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Albizia julibrissin

• INTRODUCTORY
• DISTRIBUTION AND OCCURRENCE
• BIOLOGICAL DATA AND HABITAT REQUIREMENTS
• FIRE EFFECTS AND MANAGEMENT
• APPENDIX: FIRE REGIME TABLE
• REFERENCES

INTRODUCTORY

AUTHORSHIP AND CITATION FEIS ABBREVIATION NRCS PLANT CODE COMMON NAMES TAXONOMY SYNONYMS LIFE FORM FEDERAL LEGAL STATUS OTHER STATUS
	© John Scheper, Floridata.com

AUTHORSHIP AND CITATION:
Meyer, Rachelle. 2010. Albizia julibrissin. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: https://www.fs.usda.gov/database/feis/plants/tree/albjul/all.html [ ].

Updates: On 30 January 2018, the common name of this species was changed in FEIS from: mimosa to: silktree.

FEIS ABBREVIATION:
ALBJUL

NRCS PLANT CODE [119]
ALJU

COMMON NAMES:
silktree
mimosa
silky acacia

TAXONOMY:
The scientific name of silktree is Albizia julibrissin Durazz (Fabaceae) [39,40,61,123,128]. It is fairly common to find the genus spelled Albizzia [31,32,63,92,115]. Although not the original spelling [56,73], it is likely used because the genus is named in honor of Fillippo delgi Albizzia, who introduced silktree to Tuscany, Italy [20,73]. Some systematists include silktree in the Mimosaceae family [7].

Infrataxa:
Albizia julibrissin var. rosea (Carr.) Mouillef, Hardy silk-tree albizia
Albizia julibrissin var. mollis Benth, Abyssinia silk-tree albizia [123]

These infrataxa are rarely distinguished in the literature and are not referred to in this review.

DISTRIBUTION AND OCCURRENCE

GENERAL DISTRIBUTION HABITAT TYPES AND PLANT COMMUNITIES
	Silktree on a disturbed site near the Tallapoosa River, Alabama. Photo © James H. Miller, USDA Forest Service.

Silktree is native to Asia [26,30,40,54,56,83,123,128,131], occurring from Iran to Japan [9,20,41,74,104]. It is often asserted that silktree was introduced to the United States as an ornamental in 1745 [8,9,25,83,111,123]. However, according to Cothran [20], it was brought to North America about 1785 and was first offered for sale in 1807. By the 1950s silktree was established locally in Georgia [29]. In 1972 it was a new record in Oconee County, South Carolina [38]. By 1992 silktree was considered common in disturbed areas of the Chauga River Gorge in Oconee County [114]. It was first reported in the flora of Illinois from 1956 to 1978 [48]. Silktree was described as "newly documented" in the Washington, DC area in 1995 [33], and, according to Connelly [19], was first reported in the flora of Connecticut in 2008. A 1994 guide to plants of Butte County, California, lists silktree as occurring in north-central Sacramento Valley and southern portions of the Cascade Range [95]. The distribution and impacts of silktree are best documented in the southeastern United States. Based on Southern Forest Inventory and Analysis data collected from 2001 to 2008, silktree is most common in north-central Alabama [86].

Silktree distribution. Map courtesy of USDA, NRCS. 2018. The PLANTS Database. National Plant Data Team, Greensboro, NC [119] [2018, January 30].

Silktree appears to be most common in disturbed communities. It is noted in oak-hickory (Quercus-Carya spp.), pine (Pinus), mixed pine-hardwood, riparian forests, and grasslands.

Silktree occurs in oak-hickory, pine, and mixed pine-hardwood communities in the Southeast. Silktree was considered a potentially high threat to oak-hickory woodlands but its threat status in pine habitats was unknown [109]. In Tennessee, it occurred infrequently in oak-hickory upland woods [59] and was present in mature, second-growth oak-hickory forest [110]. It occurred at low density in the sapling layer of an old-growth, longleaf pine (P. palustris) forest in Alabama where fire had been excluded for at least 45 years [67,121]. In Great Smoky Mountains National Park in Tennessee, silktree was often associated with Virginia pine (P. virginiana). Several other tree and shrub species commonly associated with silktree in the park are listed by Baron and others [8]. A few silktree seedlings occurred in a loblolly pine (P. taeda) plantation in Georgia [31]. It occurred infrequently in a Florida forest dominated by sand post oak (Q. margarettiae), turkey oak (Q. laevis), and longleaf pine or slash pine (P. elliottii) [49] and was a minor component in mixed pine-hardwood forests near Macon, Georgia [125]. The understory of a recently thinned, 30-year-old loblolly pine stand in the Piedmont region of South Carolina consisted of honey-locust (Gleditsia triacanthos), black cherry (Prunus serotina), and silktree [32]. It was one of 19 overstory species, although it occurred at low density and near the edge, in a forest dominated by sweetgum (Liquidambar styraciflua), loblolly pine, and red maple (Acer rubrum) near Virginia's Atlantic coast [90].

