Specific delimitation within Magnolia has been a subject of persistent debate and disagreement among taxonomists, botanists, horticulturists, and morphologists for at least the last half of the twentieth century. Recently, molecular systematists have joined the debate (). The number of species in the genus differs according to different classification systems of the family Magnoliaceae. Several systems have been proposed for the Magnoliaceae in recent years. In his last treatment, Dandy () recognized 12 genera in the Magnoliaceae, and about 70–80 species. The extremes were shown by the classification of Nooteboom (), where only 3 genera and about 150 species in the genus Magnolia were recognized, and Liu (= Y.W. Law) (2000) recognized 16 genera in the family Magnoliaceae and about 90 species in Magnolia alone. The differences between these two systems are mainly based on the importance assigned to certain morphological features or to some molecular evidence. In Nooteboom’s classification of Magnoliaceae, little significance is attached to the position of the inflorescence and the molecular data, and he places all genera except Pachylarnax and Liriodendron in his large genus Magnolia. Liu’s classification system uses the position of the inflorescence and many fine morphological characters to recognize a number of small and even monotypic genera, such as his new genus Woonyoungia () plus the medium-sized genus Michelia. Frodin and Govaerts () treated the genus Magnolia in broad sense to contain approximately 128 species, including those previously placed in Alcimandra, Aromadendron, Dugandiodendron, Manglietiastrum, and Talauma. Based on observations of the proleptic branching and other morphological characters, Figlar () concluded that there was no reason to maintain the genus Michelia separate from Magnolia, and then he treated Michelia as a subgenus of Magnolia along with the subgenus Yulania. Recent molecular data do not support the recognition of many small genera in the family ().
Distribution of subgenera and sections
The genus Magnolia is both temperate and tropical. Here we have mainly adopted 45 taxa listed in the GRIN (Germplasm Resources Information Network) database (based on Dandy’s system) (GRIN database, 2001) plus eight endemic species from China (). Of the 53 species, varieties, and hybrids in the genus Magnolia, 45 were based on information from GRIN and 8 were from Flora Reipublicae Popularis Sinicae. According to Dandy’s () classification system, the genus Magnolia is divided into two subgenera: Magnolia and Yulania. These were further subdivided into 10 sections (not including section Maingola), with their general distribution in eastern Asia and the Americas north of the equator. In each of the subgenera, one section contains species from the temperate parts of both North America and eastern Asia.
Seven sections with 29 species and varieties and three hybrids are included in subgenus Magnolia based on Dandy’s classification. Among these sections, two of them are American (section Magnolia and section Theorhodon), five are Asian (section Gwillimia, section Lirianthe, section Oyama, section Gynopodium, and section Maingola), and one occurs both in Asia and America (section Rytidospermum).
The section Magnolia de Candolle contains only a single species, Magnolia virginiana, the type of the genus. It ranges along the Atlantic and Gulf coastal plains in the eastern United States from Massachusetts, where it is rare, to North Carolina and Texas.
The section Theorhodon Spach has six evergreen species (GRIN database, 2001). All are trees in tropical America, except for Magnolia grandiflora, which is a native of the southeastern United States along the Atlantic and Gulf coastal plains. The tropical series range from eastern Mexico (M. schiediana and M. sharpii), Guatemala (M. guatemalensis), Honduras, Costa Rica, and Panama to the mountains of southeastern Venezuela. Another series is West Indian and contains two species, M. splendens and M. portoricensis. They range from eastern Cuba, Haiti, and the Dominican Republic to western and eastern Puerto Rico.
The section Gwillimia de Candolle is a section with about eight evergreen species ranging from southern China through Indochina to the Philippines. Three species are native to China (M. championii, M. delavayi, and M. odoratissima). M. championii ranges from Hong Kong to Guangzhou, Hainan, the islands of Guangdong, Taiwan to Guangxi. M. odoratissima is restricted to Yunnan. Three species in the section are temperate. M. delavayi is also confined to Yunnan. M. paenetalauma, however, has a wider area of distribution, ranging from Guizhou, Hainan, and Guangxi in China to northern Vietnam. M. coco has the distinction of being the first Asian Magnolia to be grown in China in 1786 (). It has a wider distribution than M. paenetalauma, ranging from Zhejiang, Taiwan, Guangdong, Guangxi, and Yunnan to northern Vietnam. The other two species are from Hainan and Vietnam (M. albosericea) and from Yunnan, Burma, Thailand, and Laos (M. henryi).
