近30年青海三江源西部干旱区草地退化特征的遥感分析

使用自20世纪70年代末~2004年的3期遥感图像(70sMSS、90sTM和2004年TM/ETM),研究了近30年青海三江源西部干旱区草地退化的格局与过程,结果表明:整个研究时段内,草地退化发生面积占总草地面积的10%左右,且以草地覆盖度轻度下降为主,其退化面积占总草地退化面积的80%以上,其次是轻度沙化/盐化,相应比重占10%以上。草地退化面积呈东南向西北减少趋势,退化程度呈东南向西北降低格局,而退化类型也由复合型向单一型过渡。整个时段内,草地退化面积呈增加趋势,尤其是沙化面积增加较快。不同海拔、坡度和坡向间草地退化面积差异显著。海拔4 800~5 100m范围内为草地退化发生的主要分布区,4 500~4 800m和5 100m以上草地退化基本相当。退化率较高的坡度级别介于2~8°之间,类似于草地面积随坡度变化的基本趋势。不同坡向呈阴坡、半阴半阳坡高于阳坡的态势。脆弱的基底、极端气候年际间周期波动等限制性因子控制着草地退化的基本格局,人为扰动则决定草地退化的强度和速度。总体看来,研究区地带性自然条件决定的荒漠化草地已占据主导地位,人类活动相对较弱,草地退化程度增加趋势不明显,新的草地退化现象不十分突出。

Remote Sensing-based Characteristics of Grassland Degradation in the Arid Western Three-River Sources Regions of Qinghai in the Past 30 Years

Using three-phase remote sensing images (i.e., 1970sMSS, 1990sTM and 2004TM/ETM) and field investigation, this paper reconstructed spatial and temporal characteristics of grassland degradation across the arid western Three-River Sources regions of Qinghai for resent 30 years. The results indicated that the proportions of grassland degraded area to total grassland area were approximately 10% of the whole study stages. The area of slight degraded grassland coverage to total grassland degraded area was 80%, followed by slight desertification and salinization of grassland, accounting for 10%. At spatial patterns, the reduced tendency of grassland degraded area was along a southwest to northwest direction in the study site. In this orientation, the grassland degradation degree presented declined stages, and the grassland degradation types were transiting from complexity to single. At temporal possess, the increasing tends were observed, compared with the area of grassland degradation of two stages, especially grassland desertification. Significant differences are among grassland degraded areas under different altitudes, slope gradients and aspects. A number of grassland degraded areas are mainly distributed in the altitude of 4800-5100m, and the grassland degradated area at 4500-4800m was not obviously different to that above 5100m. Furthermore, the proportions of grassland degraded area to total grassland area occurred at the slope gradient of 2-8^o, and were similar to the distribution of grassland area under different slope gradients. At the same time, there was more grassland degraded area in shady and on subsunny slope than on sunny slope. The reasons for these results were due to the limited factors, including vulnerable environment and interannual periodic fluctuations of extreme climate which controlled the trends and processes of grassland degradation, while human disturbances, as incentive forces, determined the intensity and speed of grassland degradation. In short, grassland desertification associated with natural conditions has been in a dominant position in the study site. Human activities were relatively weakening. The increasing trends of grassland degradation were not pronounced, and new grassland degradation phenomenon was not very obvious.