In north-central California, silktree was listed as occupying foothill woodland communities [95]; given the location, the vegetation was likely dominated by oaks.

Silktree often occurs in riparian areas and floodplain communities. It has been reported in these habitats in Maryland [102], Washington, DC [33], Tennessee [6,8], and North Carolina. In North Carolina silktree was reported in riparian areas with sycamore (Platanus occidentalis), sweetgum, yellow-poplar [91,122], red maple, and several oak and hickory species [122]. In a wetland created in Tuscaloosa, Alabama, it occurred with loblolly pine, willow (Salix sp.), and saplings of red maple and sweetgum [47]. Silktree was found around springs and in sinkholes in upland woodlands of Florida [49] and occurred in riparian woodlands of north-central California [95]. In a constructed wetland in New Jersey, it occurred in an area dominated by marsh species such as marsh seedbox (Ludwigia palustris), purple loosestrife (Lythrum salicaria), yellowseed false pimpernel (Lindernia dubia), and common rush (Juncus effusus) [72]. Silktree has been reported in riverbank communities in subtropical forests in the foothills of Garhwal Himalayas, India, part of silktree's native range [65].

The extent to which silktree can establish in grasslands is unclear. A review notes its occurrence in grasslands [23]. In Kentucky, silktree occurred in the ecotone between an oak-hickory forest and a cool-season grassland that established following logging [113], but Stocker and Hupp [109] note that it is not invasive in grasslands.

Silktree is frequent in disturbed communities such as those found along roadsides and in old fields. For information on disturbed sites where it occurs, see Successional Status. See the table below for species that occur with silktree in study areas that have experienced light to severe disturbance.

BOTANICAL AND ECOLOGICAL CHARACTERISTICS

GENERAL BOTANICAL CHARACTERISTICS SEASONAL DEVELOPMENT REGENERATION PROCESSES SITE CHARACTERISTICS SUCCESSIONAL STATUS
	Silktree flowerhead. Photo by Dan Tenaglia, missouriplants.com

Botanical description Raunkiaer life form
	Silktree leaves and fruit. Chuck Bargeron, University of Georgia, Bugwood.org

Botanical description: This description provides characteristics that may be relevant to fire ecology and is not meant for identification. Keys for identification are available (e.g., [39,40,123,128]).

Aboveground description: Silktree is a deciduous [40], nitrogen-fixing [83,89,111,126] tree or shrub [61,126] with thin [9,83], nearly smooth [9,74,83] bark. It has a broad crown [30,39,40,54,74,123] and single or multiple [9,83], short trunks [40,126]. It ranges from 10 to 50 feet (3-15 m) tall [9,20,30,39,83,123,128]. The champion silktree, last measured in 2006, was 64 feet (20 m) tall and had an 80.4-foot (24.5 m) spread. Its circumference at 3 feet (1 m) above ground was 103.2 inches (262 cm) [2]. Average circumference of silktree in the subtropical forests in the foothills of Garhwal Himalayas was 76.9 inches (195.2 cm) [65]. Silktree has alternate leaves, 4 to 20 inches (15-38 cm) long [9,39,40,74,83,123,126] and up to 6 inches (15 cm) wide [40,126]. The leaves are bipinnately compound [9,20,30,40,83,123,128] and move in reaction to light [26,30] and touch [26]. The oblong leaflets [39,74,123,126] are 5 to 15 mm long [40,74,83,126,128] and 2 to 5 mm wide [30,126]. The showy flowerheads [9,126] occur in clusters [26,83] at the ends of branches [9,39,83,126]. Each head has 15 to 25 sessile flowers [9,83] from 1 to 2 inches (2.5-6 cm) long [9,30,74,83]. Silktree's fruits are flattened legumes [26,74,83,123,126,128] from 3 to 8 inches (8-20 cm) long [20,26,39,83,123,128] and 0.6 to 1.2 inches (1.5-3 cm) wide [39,40,54,123,126,128]. They contain 5 to 16 seeds [41,83] that are about 6 to 12 mm long, half as wide [9,40,123], and have hard seed coats [126].

Life span, population dynamics, and stand structure: Silktree is apparently short lived in the United States [9,26,40,96], due to a soil-borne fungus (Fusarium) that infects the root system and causes wilting and eventually death (DeWolf 1968 as cited in [96]). Researchers studying silktree populations in and around Athens, Georgia, suggest that in the southeastern United States, silktree is characterized by frequent colonization and extinction of local populations, and that silktree often senesces within 10 to 20 years. However, they indicate that thinning and extinction of silktree populations only occurs on very shady sites or after the canopy closes [96]. Baron and others [8] report an average life span of around 30 years. The minimum age of silktree reported in Korea ranges from 30 to 45 years or more [53].

