production also results.[20] Michael Dirr, a noted American horticulturalist and professor at the University of Georgia, reported meeting, in 1982, a grower who could not find any buyers. He further writes (his emphasis):
For most landscaping conditions, it has no value as there are too many trees of superior quality; for impossible conditions this tree has a place; selection could be made for good habit, strong wood and better foliage which would make the tree more satisfactory; I once talked with an architect who tried to buy Ailanthus for use along polluted highways but could not find an adequate supply [...]
— Michael A. Dirr, Manual of Woody Landscape Plants[65]
In Europe, however, the tree is still used in the garden to some degree as its habit is generally not as invasive as it is in America. In the United Kingdom it is especially common in London squares, streets, and parks, though it is also frequently found in gardens of southern England and East Anglia. It becomes rare in the north, occurring only infrequently in southern Scotland. It is also rare in Ireland.[66] In Germany the tree is commonly planted in gardens.[31] The tree has furthermore become unpopular in cultivation in the west because it is short-lived and that the trunk soon becomes hollow, making trees more than two feet in diameter unstable in high winds.[61]
A few cultivars exist, but they are not often sold outside of China and probably not at all in North America:
Nearly every part of A. altissima has had various uses in Chinese traditional medicine,[10] although there is no high-quality clinical evidence that it has an effect on any disease.
A tincture of the root bark was thought useful by American herbalists in the 19th century.[14] It contains phytochemicals, such as quassin and saponin, and ailanthone.[69] The plant may be mildly toxic.[1] The noxious odours have been associated with nausea and headaches, and with contact dermatitis reported in both humans and sheep, which developed weakness and paralysis. It contains a quinone irritant, 2,6-dimethoxybenzoquinone, as well as quassinoids.[69]
In addition to the tree of heaven's various uses, it has also been a part of Chinese culture for many centuries and has more recently attained a similar status in the west. Within the oldest extant Chinese dictionary, the Erya, written in the 3rd century BCE, the tree of heaven is mentioned second among a list of trees. It was mentioned again in a materia medica compiled during the Tang dynasty in 656 CE. Each work favoured a different character, however, and there is still some debate in the Chinese botanical community as to which character should be used. The current name, chouchun (Chinese: 臭椿; pinyin: chòuchūn), means "stinking tree", and is a relatively new appellation. People living near the lower Yellow River know it by the name chunshu (simplified Chinese: 椿树; traditional Chinese: 椿樹; pinyin: chūnshù), meaning "spring tree". The name stems from the fact that A. altissima is one of the last trees to come out of dormancy, and as such its leaves coming out would indicate that winter was truly over.[10]
In Chinese literature, ailanthus is often used for two rather extreme metaphors, with a mature tree representing a father and a stump being a spoiled child. This manifests itself occasionally when expressing best wishes to a friend's father and mother in a letter, where one can write "wishing your ailanthus and daylily are strong and happy", with ailanthus metaphorically referring to the father and daylily to the mother. Furthermore, one can scold a child by calling him a "good-for-nothing ailanthus stump sprout", meaning the child is irresponsible. This derives from the literature of Zhuangzi, a Taoist philosopher, who referred to a tree that had developed from a sprout at the stump and was thus unsuitable for carpentry due to its irregular shape. Later scholars associated this tree with ailanthus and applied the metaphor to children who, like stump sprouts of the tree, will not develop into a worthwhile human being if they don't follow rules or traditions.[70]
The 1943 book A Tree Grows in Brooklyn by Betty Smith uses the tree of heaven as its central metaphor, using it as an analogy for the ability to thrive in a difficult environment. At that time as well as now, ailanthus was common in neglected urban areas.[20][71] She writes:
There's a tree that grows in Brooklyn. Some people call it the Tree of Heaven. No matter where its seed falls, it makes a tree which struggles to reach the sky. It grows in boarded up lots and out of neglected rubbish heaps. It grows up out of cellar gratings. It is the only tree that grows out of cement. It grows lushly...survives without sun, water, and seemingly earth. It would be considered beautiful except that there are too many of it.
— A Tree Grows in Brooklyn, Introduction
In William Faulkner's novel, Sanctuary, a "heaven-tree" stands outside the Jefferson jail, where Lee Goodwin and a "negro murderer" are incarcerated. The tree is associated with the black prisoner's despair in the face of his impending execution and the spirituals that he sings in chorus with other black people who keep a sort of vigil in the street below:
...they sang spirituals while white people slowed and stopped in the leafed darkness that was almost summer, to listen to those who were sure to die and him who was already dead singing about heaven and being tired; or perhaps in the interval between songs a rich, sourceless voice coming out of the high darkness where the ragged shadow of the heaven-tree which snooded the street lamp at the corner fretted and mourned: "Fo days mo! Den dey ghy stroy de bes ba'yton singer in nawth Mississippi!"[72] Upon the barred and slitted wall the splotched shadow of the heaven-tree shuddered and pulsed monstrously in scarce any wind; rich and sad, the singing fell behind.[73]
In the 2013 book Teardown: Memoir of a Vanishing City by Gordon Young, the tree is referenced in a description of the Carriage Town neighborhood in Flint, Michigan.
Festive Victorian-era homes in various stages of restoration battled for supremacy with boarded-up firetraps and overgrown lots landscaped with weeds, garbage, and "ghetto palms," a particularly hardy invasive species known more formally as Ailanthus altissima, or the tree of heaven, perhaps because only God can kill the things. Around the corner, business was brisk at a drug house where residents and customers alike weren't above casually taking a piss in the driveway.[74]
Ailanthus is also sometimes counter-nicknamed "tree from hell" due to its prolific invasiveness and the difficulty in eradicating it.[71][75] In certain parts of the United States, the species has been nicknamed the "ghetto palm" because of its propensity for growing in the inhospitable conditions of urban areas, or on abandoned and poorly maintained properties, such as in war-torn Afghanistan.[76][77]
Until 26 March 2008, a 60-foot-tall (18 m) member of the species was a prominent "centerpiece" of the sculpture garden at the Noguchi Museum in the Astoria section in the borough of Queens in New York City. The tree had been spared by the sculptor Isamu Noguchi when in 1975 he bought the building which would become the museum and cleaned up its back lot. The tree was the only one he left in the yard, and the staff would eat lunch with Noguchi under it. "[I]n a sense, the sculpture garden was designed around the tree", said a former aide to Noguchi, Bonnie Rychlak, who later became the museum curator. By 2008, the old tree was found to be dying and in danger of crashing into the building, which was about to undergo a major renovation. The museum hired the Detroit Tree of Heaven Woodshop, an artists' collective, to use the wood to create benches, sculptures and other amenities in and around the building. The tree's rings were counted, revealing its age to be 75, and museum officials hoped it would regenerate from a sucker.[78]
Ingo Vetter, a German artist and professor of fine arts at Umeå University in Sweden, was influenced by the idea of the "ghetto palm" and installed a living ailanthus tree taken from Detroit for an international art show called Shrinking Cities at the Kunst-Werke Institute for Contemporary Art in Berlin in 2004.[76][77]
Ailanthus altissima /eɪˈlænθəs ælˈtɪsɪmə/, commonly known as tree of heaven, ailanthus, varnish tree, or in Chinese as chouchun (Chinese: 臭椿; pinyin: chòuchūn), is a deciduous tree in the family Simaroubaceae. It is native to northeast and 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 15 metres (49 ft) in 25 years. While the species rarely lives more than 50 years, some specimens exceed 100 years of age. Its suckering ability allows this tree to clone itself indefinitely. 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 harboring of the also destructive and invasive spotted lanternfly.
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