There is only one species, M. pterocarpa, in the section Lirianthe (Spach) Dandy. This species is a tropical tree distributed from Bhutan to India-Assam, Nepal, Burma, and Thailand.
The section Oyama Nakai is a group of deciduous shrubs or small trees with four species confined to temperate eastern Asia. In this section, M. sieboldii is the easternmost species, and the oldest one in cultivation. It occurs in Korea, Honshu, Kyushu, and Shikoku in Japan, and northeastern China and Anhui, Guangdong, Liaoning, Sichuan, and Yunnan in China. Two endemic Chinese species are in this section: M. sinensis occurs only in western Sichuan, and M. wilsonii has a wider range, extending from Sichuan to northern Yunnan and Guizhou. M. globosa is the most westward-ranging species in this section, extending from eastern Nepal along the eastern Himalaya to Sichuan, Xizang (Tibet), and extreme northwestern Yunnan in China.
The small Asian section Gynopodium Dandy includes a few species ranging from southeastern Xizang, northwestern Yunnan (M. nitida) and northeastern Upper Burma, Vietnam, to southeastern China and Taiwan.
The section Rytidospermum Spach is the only section occurring in both Asia and America in the subgenus. It contains about nine species of deciduous trees. Three series have been recognized in Dandy’s classification system, one Asian and the other two American, and each contains two or three species. The Asian series contains Magnolia hypoleuca, Magnolia officinalis and Magnolia hypoleuca, the last one being a native Japanese species and the easternmost of the Asian species of this section. The Chinese species Magnolia officinalis ranges from southern Shanxi to southeastern Gansu, southeastern Henan, western Hubei, southwestern Hunan, central and eastern Sichuan to northern Guizhou. Its variety, Magnolia officinalis var. biloba, appears to occur wild in eastern China (eastern Anhui, western Zhejiang, Mt. Lushan in northern Jiangxi, southern Hunan, southern and central Fujian, northern Guangdong, and northern and northeastern Guangxi). M. rostrata grows in northwestern Yunnan and adjacent southeastern Xizang and in northeastern Upper Burma. One of the American series in this section includes three species, M. tripetala, M. fraseri, and M. pyramidata. M. tripetala is indigenous to the Appalachian and Ozark Mountains of the eastern United States (Alabama, Arkansas, Florida, Georgia, southern Indiana, Kentucky, Maryland, Mississippi, North Carolina, southern Ohio, eastern Oklahoma, Pennsylvania, South Carolina, Tennessee, Virginia, and West Virginia), and is now introduced and naturalized to New York and Massachusetts. M. fraseri has a more restricted distribution in the southern Appalachian Mountains (northern Georgia, eastern Kentucky, western North Carolina, northwestern South Carolina, western Virginia, and West Virginia). M. pyramidata grows on the coastal plain of the southeastern United States (Alabama, northwestern Florida, Georgia, Louisiana, southern Mississippi, southern South Carolina, and southeastern Texas). The other American series comprises two species: M. macrophylla is native of the southeastern United States in the southern Appalachian and Ozark Mountains (Alabama, northeastern Arkansas [perhaps extirpated], Georgia, Kentucky, Louisiana, Mississippi, North Carolina, southern and central Ohio, Tennessee, and western Virginia). M. ashei is closely related to M. macrophylla and occurs only on the coastal plain of northwestern Florida. Three hybrids in this section, M. × thompsoniana (= M. tripetala × Magnolia virginiana), M. × wiesneri (= Magnolia hypoleuca × M. toringo) and M. × vitchii (= M. campbellii × M. denudata) have long been cultivated.
Subgenus Yulania comprises three sections with about 19 species (including one variety) and two hybrids, based on Dandy’s system. Two of the three sections are temperate Asian. The other one is another disjunct section between Asia and America in the genus Magnolia.