Silktree may form dense, even-aged stands in some areas. Sites that are greater than about 650 feet (200 m) from mature silktree trees appear to be colonized by 1 or a few individuals. Once these individuals flower and produce seed, the site becomes populated by even-aged recruits. Sites with many mature trees nearby are generally just colonized by dense, even-aged stands [96]. A field survey in Great Smokey Mountains National Park in the summer of 1975 found silktree on 87 sites, 16 of which included mature trees or clumps of mature trees. Two of these sites had a large number of saplings. The remaining 71 sites included only seedlings, with as many as 1,000 seedlings in the same vicinity [8].

Fruits first appear in June [83] and mature from August [9,54] to November [96,98,129]. According to Parrotta and others [98], seeds disperse from September to November. A description of silktree notes that the legumes split open in winter [83]. Fruits may remain on the tree into winter [9,20,83] or spring [54].

In a greenhouse experiment silktree stopped growing when daylight hours were short and grew vigorously with daylight of 14 hours or longer [92].

• Pollination and breeding system
• Seed production
• Seed dispersal
• Seed banking
• Germination
• Seedling establishment and plant growth
• Vegetative regeneration

Pollination and breeding system: Silktree is monoecious or andromonoecious and pollinated by insects [53]. Flowers are visited by bees, butterflies [41,61], and hummingbirds [41,54,61]. It is not clear whether silktree flowers are perfect [41,123] or if the apical flower of each head is perfect and the rest are staminate [40]. Pardini and Hamrick [97] cite Elias (1980) as describing silktree flowers as andromonoecious, but disagree based on observations of individual inflorescences commonly producing 3 to 9 fruits.

Silktree is self-incompatible [41,53]. Inbreeding occurs occasionally [53]. For information on pollen donor variability, see Irwin and others [55] and Pardini and Hamrick [97]. Pardini and Hamrick [96] provide information on spatial genetic structure of silktree populations.

Seed production: Observations by Pardini and Hamrick [96] suggest that silktree begins producing seed at an early age. Several authors note that silktree produces many seeds [8,9,25,83,96,104,126], and one source reports that it produces 8,000 seeds/year (Wick and Walters 1974 cited in [89]).

Seed dispersal: Silktree seeds are primarily dispersed by gravity [96] and secondarily by wind, water, and animals. Several sources suggest that the thin, papery legumes are wind dispersed [41,90,96,105]. Observations in Georgia suggest that wind can carry silktree seeds at least 300 feet (~90 m) [96]. Seeds may travel long distances in high winds [98]. An experiment to determine the wind-dispersal capability of several species found that silktree legumes could disperse from the parent tree in a 6 mile (10 km) per hour horizontal breeze [78]. Reviews suggest that silktree seeds are also transported by water [9,83] and animals [83]. Silktree seeds may also be dispersed in contaminated fill dirt [9].

Seed banking: Several authors note that silktree seeds have a hard, impermeable seed coat [31,129] that requires scarification to break dormancy (see Germination, below), suggesting that they may remain viable in the soil for long periods. However, seed bank studies were lacking as of 2010. Several reviews cite studies that found a small percentage of silktree seeds kept in dry storage remained viable for 70 [21] to nearly 150 years [22,93,115]. A study by Wick and Walters (1974 cited by [9,98,129]) found 90% viability of silktree seeds stored in loosely corked bottles for 5 years. A flora from the southeastern United States reports that silktree seeds remain viable for "several years" in soil [30]. Silktree seed was not detected in soil samples taken from a forested site where it occurred in the overstory at low density near the edge [90].

Germination: Silktree seeds are dormant due to their hard seed coat and must be scarified to germinate [27]. Exposure to heat [42,99] and fungi [42], soaking in water [89,98] or acid [27], or other damage to the seed coat can break silktree seed dormancy. For more information on the effects of heat on germination rates, see Fire adaptations and plant response to fire. The presence of fungi in soil resulted in a 30.1% germination rate in silktree seeds that had not been previously scarified, significantly (P<0.05) greater than the 11.4% germination rate of unscarified seeds planted in sterilized soil [42]. Soaking in water resulted in 79.6% of silktree seeds germinating over a 4-year period [89]. Manual snipping of part of the seed coat resulted in 93% germination after 7 days in one laboratory study [99] and 90% germination after an unspecified time in another [44]. Mechanical scarification resulted in germination rates from 89.7% to 98.4% [42]. Other methods for stimulating silktree germination include hot water [99] and sulfuric acid [10,98,129]. Silktree seed germination following 8 months at 54 °F (12 °C) was only 2.5% [89], suggesting that cool temperatures do not promote silktree germination. According to a flora of Texas, generally one-quarter to one-third of silktree seeds germinate [123].

