In the present paper, the distribution center, the place and time of origin, the
way of dispersal and the formation of modern distribution patterns of the Taxodiaceae were
discussed based upon studies of phylogeny, fossil history and modern distribution, with reference to the paleogeography and paleoclimatology.
The Taxodiaceae, containing 9 genera, 12 species and 3 varieties, is a small family of
subtropical to Warm temperate trees with highly disjunct and restricted distribution, occurlng in East Asia, North America and Australia. Of these nine genera, seven are monotypic,
i. e., Cryptomeria, Glyptostrobus, Sequoiadendron, Sequoia, Metasequoia, Cunninghamia
and Taiwania, while the other two are ohgotypic, i. e. , Taxodium consisting of 2 species
and Athrotaris containing 3 species. Moreover, none of the genera is distributed on more
than one continent. Athrotaxis, endemic to Tasmania, is the only genus that is found in the
Southern Hemisphere. Metasequoia and Glyptostrobus are endemic to China. Cryptomeria is
found in China and Japan. Cunningharnia is widely distributed in China and northern Viet
Nam. Taiwania is native in the Chinese mainland, Taiwan and northern Myanmar. Se
quoiadendron and Sequoia are endemic to western America. Taxodium is distributed in
southeastern North America, Mexico and Guatemala.
According to Takhtajan's (1986) regionalization of the world flora, the number of
species in every region of the world was statistically surveyed. East Asia Region, with 5
genera and 5 species, is most aboundant in number of genera and species. In China, accord
ing to Wu's ( 1979 ) regionalization of Chinese flora, Sino-Japanese forest subkingdom, with
S genera and 5 species, is richest in genera and species,Sino Himalayan forest subkingdom,
with 3 genera and 3 species, ranks second. The region ranging from the Yangtze river,
Qinling Mts. to South China (the southern part of East Asia Region) is the modern distribution center of the Taxodiaceae, where the genera are found to reflect basically main phylogenetic stages of the family, including the most primitive genus Cryptomeria, more primitive
genus Glyptostrobus, relatively advanced genus Metasequoia, the more advanced genus Cunningharnia and the most specialized genus Taiwania.
Despite the limited numbers of extant genera and species, and their restricted geographical distribution, the Taxodiaceae, with a great number of fossil remains, was a very large
group of plants in geological times and once played an important role in the forest vegetation
of the Northern Hemisphere. The fossil distribution maps of all the living genera and several
important extinct genera were drawn. The developments of these genera and the family
were discussed. The earliest appearance of the Taxodiaceae was in the deposits of the Middle
Jurassic in Eurasia, and it began to occur in North America during the Early Cretaceous. In
the Cretaceous times, the Taxodiaceae, contaning all the modern genera and a large number
of fossil genera with most of them extinct after the Mesozoic, attained its greatest diversity.
During the Middle-Late Mesozoic and the Early Tertiary, the Taxodiaceae was highly diverse and widely distributed throughout the Northern Hemisphere , reaching the higher latitude region of the Arctic Circle, such as New Siberian Islands, Greenland, Spitzbergen ,
Franz Josef Land and Ellesmere˚˚ Islands at about 82˚ N. Most of the extant genera once had
three or two distribution centers: Glyptostrobus, Taxodiurn and Sequoia in East Asia, western North America and Europe; Metasequoia in East Asia and western North Americas
Cryptomeria, Cunninghamia and probably Taiwania in East Asia and Europe; and Sequoiadendron in Europe, North America and East Asia. The extant genera, especially
Glyptostrobus, Taxodium, Sequoiadendron, Sequoia and Metasequoia, were very important
elements of the temperate flora in the Northern Hemisphere during the Late Cretaceous and
Later than in East Asia and North America, most of the living genera began to appear
in Europe during the Early Tertiary, where taxodiaceous plants increased gradually from the
Paleocene to Miocene, but subsequently underwent a drop, as other conifers did. The Taxo
diaceae was also more diverse and more widespread in the Southern Hemisphere in the past
than at present.
The modern distribution of the Taxodiaceae seems to provide very little information as
to the determination of its origin place. Based primarily on the fossil evidence, the author
considers that the northeastern part of East Asia at higher middle latitudes (about 35˚-60˚
N), embracing northeastern and northern China, southeastern Siberia, Korea, Japan and
Sakhalin etc. , might have been the area of the origin and early differentiation of the Taxodiaceae. The origin may date back to the Early Jurassic or the Late Triassic.
After appeared on the earth, the Taxodiaceae dispersed westward to Europe and spread
to the Southern Hemisphere by crossing the Tethys Sea because of the less distinct zonation
of climate. In the Early Cretaceous, when Lanrasia had not broken up completely, the Taxodiaceae migrated from the Europe westward to North America, where the absence of
Jurassic records might have been the result of its dry environment. Many early genera underwent extinction during the Cretaceous. Most of the extant genera, including Cryptomeria, Glyptostrobus, Taxodium, Sequoia, Metasequoia, Cunninghamia and Taiwan&, and
some fossil genera, might be differentiated from their ancestral complex in the northeastern
part of East Asia during the Early Cretaceous or the Late Jurassic. They dispersed to the
northeastern Siberia and then migrated eastward to western North American via Bering
Land Bridge in the Late Cretaceous, where they flourished and formed a secondary distribution center. As the Mid-Continental Seaway through North American, a successful phytogeographic barrier during the Late Cretaceous, disappeared in the Latest Cretaceous, the
Taxodiaceae spread to eastern North American and then by North Atlantic Land Bridge,
which existed during the Paleocene and Early Eocene, arrived in Europe, where they
thrived and formed another secondary distribution center. From the Late Jurassic to the end
of Eocene, Asia and Europe were separated by the Turgai Straits, which formed the barrier
to plant migration and dried up completely in the Early Oligocene. As a result, a few genera, i. e. , Glyptostrobus, Taxodium, Sequoia and Metasequoia, did not reach central Asia from
East Asia until the Late Eocene or Oligocene and probably spread further westward to Europe, but the dispersal route is far less important for the living genera of the Taxodiaceae.
The birthplaces of Sequoiadendron and Athrotaxis are still unclear, but it is certain that Sequoiadendron spread in the Northern Hemisphere through the Bering Land Bridge and North
Atlantic Bridge and Athrotaxis dispersed in the Southern Hemisphere through the Antarctic.
During the Late Cretaceous and Early Tertiary, the living genera were most widespread
in the Northern Hemisphere, with Glyptostrobus, Taxodium, Sequoiadendron, Sequoia,
Metasequoia and Taiwania reaching the Arctic region, Cryptomeria and Cunninghamia arriving at the northern boundaries of their historical distributions respectively as well. Because
of the tectonic movements and climatic changes during the Late Tertiary, especially the climatic deterioration, the Taxodiaceae withdrew from the regions at higher latitudes in the
Northern Hemisphere gradually and was forced to migrate southward, its disappearance
from central Asia and eastern Europe might be caused by their dry climates. During the
Quaternary glacial period, Canada, northeastern America, central and western Europe and
northern Asia were covered by ice sheet, the Taxodiaceae retreated to low and lower middle
latitudes, where it was confined to much more localized areas. Finally, the Taxodiaceae became extinct in Europe entirely, but the southern part of East Asia, where there was relatively less influence by glaciation owning to its great diversity in topography and the climatic
conditions were more favorable, became the most important center for the survival of the
Taxodiaceae, while southern North America and Central America is another center of survival. Probably, it was also the Quaternary glaciation that contracted the distribution ofAthrotaxis to Tasmania in the Southern Hemisphere.
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