The formation and differentiation of the Leguminosae flora in Xizang (Tibet)

Abstract

Abstract:

Xizang (Tibet) is rich in Leguminosae flora, comprising 41 genera and 254 species

so far known, exclusive of the commonly cultivated taxa (including 11 genera and 16

species). There are 4 endemic genera (with 8 species), 10 temperate genera (with 175

species) and 19 tropical genera (with 46 species) as well as the representatives of those

genera whose distribution centers are in East Asia-North America, Mediterranean

and Central Asia.

1. There are altogether 4 endemic genera of Leguminosae in this region. Accord-

ing to their morphological characters, systematic position and geographical distribution,

it would appear that Salweenia and Piptanthus are Tertiary paleo-endemics, while

Straceya and Cochlianths are neo-endemics. Salweenia and Piptanthus may be some

of more primitive members in the subfamily Papilionasae and their allies are largely

distributed in the southern Hemisphere. The other two genera might have been derived

from the northern temperate genus Hedysarum and the East Asian-North American

genus Apios respectively, because of their morphological resemblance. They probably

came into existanc during the uplifting of the Himalayas.

2. An analysis of temperate genera

There are twelve temperate genera of Leguminosae in the region, of which the

more important elements in composition of flora, is Astragalus, Oxytropis and Cara-

gana.

Astragalus is a cosmopolitan genus comprising 2000 species, with its center

distribution in Central Asia. 250 species, are from China so far known, in alpine zone of

Southwest and Northwest, with 70 species extending farther to the Himalayas and

Xizang Plateau.

Among them, there are 7 species (10%) common to Central Asia, 12 species (15.7%)

to Southwest China and 40 species (60%) are endemic, it indicates that the differentia-

tion of the species of the genus in the region is very active, especially in the subgenus

Pogonophace with beards in stigma. 27 species amounting to 78.5% of the total species of

the subgenus, are distributed in this region. The species in the region mainly occur in

alpine zone between altitude of 3500—300 m. above sea-level. They have developed into

a member of representative of arid and cold alpine regions.

The endemic species of Astragalus in Xizang might be formed by specialization of

the alien and native elements. It will be proved by a series of horizontal and vertical vicarism of endemic species. For example, Astragalus bomiensis and A. englerianus are

horizontal and vertical vicarism species, the former being distributed in southeast part

of Xizang and the latter in Yunnan; also A. arnoldii and A. chomutovii, the former

being an endemic on Xizang Plateau and latter in Central Asia.

The genus Oxytropis comprises 300 species which are mainly distributed in the

north temperate zone. About 100 species are from China so far known, with 40 species

extending to Himalayas and Xizang Plateau. The distribution, formation and differ-

entiation of the genus in this region are resembled to Astragalus. These two genera are

usually growing together, composing the main accompanying elements of alpine mea-

dow and steppe.

Caragana is an endemic genus in Eurasian temperate zone and one of constructive

elements of alpine bush-wood. About 100 species are from China, with 16 species in Xi-

zang. According to the elements of composition, 4 species are common to Inner Mon-

golia and Kausu, 4 species to Southwest of China, the others are endemic. This not only

indicates that the species of Caragana in Xizang is closely related to those species of

above mentioned regions, but the differentiation of the genus in the region is obviously

effected by the uplifting of Himalayas, thus leading to the formations of endemic species

reaching up to 50%.

3. An Analysis of Tropical Genera

There are 19 tropical genera in the region. They concentrate in southeast of Xizang

and southern flank of the Himalayas. All of them but Indigofera and Desmodium are

represented by a few species, especially the endemic species. Thus, it can be seen that

they are less differentiated than the temperate genera.

However, the genus Desmodium which extends from tropical southeast and northeast

Asia to Mexio is more active in differentiation than the other genera. According to Oha-

Shi_,s system about the genus in 1973, the species of Desmodium distributed in Sino-Hima-

laya region mostly belong to the subgenus Dollinera and subgenus Podocarpium. The

subgenus Dollinera concentrates in both Sino-Himalaya region and Indo-China with 14

species, of which 7 species are endemic in Sino-Himalaya. They are closely related to

species of Indo-China, southern Yunnan and Assam and shows tha tthey have close con-

nections in origin and that the former might be derived from the latter.

Another subgenus extending from subtropical to temperate zone is Podocarpium.

Five out of the total eight species belonging to the subgenus are distributed in Sino-

Himalaya and three of them are endemic.

An investigation on interspecific evolutionary relationship and geographic distribu-

tion of the subgenus shows that the primary center of differentiation of Podocarpium

is in the Sino-Himalaya region.

Finally, our survey shows that owing to the uplifting of the Himalayas which has

brought about complicated geographic and climatic situations, the favorable conditions

have been provided not only for the formation of the species but also for the genus in cer-

tain degree.

Li Pei-Qiong, Ni Chi-Cheng. The formation and differentiation of the Leguminosae flora in Xizang (Tibet)[J]. .

The formation and differentiation of the Leguminosae flora in Xizang (Tibet)

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