Spatial Pattern of Biological Soil Crusts and its Changes in Gurbantunggut Desert during the 1970 to the 2000

The biological soil crusts,especially lichen crusts,are well developed in Gurban-tunggut Desert,the largest fixed and semi-fixed sandy desert in China.Together with vascular plant,biological soil crusts become an important factor relating to sandy surface fixation.The objective of this paper is to analyze the variation of the biological soil crust distribution in Gurbantonggut desert from the 1970s through to the 2000s.In this study,43 samples including lichen dominated biological crust,bare sand,litter and green vegetation were collected.The reflectance spectra of these samples were measured and a new biological soil crust index(BSCI)was used to detect and map the lichen-dominated crusts by remotely sensed data including four Landsat ETM+ images,four Landsat TM images and five Landsat MSS images,acquired in the 1970s,the 1980s and the 2000s respectively.Based on the maps of the biological soil crust distribution in the 1970s,the 1980s and the 2000s,we calculated the landscape indexes of biological soil crust using software Fragstats and analyzed the variations of the crust distribution and their landscape features.The results showed that:(1)the biological soil crust was mainly distributed in the southeast part of this desert,but became gradually sparse toward northern and western parts of this desert.The crusts in the southeast part decreased remarkably,while in the north and central parts of the desert it developed very well during the 1970s to the 1980s.(2)There was an increasing trend for crusted areas during the 1970s to the 1980s in general,and the crusted areas displayed a decreasing trend during the 1980s to the 2000s.The climate change and increasing human activities resulted in such a trend.(3)An increasing landscape fragmentation,irregularity of patch shape and bigger isolation between patches were observed during the 1970s to the 2000s because of the significant human disturbance.