青藏高原冻融荒漠化的若干问题——以藏西-藏北荒漠化区为例

Some Problems of Freeze-thaw Desertification in the Tibetan Plateau: A Case Study on the Desertification Regions of the Western and Northern Plateau

冻融荒漠化是高海拔地区特有的土地退化过程, 依据生态基准面理论将青藏高原冻融荒漠化划分为极重度、重度、中度和轻度4个等级. 藏西-藏北区冻融荒漠化面积达44303.73 km2, 多呈斑块状零星分布和片状分散分布. 高原冻土区具有冻融荒漠化形成的环境基质, 近40 a区域气候持续变暖, 人为活动频率与强度加剧, 鼠类活动猖獗, 使浅层多年冻土的冻融过程加剧, 从而形成冻融荒漠化土地. 其形成过程主要有多年冻土季节融化层增厚-地下水位下降-地表土壤干燥化、地表覆盖改变或地下融水增加-冻土融冻界面热融-地表沉陷破碎、冻融作用过程和斜坡过程受到强化等. 预测未来20～30 a冻融荒漠化继续发展, 程度加重. 防治冻融荒漠化的对策主要是减轻草场压力、恢复草地植被, 加强鼠害防治和合理布局开发工程、采用工程与生物措施等.

As a process of land degeneration, freeze-thaw desertification takes place in the high altitude regions due to increase in the depth of seasonal thaw layer, which causes an intensive freeze-thaw or sloping process in surface earth. The freeze-thaw desertification always results in the soil and vegetation degenerations. According to the ecological datum plane theory, the freeze-thaw desertification in the plateau can be classified into four types with different landscape indicators and characteristics, i.e. extremely severe, severe, medium and slight degrees. The land area of the freeze-thaw desertification in the western and northern Tibetan Plateau amounts to 44303. 73 km^2 and occupies 9.61% of the whole area of the northern and western plateau. The freeze-thaw desertification lands embed in sheet and spot shape in the study area. Long-term freeze-thaw process, low temperature instability of permafrost and covering of sparse vegetation will be thought as the environmental background of the freeze- thaw desertification in the plateau. Over the last 40 years, the air temperature in the Tibetan Plateau increases with a rate of 0. 26℃/10a. Moreover, the number of population and livestock on the Tibetan Plateau is growing rapidly and grasslanc has long been overgrazed. In addition, mice and rabbits have also been rampant, having intensified the freeze-thaw processes of the ground surface and enlarged the thawing area, forming the desertification land ultimately. Seasonal thawing layer increasing leads to the drop of groundwater level and the desiccation of surface soil, resulting in the desertification process on the surface and increased recharge to the ground runoff, which will in turn thaw the freeze-thaw interface of the permafrost.The surface soil is sinking and then the slope process is intensified. These are the main processes for the formation of the freeze-thaw desertification. Analyzing the climate data found that the freeze-thaw desertification on the Tibetan Plateau would continue in the coming 20 to 30 years, with a rate similar to that of the last decades but the extent would be more severe. The estimated area of the desertification land on the northern and western parts of the plateau would reach 49334. 78km^2, 50471. 22 km^2, and 51633. 85 km^2 by the year 2010, 2020 and 2030, respectively. Restoring the grass vegetation, reducing the capacity pressures on the grassland through carrying out the ecological construction and developing substitute industries are the major countermeasures preventing the freeze-thaw desertification. The activities of mice and rabbits should also be monitored in order to prevent the burrowing processes and protect the frozen land environment. Biological and engineering measures could also be considered.