Trees, to 20 m tall; crown broad, umbelliform. Bark brown-gray. Twigs conspicuously lenticellate, viscid glandular when young. Leaf blade cordate, ca. 40 cm, abaxially densely to sparsely hairy, adaxially sparsely hairy, apex acute. Thyrses pyramidal to narrowly conical, to 50 cm; cymes 3- or 4-flowered; peduncle 1-2 cm, nearly as long as pedicels. Calyx shallowly campanulate, ca. 1.5 cm, outside tomentose; lobes to 1/2 or more calyx length, ovate-oblong. Corolla purple, funnelform-campanulate, 5-7.5 cm, ridged ventrally, outside glandular, inside glabrous; limb ca. 4.5 cm in diam. Stamens to 2.5 cm. Ovary ovoid, glandular. Style shorter than stamens. Capsule ovoid, 3-4.5 cm, densely viscid-glandular hairy; pericarp ca. 1 mm; persistent calyx lobes flat. Seeds 2.5-4 mm including wing. Fl. Apr-May, fr. Aug-Sep.
Princesstree may increase fuel loads locally but may not increase fire hazard and thus not affect FIRE REGIMES. Princesstree's growth habit suggests that it may contribute substantially to fuel loads. Princesstrees may produce large amounts of litter. The brittle branches break easily even when green, and branch die-back from frost is common, so the branches, large leaves, and numerous seed capsules accumulate under the tree canopy (e.g., [37,81,115,126]). However, princesstree is not considered a fire hazard . Paulownia wood has low thermal (0.063-0.086 Kcal m‾ ¹ hr‾ ¹ °C‾ ¹) and temperature conductivity (0.000561-0.000631 m‾ ¹ hr‾ ¹) and thus very high heat insulation properties and low combustibility relative to other species . Relatively high moisture content and low ignitability of chemicals in the plant partially explain its low combustibility and consumption in fire. Li and Oda  studied the characteristics of princesstree wood in the laboratory and found that "the thermal conductivity of princesstree is lowest among all types of wood", although the authors did not provide relative numbers. The porous microstructure of princesstree wood and its chemical composition help explain its light weight, relatively low combustibility, and "flame retardant" characteristics compared to other types of wood . For more information on the cellular structure and physical characteristics of princesstree wood see Zhu and others  and Hu [62,63].
Since royal paulownia is a naturalized exotic that is only now being planted, little is known about its silviculture. Like most pioneer species, however, it is classed as intolerant of shade and competing vegetation.
Do not plant princess tree. Young plants can be pulled by hand; cut larger trees at ground level with power or manual saw, preferably prior to seed formation to prevent further spread. Systemic herbicides containing glyphosate or triclopyr are effective and can be applied to cut stumps or to bark.
Highly shade-intolerant, princesstree requires large-scale disturbances such as fire, landslides, flood scour, or other land scarification for optimal stand establishment . The small, wind-dispersed seeds germinate almost exclusively on open sites with exposed mineral soil [18,63]. These traits are similar to those of many native fire-dependent species, such as Table Mountain pine and pitch pine . Princesstree may grow rapidly after fire. For example, in oak-pine forest in Linville Gorge Wilderness Area dominated by scarlet oak, chestnut oak, pitch pine, and Table Mountain pine, seedlings grew to 14 feet (4 m) tall 4 years after fire . Furthermore, princesstree's ability to sprout from adventitious buds along its bole, root crown, and/or roots after partial to complete top-kill  likely allows it to persist after fire. Thus, prescribed fire meant to enhance regeneration and maintenance of native, fire-dependent forest species may also create conditions suitable for princesstree regeneration.
Establishment of princesstree after wildfires has been reported in the fire-dependent Table Mountain pine-pitch pine forests of the southern Appalachians (e.g., [40,84,111]). On the western rim of the Linville Gorge Wilderness Area, Dumas and others  examined the effects of a fall 2000 wildfire in primarily xeric oak-pine forest. This area had not been burned in 50 years and had succeeded to a mixed-hardwoods community with a dense mountain-laurel (Kalmia latifolia) understory . Throughout Linville Gorge Wilderness Area, fire severity ranged from low-severity surface fire to high-severity crown fire (see Kuppinger  for more information). In this area, the fire was a low-severity surface fire, with crowning mainly restricted to stands of pitch pine and Table Mountain pine that had been killed by southern pine beetle. Two years after fire, little overstory mortality was evident in healthy stands. Most of the understory, composed primarily of mountain-laurel, was top-killed and had subsequently sprouted. The fire reduced surface organic horizons nearly 50% in burned plots relative to unburned plots and increased light penetration about 15%, resulting in greater soil temperature extremes but no differences in available nitrogen or phosphorus. Rates of soil respiration and litter decomposition tended to be lower in burned than unburned plots. This low-intensity surface fire caused postfire basal sprouting of trees and shrubs, increased species richness in the herb layer, and allowed the establishment of pitch pine and princesstree seedlings. Princesstree had not been detected in the 12,002-acre (4,857 ha) Wilderness Area prior to the fire, but seedlings had "rapidly increased in number and height" by postfire year 4 . Potential brevity of princesstree seeds in the soil seed bank and susceptibility of seeds to fire-induced mortality suggests that princesstree may have established in the Linville Gorge Wilderness Area from seed dispersed after the fire rather than from dormant seed present in the soil seed bank prior to the fire (see Fire Management Considerations and Seed banking) [79,111].