According to the Woody Plant Seed Manual, planting seeds no more than 1 inch (2 cm) deep in loose moist soil in full sunlight favors silktree germination [98].

Seedling establishment and plant growth: Under appropriate conditions silktree seedlings may have high survival rates. Silktree survival on acid surface-mine spoils in Kentucky averaged 84% [100]. No mortality was observed in silktrees planted on a landfill in South Korea [63]. In a trial to determine silktree's potential as domestic goat forage, 96.5% of silktree seedlings planted in experimental plots in March survived their first year [1]. However, in a test to determine silktree's usefulness as livestock forage in the Louisiana coastal plain, very few seedlings established during a period with below-average precipitation [99]. Due to the rarity of silktree saplings (>4 inches (10 cm) tall) in a 1975 field survey of Great Smoky Mountains National Park, the authors suggested that few silktree seedlings survive their first year, except in open areas and along roads, mostly likely due to mowing and shading from forest trees [8].

Increased cover of silktree following prescribed fire in western Tennessee was likely due to silktree seedlings establishing from the soil seed bank [110]. See Plant response to fire for details of this study.

Circumstantial evidence suggests that silktree seedling establishment is aided by disturbance (e.g., [8,77,102]). For example, silktree occurred on loosened skid trails and was among the dominant species in compacted skid trails 7 months after selective harvesting in an oak-hickory forest at Oak Ridge, Tennessee. Silktree did not occur in similar undisturbed microsites in this study [77]. See Successional Status for more information on the association of silktree with disturbed areas.

Silktree grows quickly under favorable site conditions [9,26,98,111,126]. One-year-old, wilt-resistant clones were 2 to 4 feet (0.6-1.2 m) tall when planted in a North Carolina pasture. During the first growing season, they grew 14 mm/day and increased in height by 91%; in 6 years, they grew to 20 feet (6 m) tall and 6 inches (15 cm) in diameter [1]. On a landfill in South Korea, silktree reached an average height of 8.2 feet (2.49 m) after 3.6 years [63]. Silktree in acid surface-mine spoils in Kentucky grew to an average height of 4.9 feet (1.5 m) over 4 years [100]. Silktree grew well with long photoperiods in a greenhouse [92]. Experimental exposure to sassafras (Sassafras albidum) leachates reduced silktree root growth [36].

Vegetative regeneration: Several reviews note silktree's ability to reproduce vegetatively [104,111] by sprouting from roots [8,9,27]. A review notes that silktree colonies form from root sprouts [83].

Silktree sprouting after cutting or damage is commonly reported [9,25,111,126]. Silktree trees killed by Fusarium wilt disease may create a mass of root sprouts. In some areas of Georgia and Tennessee, roadside silktrees survive only as root sprouts, as the main portion of the tree has been killed by the wilt disease [27]. Sprouts may grow over 3 feet (1 m) in a single season [9,25]. However, observations at Great Smoky Mountain National Park led managers to suggest that silktree seedlings (2.5 to 4 inches (6-10 cm) tall) do not sprout after mowing [8]. For more detail on silktree response to cutting or other treatments, see Control.

Moisture: Since it occurs on wet to dry sites [83], silktree does not appear sensitive to moisture conditions [111]. It occurred in a constructed wetland in New Jersey that was dominated by several marsh species [72] and occurs near rivers that are frequently flooded [6,102]. In silktree's native range it occurs in moist scrub and woodland areas [126]. Moist soil may favor silktree seedling establishment [98]. Silktree has been reported in mesic sites [124]. Establishment of silktree was rare on the Louisiana coastal plain during a period with 75% of the long-term average precipitation [99]. However, Vines [123] and Weber [126] note that silktree is resistant to drought, and silktree has been observed in xeric areas [6,8].

Soil: Although descriptions in the literature are limited, reviews state that silktree occurs in a wide range of soil conditions [9,111]. Weber [126] states that silktree is adapted to "poor soil", and Moore [89] notes that silktree's nitrogen-fixing capability enables it to grow well on infertile soil.

Silktree occurs on sites with acidic to moderately alkaline soil pH and grows on acid surface-mine spoils. At a landfill in south Korea, silktree grew on sites with pH ranging from 5.67 to 7.94 [63]. Silktree grew on surface-mine spoils in Kentucky where pH ranged from 4.0 to 7.1. Soluble salt concentrations on these sites ranged from 0.205 to 0.243 mmhos/cm, and phosphorus concentrations ranged from 1.6 to 8.1 ppm. Silktree's growth and survival were generally better on sites with high phosphorus concentrations. In areas with low phosphorus concentrations, greater growth occurred on sites with neutral pH [100].

