黄土高原白羊草草原的基本特征及其地理学意义

黄土高原的白羊草草原具有水平地带性特征，是第三纪亚洲内陆热带性质的草原随着干旱的发展和气候的变冷逐渐向外推移的结果。因而该草原是次生的，残遗的，本区现在的分布区正是欧亚内陆外围残遗的分布区的组成部分。该草原分布通常与森林相结合或形成群落复合体，构成森林草原的自然景观。该草原西部和北部边缘应是中亚与东亚植物区系的自然界限，即恒山－雁门关－管涔山－白于山－六盘山－天水－武山。该草原的建群种和优势种主要

CHARACTERISTIC ON THE STEPPE OF BOTHRIOCHLOA ISCHAEMUM IN LOESS PLATEAU AND ITS GEOGRAPHICAL SIGNIFICANCE

The steppe of Bothriochloa ischaemum in Loess Plateau are the characters of thezone. It is result that this subtropical steppe in Loess Plateau were moved from Eurasianinland to periphey along with the intenifying drought and the changing cold of the climate after the end of Tertiary,so this steppe is secondary and relict. The present distribution in this area is a part of the relic area in the periphery of the Eurasian inland.This steppe usually united with the forest or have come community complex. Finally,it shows that the forest-steppe in this area reprensent obviously the regional landscape.Meanwhile, this distributional boundaries of the North and West parts should bethe natural border-line between centre Asian and east Asian floristic. This line is locatedin Hengshan-Yanmenguan-Guancenshan.Because the edificatores and dominants are temperate-subtropical area, so thissteppe presents subtropical characters. But it is also a lot of xerophilous species of thetemperate steppe showing the trastional characters in Loess Plateau.