Origin, Evolution and Distribution of the Taxodiaceae-文献传递-植物通论文库

作者：Yu Yong-fu

期刊：植物分类学报 1995年 4期

Keywords:Taxodiaceae, Modern distribution, Fossil distribution, Origin, Evolution, Dis- persal,

Abstract:

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 ref-

erence 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, occur-

lng 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 distri-

bution center of the Taxodiaceae, where the genera are found to reflect basically main phylo-

genetic stages of the family, including the most primitive genus Cryptomeria, more primitive

genus Glyptostrobus, relatively advanced genus Metasequoia, the more advanced genus Cun-

ningharnia and the most specialized genus Taiwania.

Despite the limited numbers of extant genera and species, and their restricted geographi-

cal 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 di-

verse and widely distributed throughout the Northern Hemisphere , reaching the higher lati-

tude 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, west-

ern North America and Europe; Metasequoia in East Asia and western North Americas

Cryptomeria, Cunninghamia and probably Taiwania in East Asia and Europe; and Se-

quoiadendron 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

Tertiary.

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 Taxodi-

aceae. 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 Tax-

odiaceae 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 un-

derwent extinction during the Cretaceous. Most of the extant genera, including Cryptomeri-

a, 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 distribu-

tion center. As the Mid-Continental Seaway through North American, a successful phyto-

geographic 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 gener-

a, 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 Eu-

rope, 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 Se-

quoiadendron 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 arriv-

ing 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 cli-

matic 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 be-

came extinct in Europe entirely, but the southern part of East Asia, where there was rela-

tively 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 sur-

vival. Probably, it was also the Quaternary glaciation that contracted the distribution of

Athrotaxis to Tasmania in the Southern Hemisphere.

Origin, Evolution and Distribution of the Taxodiaceae

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