Soil textures reported from sites with silktree are generally coarse. In the South Carolina Piedmont, silktree occurred in loamy sand [32], and at a landfill in South Korea, silktree occurred in sandy loam [63]. Silktree has been reported in coarse soil at a constructed marsh in New Jersey [72] and in mixtures of sand, gravel, and boulders next to a river in northern Tennessee [6].

Topography: Silktree has been reported in flat areas [8,32] and on steep slopes [8]. It was reported "on a level area" in the South Carolina Piedmont [32] and on slopes ranging from <10° to 90° in Great Smoky Mountains National Park [8].

SUCCESSIONAL STATUS:
Silktree is considered a pioneer species in its native Japan [94]. In North America, silktree is generally considered an early- to midsuccessional species, given its reported affinity for disturbed, open areas (e.g., [8,40,41,51,52,81]) such as old fields (e.g., [50,95,112]), old homesites (e.g., [8,18,132]), roadsides (e.g., [8,9,18,56,81,130]), and woodland edges (e.g., [33,40,82,111,126]). It was a "pioneer invader" that dominated stripmine overburden soils in experimental plots in Florida by the second growing season [11]. Silktree may establish or persist in later stages of succession on some sites [67,102].

Silktree may establish or increase on recently burned, thinned, or logged forest sites, and commonly occurs in forest edge habitat. Silktree had low canopy cover (<1%) in a mature, second-growth oak-hickory forest in western Tennessee, but its cover increased after fire [110] (see Plant response to fire). It occurred in the understory of a recently thinned, 30-year-old loblolly pine stand in South Carolina [32]. Silktree seedlings established in skid trails after selective harvesting in an oak-hickory forest in Tennessee [77]. A few silktree individuals occurred near the edge of a midsuccessional forest bordering an old agricultural field near Virginia's Atlantic coast. The authors speculate that they established following clear-cutting for agriculture [90]. In Kentucky, silktree occurred (but was rare) in an oak-hickory forest edge bordering a cool-season grassland [113]. Despite its common occurrence along forest edges, an analysis of edges in North Carolina forest did not list silktree among the species that were considered good edge indicators [80].

Silktree may occur in late-successional forests under some circumstances. Silktree occurred at low density in the sapling layer of an old-growth, longleaf pine forest in Alabama where fire had been excluded for at least 45 years [67,121]. No additional information was given regarding characteristics of microsites where silktree occurred in this forest; however, its establishment was likely due to anthropogenic influences, given the location within the city of Flomaton, Alabama. Silktree had <1% cover in the understory of a completely forested, undisturbed reference site on an island in the Potomac River [102] (see Shade tolerance, below for more details).

Shade tolerance: Silktree tolerates partial shade [9,83,124] but is generally considered intolerant of shade [8,80,89]. Full sunlight may promote flowering [41], germination, and seedling establishment [98]. Silktree often occurs in open areas [30,72,104,111,114] and rarely occurs under a full canopy [9]. In Japan, it was an indicator species of habitats with the highest percent of photosynthetically active photon flux density at ground level [94]. A study in western North Carolina showed silktree was more likely present in watersheds with less forest cover [64]. None of the silktree trees near a yellow-poplar successional forest in Great Smoky Mountains National Park were growing under a closed canopy [8]. Silktree occurred on a completely forested island in the Potomoc River where sunlight intensity of averaged 16% of full sunlight at 3 feet (1 m) above ground [102]. It is unclear where silktree occurred on this site, or what growth stage it was in, but it may have occurred in small nautral canopy gaps or edges that resulted from flooding.

FIRE EFFECTS AND MANAGEMENT

• FIRE EFFECTS
• FUELS AND FIRE REGIMES
• FIRE MANAGEMENT CONSIDERATIONS

FIRE EFFECTS:
Immediate fire effect on plant: As of 2010, no information on the direct impacts of fire on silktree was available. Given its ability to sprout from the roots following injury or top-kill (see Vegetative regeneration and Control), silktree is likely only top-killed by fire.

Silktree seed in the soil is likely to survive fire, and heat from fire may scarify silktree seeds and increase germination rates. In the laboratory, exposing silktree seeds to open flame for 1 to 3 seconds [42] and dry, 176 °F (80 °C) heat for 5 to 60 minutes [99] increased germination rates (see Postfire germination).

Postfire regeneration strategy [108]:
Tree with sprouting root suckers
Ground residual colonizer (on site, initial community)
Initial off-site colonizer (off site, initial community)
Secondary colonizer (on- or off-site seed sources)

Fire adaptations: As of 2010, no information was available that specifically addressed fire adaptations in silktree. Given its ability to sprout from the roots following injury or top-kill and the rapid growth of sprouts (see Vegetative regeneration and Control), once established on a site, silktree is likely to persist and may spread even under a regime of frequent fire. Silktree commonly occurs on open, disturbed sites (see Successional Status). Sites with these postfire attributes may be favorable for seedling establishment. Silktree produces seed with a hard, impermeable seed coat that requires scarification for germination (see Seed banking), so seedlings may establish from the soil seed bank after fire. Seedlings may also establish from off-site seed sources, as silktree seed has the potential to disperse long distances.

