Abstract:

The Helan Mountains (38°10′－39°30′　N and 105°45′－106°45′　E) is situated on the eastern edge of the Alashan Plateau and the western edge of the Yinchuan Plain and extend about 270 km from north to south and about 20－40 km east to west. Its general altitude ranges from 2 000 m to 3 000 m with the highest summit at 3 556 m above sea level and relative elevations are 1 500－2 000 m. The Helan Mountains form an important boundary of climate and vegetation in northwest China: the eastern side of the Helan Mountains belongs to steppe climate and steppe vegetation and desert climate and desert vegetation characterize the west resulting in two different biomes. The mountains are a core area of the Alashan-Ordos biodiversity center that is among the top eight centers of biodiversity in China. It is a rich source of plants for the arid west and is an important pivotal point that connects the floras of the Qinghai-Xizang Plateau, the Mongolian Plateau and North China. Hence, it is very important to study the biodiversity of the Helan Mountains. The diversity and spatial distributional characteristics of plant communities are discussed in this paper. Based on our observations and research over many years, we have classified 11 vegetation types, and 55 formations in the Helan Mountains. The vertical zonation of the vegetation is strongly developed: vegetation belts can be divided into the desert belt (below 1 600 m asl), the steppe belt (1 600-1 900 m asl), the coniferous forest belt (1 900-3 100 m asl), and the alpine shrub or alpine meadow belt in the alpine or sub-alpine zone (>3 100 m asl). There also is a strong differentiation of vegetation on sunny and shady slopes. In the steppe belt of low hillsides, steppe communities inhabit sunny slopes but mesophilous shrub occur on shaded slopes. In the coniferous forest belt in the mid-elevation zone, the community of Picea crassifolia is distributed widely on shaded slopes but open forests of Ulmus glaucescens and Juniperus rigida or other mesophilous shrub occur on the sunny slopes. At 3 000 m and upwards, the vegetation of sunny and shade slopes is similar. The vegetation also is differentiated in an east-west and north-south direction resulting in some unique communities. The climate is warm and dry on the east side of the Helan Mountains and some thermophilic plants such as Zizyphus jujuba var. spinosa and Ostryopsis davidiana are distributed on the eastern side only. On the western side, the climate is cool and wet and there is a greater proportion of forests. The mid-elevation zone is the main body of the Helan Mountains, and the vegetation comprised primarily of forests and mesophilous shrub. The degree of desertification is very distinct in both the north and south segments of the mountains, but the communities are different. In the north, Ammopiptanthus mongolicus, Salsola laricifolia and Tetraena mongolica are dominant whereas Ephedra rhytidosperma, Syringa pinnatifolia var. alashanensis are dominant in the south. Furthermore, there are four endemic communities with Syringa pinnatifolia var. alashanensis, Ephedra rhytidosperma, Leptodermis ordosica and Hippolytia alashanensis in the Helan Mountains.