A brief history of taxonomic studies
The genus Magnolia L. ranges widely in eastern and southeast Asia and in the New World in the southeastern United States, the West Indies, Mexico, and Central America to northern South America. Because of its remarkable discontinuous distribution between eastern Asia and eastern North America, Magnolia has attracted the attention of biologists. The first published reference to the similarities of species between eastern Asia and eastern North America appears in a Linnaean dissertation published in the mid-eighteenth century (). Thunberg (1784) made a brief mention that many Japanese plants occurred in Europe, America, and the East Indies, but particularly in the northern, and adjacent, vast Chinese region. Among the plants he cited was Magnolia glauca L., which he believed to be in the two areas. Later, in his travels in the United States, Luigi Castiglioni noted the affinities of many American plants (including Magnolia) that he had observed growing with those in Japan. Since then, many authors, such as Pursh (1814), Nuttall (1818), Gray (1840), Miquel (1867), Engler (1879) and Diels (1900), and many recent workers — Hu (1935), Hara (1966), Li (1952), Wood (1971), Raven (1972), Thorne (1972), Boufford (1992), and Sewell et al. (1993) — have studied this distribution pattern in detail. Magnolia, as a typical disjunct genus between eastern Asia and eastern North America, and the largest genus in the family Magnoliaceae, has a rather clear geographical distribution. However, uncertainties and controversies about the delimitation of the genus have existed for a considerable time because of the overlap in characters between it and other traditional genera of Magnoliaceae ().
The formation of distribution patterns
Paleobotany of Magnolia
The emergence of fossils that are convincingly Magnolia was in the Early Cretaceous Pseudofrenelopsis-Angiosperms Assemblage in what is regarded as Aptian-Albian in age (). The earliest fossils of this genus, found in northeastern China (Jilin), have been named Archimagnolia rostrato-stylosa Tao et Zhang. The Late Cretaceous was an important stage of angiosperm development. Although reliable fossils of Magnolia were found as early as the Early Cretaceous, they were in only a subordinate status in the vegetation because gymnosperms still predominated in the Early Cretaceous floras. Since the Late Cretaceous, however, especially in the Turonian, angiosperms (including magnolias) gradually became dominant (). Many more fossils of magnolias, such as Magnolia sp. and Magnolioxylon were found in Japan, in the far eastern areas of Russia, and in North American Late Cretaceous floras (). In the Early Tertiary, many fossils of Magnolia and Magnoliastrum, were found in widespread regions of the world. The fossil sites ranged from Asia (southeastern Asia, Yunnan, Xizang, Shandong and northeastern China) (), Europe () to North America ().
Early Cretaceous (Barremian to Albian) Magnolia pollen, so-called Magnolia-Magnolipollis, was reported from the Early Cretaceous Xinmingbao Group in several locations of Gansu by Hsu et al. () and Ye and Zhang (), which is earlier than the fossil Archimagnolia. Considerable deposits of Magnolia pollen have been found in widely scattered strata of Cretaceous and Tertiary age in mainland China (Gansu, Xinjiang, Qinghai, Jilin, Heilongjiang, Yunnan, Guangdong, Sichuan, Hubei, Shaanxi, and eastern China) ().
On the basis of current information from megafossils and pollens, the early evolution of the genus Magnolia could have taken place during the Early Cretaceous or even earlier. The development of the genus may have reached a rather high level at the time.
Present distribution centers of Magnolia
The geographical distribution of the species of Magnolia is summarized in Table Geographical distributions of Magnolia. This genus is widely dispersed throughout the temperate Northern Hemisphere (). Its greatest concentration of species is in southeastern Asia, in the region extending from the eastern Himalaya eastward to China and southward to Java (). A detailed analysis of the geographical distribution of Magnolia by areas follows (see Table Geographical distributions and species numbers of subgenera and sections of Magnolia).
Table Geographical distributions of Magnolia
|Temperate Asia||Tropical Asia||China||North America||South America|
|M. acuminata var. subcordata||+|
|M. officinalis var. biloba||+||+|
|M. X loebnen||+|
|M. X soulangeana||+|
|M. X thompsoniana||+|
|M. X veitchii||+|
|M. X wiesnen||+|
Table Geographical distributions and species numbers of subgenera and sections of Magnolia
|Temperate Asia||Tropical Asia||China||North America||South America|
Asia-Temperate: This area, where the greatest concentration of species occurs and which has been a major center of speciation, is undoubtedly the present distribution center of Magnolia. The region is also important as an area of local endemism. Species diversity is greatest from the eastern Himalaya to eastern Asia. There are about 31 species and varieties distributed either widely or locally in this area. These species represent 58.5% of all species in the genus and discussed in this chapter. Three sections of Magnolia are restricted to this area: section Oyama (subgenus Magnolia), section Yulania and section Buergeria (subgenus Yulania).
Asia-Tropical: The diversity of the genus Magnolia is not as rich in tropical Asia as in temperate Asia. In total, about eight species and varieties, i.e., 15.1% of the taxa in the genus, are distributed in the Asian tropics. All species of this area belong to subgenus Magnolia and almost all of them are in three sections: section Gwillimia, section Lirianthe and section Gynopodium. One species, Magnolia rostrata, is in the disjunct section Rytidospermum. The section Lirianthe is a small, typically tropical Asian section with only one species, M. pterocarpa, which ranges within a confined area in Bhutan, India-Assam, Nepal, Burma, and Thailand.
North America: This area has the same species richness for Magnolia as the tropical Asian area. About nine species and varieties (17% of all the taxa of Magnolia) are divided among four sections and two subgenera. Of these, the distribution patterns of two sections (section Rytidospermum and section Tulipastrum) are disjunction on different continents in the Northern Hemisphere; one (section Theorhodon) ranges widely from South to North America; the fourth section, section Magnolia, is confined to North America. This area represents another center of diversity for the genus Magnolia.
South America: This area has the smallest number of naturally occurring species in the genus (only five species or 9.4% of the genus). The species of South America are confined to section Theorhodon in subgenus Magnolia.
China: Although China is probably not a natural geographical distribution area, without doubt it is very important as an area of local endemism, with 19 endemic taxa of Magnolia (35.9% of the genus). The endemic species in China are separated into six sections and two subgenera: three sections (section Gwillimia, section Oyama, and section Rytidospermum) in the subgenus Magnolia and three sections (section Yulania, section Buergeria, and section Tulipastrum) in the subgenus Yulania. They are also separated into two sections with disjunction ranges between eastern Asian and North American. China is therefore the center of the present day distribution of Magnolia in Asia.
Selections from the book: “Magnolia. The genus Magnolia”. Edited by Satyajit D. Sarker and Yuji Maruyama. Series: “Medicinal and Aromatic Plants — Industrial Profiles”. 2002