Plant response to fire: The very limited information available on silktree's response to fire suggests that it persists and may increase in abundance after fire. Only 3 fire studies in the available literature (as of 2010) include information on silktree, and only 1 of these provides information on silktree abundance before and after fire. In 1993, silktree occurred in the Flomaton Natural Area, a remnant stand of presettlement longleaf pine forest where fire had been excluded for over 40 years. Restoration efforts at Flomaton included prescribed fires in winter or spring of 1995 and 1996, midstory hardwood removal in early spring of 1996, and a 3rd prescribed fire in late spring of 1997. A survey was conducted in October 1997, and silktree was still present [68,121]. No information was provided regarding silktree size, abundance, or microsite location within the stand, and no additional information will be forthcoming from this site. In January 2008 the Flomaton Natural Area was clear-cut after Champion sold the land to a private landowner who cut down the trees, pulled out the stumps, and put in a trailer park. In the Piedmont National Wildlife Refuge near Macon, Georgia, silktree was a minor species on both unburned mixed pine-hardwood sites and on sites that were burned in winter every 4 or 5 years for 20 years, when surveyed 4 years after the last fire. The frequency of silktree on unburned transects was less than 2%, and frequency on burned transects was less than 1% [125]. No additional information was provided.

In a mature, second-growth, oak-hickory forest in Tennessee where fire had been excluded for at least 30 years, silktree cover increased following spring thin-and-burn and burn-only treatments and did not increase on control plots. The prescribed fire consumed all surface fuel on about 2.5 acres (1 ha) in 2 hours and had a maximum flame length of 1.6 feet (0.5 m). The table below shows increases in silktree on plots that were burned or thinned then burned, while cover on control plots remained low [110].

The author speculated that increases in silktree cover were due to fire-stimulated germination from the soil seed bank [110]. Postfire germination could be due to seed scarification by fire, changes in seedbed characteristics, or both.

Postfire germination: Laboratory studies suggest that fire may result in increased silktree germination by scarifying the seed. In the laboratory, brief exposure to open flame [42] and dry heat (176 °F (80 °C)) [99] stimulated silktree germination. The table below shows increased germination in silktree seeds exposed to open flame for 1 to 3 seconds [42]:

Hand-scarified silktree seeds germinated faster than heat-scarified silktree seeds, but after 240 days silktree seeds exposed to heat of 176 °F (80 °C) for 5 to 60 minutes had similar germination rates as hand-scarified seeds. Silktree seeds exposed to 176 °F for 4 hours had higher germination rates than those exposed to 176 °F for 1 minute [99].

Fire regimes: No information was available (as of 2010) on fire regimes in plant communities where silktree is native. In its nonnative North American range, information regarding plant communities in which silktree is invasive is also lacking. Silktree is most often described as occurring in areas of anthropogenic disturbance and along edges of native, second-growth forests that were either logged or cleared for agriculture at some earlier time (see Habitat Types and Plant Communities), and where presettlemet fire regimes are no longer functioning. Many of the vegetation types in which silktree occurs (oak-hickory, pine, mixed pine-hardwood) have presettlement fire regimes characterized by relatively frequent, low-severity fires. Silktree also occurs in riparian forests, where presettlement fire regimes were thought to be characterized by infrequent fires (see the Fire Regime Table). 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".

Given its regeneration strategies and successional status (see Fire adaptations), silktree seems well adapted to establish after fire and to persist under a regime of frequent fire. Silktree occurs in communities that are managed with frequent fire [49,121,125], but it also occurs in areas where fire has been excluded for several decades [91,110,121].

FIRE MANAGEMENT CONSIDERATIONS:
Available evidence suggests that a single fire will not control silktree and may promote its establishment.

Potential for postfire establishment and spread: Silktree is likely to persist by sprouting and, if a seed source is available, establish from seed after fire. Although data regarding silktree's response to fire are lacking, silktree is known to sprout following injury or top-kill (see Vegetative regeneration and Control), and evidence suggests that seed germination may be stimulated by heat scarification (see Plant response to fire). Additionally, silktree seems to grow best on disturbed sites with open canopies (see Successional Status). These traits suggest that burned sites should be monitored for postfire establishment and spread of silktree if it occurs onsite or nearby.