There are approximately nine species in the beautiful garden section Yulania (Spach) Dandy, which occur in temperate eastern Asia from the eastern Himalaya to eastern China. Magnolia denudata is the oldest species in cultivation and native of eastern China, but cultivated in many regions of the country, such as Anhui, Fujian, Guangdong, Guizhou, Hunan, Jiangsu, and Zhejiang, as well as in Japan. M. sprengeri replaces M. denudata in western Hubei, western Henan, eastern Sichuan, Guizhou, and Yunnan in China. Two of the other westward-ranging species in Asia are M. dawsoniana and Magnolia sargentiana. Both were found only in western Sichuan and northern Yunnan and are very rare. Another species, M. campbellii is the westernmost representative of the section Yulania and ranges from Nepal along the eastern Himalaya (Bhutan, India-Assam, Sikkim, and northern Burma) to western Yunnan and Xizang. Treseder () noted the interesting distribution pattern in this area: “As often happens in species with this distribution (Himalayas to west China) the plants from the two extremes of the range show differences, the significance of which is open to different interpretations”. The other two native species of China are M. amoena and M. zenii. They are apparently closely allied to M. denudata, but distributed in eastern China. M. amoena occurs in Mt. Tianmushan of northern Zhejiang and M. zenii ranges from the neighborhood of Nanjing in southern Jiangsu to Henan. Two more native species of central China were recently treated by Law () in Flora Reipublicae Popularis Sinicae, M. multiflora M.C. Wang et C.L. Min () and M. elliptigemmata C.L. Guo et L.L. Huang (). Both of them have very restricted distributions in central China. M. multiflora was collected in Shaanxi and Ningxia, and M. elliptigemmata has been found in Hubei and Yunnan. A hybrid, M. × soulangeana (= M. denudata × Magnolia liliflora), also belongs to this section. This hybrid has long been cultivated in Hangzhou (Zhejiang), Guangzhou (Guangdong), and Kunming (Yunnan) in China, and has also been widely cultivated in North America. Its early flowering period, however, makes the flowers susceptible to frost damage in the spring.
The section Buergeria (Siebold et Zuccarini) Dandy is another temperate Asian section with about six species listed in the GRIN database and in Flora Reipublicae Popularis Sinicae. Three species are in Japan and Korea, one in eastern China, and the other two in central China. Magnolia kobus is the most widely distributed of the three Japanese and Korean species. It occurs in southern Japan and on the coast of southern Korea. Magnolia salicifolia has a more restricted distribution than Magnolia kobus. It was found on Honshu, Kyushu, and Shikoku in Japan. The third Japanese native species is M. stellata. It occurs only on the islands of Honshu and Kyushu. Magnolia biondii is the northernmost species in China. Its home is in central China, with a range extending from Gansu, Shaanxi, western Henan and western Hubei to eastern Sichuan. The other native Chinese species is M. cylindrica, ranging from southern Anhui to northern Jiangxi, northwestern Zhejiang, and northern Fujian. The newly recognized species M. pilocarpa Z.Z. Zhao et Z.W. Xie, has a very narrow distribution in Luotian county, Hubei (). Magnolia × loebneri Kache (= Magnolia kobus × M. stellata) has been cultivated in many areas.
The section Tulipastrum (Spach) Dandy is the second of two sections having a disjunct distribution between Asia and America. There are two deciduous species and one variety in the section. One Asian species and one American species and its variety have long been in cultivation. The Asian species Magnolia liliflora has been cultivated for both medicinal and ornamental uses in China and Japan. It is believed to have originated in eastern China and probably in the temperate region south of the Changjiang (Yangtze) River. The American species Magnolia acuminata and its variety Magnolia acuminata var. subcordata are the most widely distributed Magnolia in the Americas. Magnolia acuminata is the only species to reach Canada (extreme southeastern Ontario). Geographically, it ranges from the Appalachian and Ozark Mountains of the eastern United States to southern New York.
Selections from the book: “Magnolia. The genus Magnolia”. Edited by Satyajit D. Sarker and Yuji Maruyama. Series: “Medicinal and Aromatic Plants — Industrial Profiles”. 2002