The lead tree is native to Mexico and Central America, but it is cultivated throughout the tropics, and it has widely escaped and naturalized. In the United States, it has been reported from Arizona, California, Florida, Hawaii and Texas.
There is no doubt that L. leucocephala originates from Mexico. However, its true natural distribution is extremely difficult to ascertain in detail because it is cultivated throughout Mexico and Central America, and no unambiguously natural populations have so far been located. Thus earlier references to it being native to Guatemala and Belize are not included here, nor are the assertion from USDA-NRCS (2007) that it is native to the USA.
Subsp. leucocephala is distributed mainly in the Yucatán Peninsula, in the Mexican States of Tabasco, Campeche, Quintana Roo and Yucatán, extending south into northern Belize. Outlying occurrences include an area of central-north Veracruz, and sporadic occurrences across the Isthmus of Tehuantepec into the State of Oaxaca. Despite the great abundance and widespread occurrence of subsp. leucocephala throughout the Yucatán, it is doubtfully native there (McClay, 1990; Waage, 1990; Hughes, 1998b), possibly introduced in pre-Columbian times for its edible seeds and pods and having since spread following introduction. As an aggressive colonizer of disturbed sites, especially on calcareous soils, its present-day abundance in the Yucatán Peninsula may be as an introduced weed rather than a native species (Hughes, 1998a, b).
The natural distribution of subsp. glabrata is also unclear. Despite intensive field exploration, no unequivocally natural populations have so far been located. Within Mexico and Central America it is extremely common as a cultivated backyard, street and orchard tree and is found in the majority of villages and towns in Mexico, in all tropical and subtropical areas (wet, seasonally dry and semi-arid), except above 2000 m altitude, but is only locally naturalized.
Subsp. ixtahuacana is restricted to a small area of northern Guatemala and the immediate border zone in Mexico. As far as is known, subsp. ixtahuacana has not been introduced elsewhere.
This deciduous shrub or small tree to 25 feet tall has an open, spreading crown and smooth, pale gray-brown bark. The leaves are twice-pinnately compound, to about a foot long, with four to nine pairs of pinnae (main divisions), and usually a dimple-like gland on the rachis (main axis) between the lowermost pair. The leaflets, with a pointed tip and an uneven base, are small (about 1/2 inch long and 1/8 inch wide) and numerous, with 13 to 17 pairs per pinna. The flowers are tiny, white or pale yellowish, in dense ball-like heads about 3/4 inch in diameter. The fruits (pods) are flat, rounded or blunt at the tip or with a short, broad point, 5 to 7 inches long and an inch or less wide, and reddish brown when mature, with 10 to 20 flattened seeds.
Although many details of the history of introduction of L. leucocephala across the tropics remain obscure, it is clear that the shrubby 'Common' or 'Hawaiian' type belonging to subsp. leucocephala was introduced much earlier than the 'Giant' or 'Salvador' type of subsp. glabrata. Subsp. leucocephala is reported to have been introduced to the Philippines aboard one of the annual Spanish government galleons that sailed between Acapulco and Manila between 1521 and 1815 (Merrill, 1912). L. leucocephala is recorded in Blanco's 1845 Flora of the Philippines (cited in Merrill, 1918), but beyond this, the precise date of introduction is not known, although some speculate that it may have been introduced in the 1600s (Brewbaker et al., 1972; Brewbaker and Hutton, 1979; Pound and Martínez-Cairo, 1983). Reviewing the literature of spread in Asia, Tuda et al. (2009) suggest that introductions date back to as early as 1645 in Taiwan, approximately 1920 in continental China (Guangdong), prior to 1815 in the Philippines, prior to 1867 in Japan and during the Sukhothai Period (1238–1378) in Thailand.
By the late 1800s, subsp. leucocephala had spread or been introduced through much of Africa, Asia and the Pacific and it is now pantropical, recorded from the majority of tropical and subtropical countries. Invasive tendencies in the Pacific were also noted as early as 1943 in Nuie, and now it is the most prevalent invasive weed in the region and a serious problem on Tonga (PIER, 2007) and the Galapagos Islands.
Subsp. leucocephala is an aggressive colonizer of disturbed sites and is spreading naturally. It has been recorded as a weed in more than 20 countries scattered across all continents except Antarctica. All early (pre-1960) agronomic investigation, most flora treatments, and references to naturalization and weediness refer to subsp. leucocephala, which is pantropically distributed and much more widely naturalized than subsp. glabrata. The latter has been widely introduced outside Mexico and Central America only in the last few decades, but following active promotion of the species it is also now distributed virtually pantropically in cultivation. It is invasive in Guam, Mauritius and La Réunion (Macdonald et al., 1991). In Ghana, there are problems where the species has been introduced for alley cropping (Cobbinah JR, Forestry Research Institute of Ghana, personal communication, 2002). Binggeli (1997) lists it as a rapidly spreading weed in India. Similarly, Ghate (1991) reports that it was introduced to the Western Ghats in India in the late 1800s and is now reported to be expanding rapidly in all habitats. Small populations of the plant exist on the Galapagos Islands, Ecuador, and whereas Mauchamp (1997) did not report the species behaving invasively, it was believed that it constituted a high risk. L. leucocephala is invasive in Florida, USA but not yet thought to have altered habitats (Miller et al., 2003). A record in St Louis, Missouri, USA (Missouri Botanical Garden, 2007) is likely to be a protected botanical specimen. It is also considered invasive in Bermuda (de Silva H, Bermuda Zoological Society, personal communication, 2003).
Herbarium specimens recorded by ILDIS (2007) recorded presence on the Portuguese Atlantic island of Madeira and on the mainland although the exact locations and frequency are not known. L. leucocephala has also been positively recorded in southern Spain from field surveys (Dana et al., 2005), and was noted to be “naturalised or on the way to naturalization”; further information on spread and risk of invasion is provided. Thus, L. leucocephala could pose a risk of invasion in more frost-free areas around the Mediterranean, and other more frost-tolerant species or hybrids would also be likely to have a further increased risk of spread.