Preventing postfire establishment and spread: 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 seed into burned areas. Specific recommendations include:

• incorporate cost of weed prevention and management into fire rehabilitation plans
• acquire restoration funding
• include weed prevention education in fire training
• minimize soil disturbance and vegetation removal during fire suppression and rehabilitation activities
• minimize the use of retardants containing nitrogen and phosphorus
• avoid areas dominated by high priority invasive plants when locating firelines, monitoring camps, staging areas, and helibases
• clean equipment and vehicles prior to entering burned areas
• regulate or prevent human and livestock entry into burned areas until desirable site vegetation has recovered sufficiently to resist invasion by undesirable vegetation
• monitor burned areas and areas of significant disturbance or traffic from management activity
• detect weeds early and eradicate before vegetative spread and/or seed dispersal
• eradicate small patches and contain or control large infestations within or adjacent to the burned area
• reestablish vegetation on bare ground as soon as possible
• avoid use of fertilizers in postfire rehabilitation and restoration
• use only certified weed-free seed mixes when revegetation is necessary

For more detailed information on these topics see the following publications: [5,16,43,118].

MANAGEMENT CONSIDERATIONS

• IMPORTANCE TO WILDLIFE AND LIVESTOCK
• OTHER USES
• IMPACTS AND CONTROL

IMPORTANCE TO WILDLIFE AND LIVESTOCK:
Silktree may be a minor food source for some wildlife and has some potential as livestock feed.

Palatability and/or nutritional value: Silktree seed may provide some food for birds and squirrels [54,123], while butterflies and hummingbirds likely consume silktree nectar [54,61]. Silktree leaves may provide browse for deer and other wildlife [1,54,98]. According to Kartesz [61], silktree has been reported as toxic.

Silktree's nutritional value and growth rate give it potential as a summer browse species for livestock in the southeastern United States [17,99]. In a test of silktree as a forage species in the Louisiana coastal plain, silktree had consistently high leaf crude protein level [99]. In an analysis of silktree as forage in Arkansas, nitrogen levels met the nutritional requirements of cattle and domestic goats, and the presence of secondary metabolites were below detectable levels [17]. In a study of silktree's potential as domestic goat feed, digestibility and chemical composition were similar to alfalfa (Medicago sativa) [12]. In a domestic sheep feeding trial, silktree digestibility was 61%, and there were no signs of toxicity [13]. In an experiment on silktree as domestic goat forage, herbage mass production was adequate [1]. Other experiments suggest that silktree tolerates 2 complete defoliations during the grazing season [3] and that yield was maximized when 6 to 8 weeks of regrowth occurred between harvests [13,14]. Despite silktree's tolerance, requirements for managing defoliation are likely greater than required of currently used forage species [99]. Although domestic goats [3] and domestic sheep [3,13] eat silktree, it has been shown to have relatively low palatability compared to some available foods [1,3]. Silktree's other limitations include its potential to become invasive [99] (see Impacts) and a lack of evidence, as of 2010, that it significantly improves animal performance measures such as weight gain [3].

Cover value: Wick and Walters (1974 cited in [98]) state that silktree provides valuable cover for wildlife.

Rehabilitation planting: Silktree has been recommended as a soil builder [100] and is used in rehabilitation planting on landfills [63,105] and mine sites [11,100].

Other: Silktree may be used for timber [61,123] and as an alleycrop species and mulch in legume production [60,79]. Although mentioned as a timber plant by Kartesz [61] and apparently used in cabinetmaking in Asia [123], silktree's weak and brittle wood was noted in a review [9]. There have been some successes with experimental trials that used silktree as an alleycrop species to maintain or improve soil fertility while growing commercial crops. Silktree fixed an estimated 245 pounds of nitrogen/acre over one growing season in an experimental planting in Alabama [13]. However, its use on broad scales is not recommended [60,79]. Kartesz [61] notes that silktree is edible and useful in erosion control. Use as a biofuel [83] has also been reported.

Despite a lack of data on the impacts of silktree on native habitats, silktree is commonly considered a weed of concern in the south-central and southeastern United States. Miller [82] states that silktree is 1 of the 16 most prevalent nonnative species in subtropical forests of the southeastern United States. Managers in Alabama, Arkansas, and Kentucky consider silktree a problem weed [76]. In Texas silktree is widespread and "can aggressively invade native habitats" [26]. According to a 2008 review [96], silktree is listed as invasive in 8 southern and mid-Atlantic states. Silktree was 1 of 12 species commonly reported as a problem by federal, state, and nongovernmental land managers of the southern Appalachians [66]. In the mid-1970s, the small size, scattered spatial arrangement, and occurrence of silktree populations only on disturbed sites in Great Smoky Mountains National Park led to the conclusion that silktree had very little impact on native flora of that area, despite occurrence in riparian plant communities [8]. From 1994 to 2005, silktree was one of the 9 most common weeds in the Great Smoky Mountains National Park [127]. However, the impact of silktree establishment in this area had not been determined as of 2008. Silktree is classified as a weed that easily spreads into native communities and displaces native species in several southeastern states including Tennessee [106], Georgia [37], and Florida [35]. It frequently occurs in the central peninsula and northern regions of Florida [131]. In 2005, silktree was classified as a "significant threat" instead of a "severe threat" in Kentucky due to fewer impacts on native plant communities and fewer invasive characteristics than weeds that pose more severe threats [62]. As of 2003, silktree was considered "moderately invasive" in Virginia due to slow spread and negligible impact on ecosystem processes [124]. It had low management priority in another Virginia study area due to comparative ease of control [28].