Landscapes with high fire severity may provide quality habitat for princesstree . Kuppinger  examined postfire establishment and spread by princesstree across 5 sites in western North Carolina and eastern Tennessee that burned in 2000 and 2001. Data collected in postfire years 1 and 4 were compared. General patterns emerged, including a positive association with relatively dry, upland reaches of the landscape, a negative association with remaining vegetation cover taller than 3.3 feet (1 m), and a negative association with decreasing hill shade.
Fire severity and intensity likely affect regeneration of princesstree. Because of variations in fuels and topography, fire creates a variety of microsites within each burned area. High-severity fire produces favorable conditions for princesstree germination and establishment but killing mature trees and some seeds, particularly those in the litter. Conversely, low-severity fire may not kill mature princesstree trees or their seeds but is less likely to create conditions necessary for germination and establishment. More information is needed about the establishment and persistence of princesstree seedlings and sprouts and their ability to compete with native vegetation after fire (see Successional Status) .
Princesstree is valued in eastern Asia for its medicinal, ornamental, and timber uses [62,63]. In the United States, it has been widely planted as an ornamental (e.g., ), as a source of high-value export lumber (e.g., [10,26,57,96,106,130]), and for revegetation of land disturbed by coal mining (e.g., ). Except for its use as an ornamental, little attention was given to princesstree in the United States until the 1970s . The United States began exporting princesstree wood to Japan in 1972. The logs obtained from wild-grown trees in the United States were of high quality, making prices for princesstree logs comparable to expensive native hardwoods such as black walnut (Juglans nigra) . High prices encouraged interest in cultivating princesstree for timber production and led to much research regarding its cultivation on plantations and surface-mined lands [26,37,51,57,58,106,115,130,133]. Princesstree has since been extensively cultivated and grown in commercial plantations in the United States and throughout the world (see General Distribution) . The commercial market for princesstree in the United States likely peaked in the late 1970s and early 1980s [58,106], when it was promoted as a "magic tree"  or as an "eco-friendly multi-purpose species" .
Frequent planting and increased propagule pressure may have resulted in increased invasibility of native communities by princesstree. The 1990s marked the first time that researchers and land managers began to see princesstree establish after fire in native xeric plant communities in the southern Appalachians . Beginning in the early 1990s, focus in the literature gradually shifted from promotion to eradication of princesstree (see Impacts).
Wood products: The characteristics of princesstree wood make it suitable for a diversity of uses, and many reports have touted the unique physical and mechanical properties of princesstree wood (e.g., [38,58,62,63,106,115,123,130]). Its wood is used to make plywood and other house construction wood (other than for structural timber), paper, veneer, hand-carvings, clogs, musical instruments, furniture, and kitchen items such as rice pots, water pails, bowls, and spoons [34,37,51,57,62,63,78,106,115,133,138].
Reforestation and reclamation: Princesstree invades open, disturbed areas and often tolerates the harsh environmental conditions of surface mines . Several studies have advocated using it for reclamation [25,26,31,95,130,136]. It has been planted on surface-mined lands throughout the eastern United States, including West Virginia, Kentucky, Tennessee, and Alabama [57,106,130,143]. However, several authors describe it as having limited importance for use in revegetation on coalmined sites due to the frequent difficulties of establishing princesstree relative to other species [129,133,143]. It was unclear how common the use of princesstree in reclamation was as of this writing (2009).
Agroforestry and cropping systems: The Chinese have developed intercropping and agroforestry systems for Paulownia, including princesstree, which have been evaluated extensively in the literature (e.g., [29,34,37,38,149,154]). Species of Paulownia other than princesstree are apparently preferred for these practices .