Riparian habitats may be at greater risk of silktree invasion than other communities, likely due to regularly disturbed soils in riparian areas as well as the potential for silktree seeds to disperse in water [9]. Silktree is reportedly a "serious problem" along some streams in Tennessee [9], where it has been documented in cobble bars of the New River [6] and streambanks in Great Smoky Mountains National Park [8]. A review of southeastern weeds notes that silktree invades riparian habitats, spreads along stream networks, and can reduce native species and hardwood regeneration in riparian habitats [82]. Silktree was described as a "common pest" of the floodplain in Rock Creek Park in Washington DC [33]. It was reported on an island in the Potomac River, Maryland, that had experienced little human disturbance but had greater light penetration than mainland forests, likely due to higher velocities of previous floods on the island compared to the mainland [102]. Silktree has also been reported in riverbank communities in subtropical forests within its native range [65].

Control: Prevention has been recommended to minimize further spread of silktree, while control of established populations is generally accomplished with some combination of mechanical and chemical treatments.

Fire: The ability of prescribed fire to control silktree is likely limited and is discussed in Fire Management Considerations.

Prevention: Reducing seed sources and disturbances have been suggested to help prevent the spread of silktree. The Southern Region of the US Forest Service prohibits planting silktree on National Forest lands [117]. Using natives instead of silktree for ornamental planting has been recommend [28,111], and Swearingen and others [111] provide a list of alternate native species for planting. For an example of selecting and implementing a weed risk assessment, see Jefferson and others [58]. In Great Smoky Mountains National Park, reducing anthropogenic disturbance was suggested to limit silktree establishment [8].

Cultural: No information is available on this topic.

Physical and/or mechanical: Effective mechanical treatments typically involve repeated girdling or cutting of silktree close to the ground before seed production. Repeated cutting or cutting in combination with herbicide application is necessary to control sprouting [9,111,126]. For example, in experimental plots in North Carolina silktree coppiced in February grew to an average height of 54 inches (137.7 cm) by the end of June. A cutting height of 10 inches (25 cm) significantly (P=0.013) decreased herbage mass production compared to a cutting height of 20 inches (50 cm) [1]. Results of another experiment suggest that silktrees cut to 4 inches (10 cm) 2 or 3 times per growing season had shorter life spans than those cut at 20 inches (50 cm) or higher. Despite the difference, silktrees cut to 4 inches survived an average of 641 days [99]. Swearingen and others [111] recommend cutting silktree at ground level. Several reviews [9,111,126] recommend cutting before seed production to prevent seed dispersal.

Seedlings up to 4 inches (10 cm) have been controlled by regular mowing [8], and seedlings up to 10 inches (25 cm) can be pulled by hand [25]. Effective hand-pulling of silktree requires removal of the entire root [9,126].

Biological: Research into appropriate biological control agents was lacking as of 2008. A root fungus [9], a bruchid beetle (Bruchidae) [24,89], and a psyllid [120] apparently impact silktree to some extent, but there were no data on their potential as biological control agents.

Silktree is susceptible to a Fusarium root fungus, which causes vascular wilting and typically results in rapid mortality [9,30,40,96,123]. Silktree strains that are resistant to the fungus are available [74,116]. The use of this root fungus to control silktree could be limited, depending on the extent to which these strains have established in native plant communities.

Bruchid beetles infested 21% of silktree seeds in a germination study [89], and DeLoach [24] suggests bruchid beetles may be a useful biological control for silktree.

The introduced psyllid Acizzia jamatonica is apparently an obligate feeder of Albizia and was documented in Clarke County, Georgia, in 2006 [120].

Chemical: Herbicides are often used to control silktree sprouting following mechanical treatments [25], or as a basal bark application on larger trees. Trees larger than 3 inches (1.2 cm) in diameter may require retreatment [71]. Recommended herbicides and applications for silktree saplings and large trees are described in several reviews [9,25,71,83,126]. See the Weed control methods handbook for considerations on the use of herbicides in natural areas and detailed information on specific chemicals.

APPENDIX: FIRE REGIME TABLE

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