藏北高山嵩草草甸植被和多样性在沙漠化过程中的变化

Changes of Vegetation and Diversity of Alpine Kobresia(Kobresia pygmaea) Steppe Meadow in Desertification Process in Northern Tibet Plateau

为确定沙漠化对高山嵩草草甸植被组成、结构和物种多样性的影响,了解高寒区草甸沙漠化的原因,选择西藏那曲安多县南部沙漠化严重区域为调查区,按照沙漠化的不同程度设置样地,系统调查了轻度、中度、重度和极重度沙化草甸的植被变化,结果表明:中度、重度和极重度沙化区的植被与轻度沙化草甸有显著的差异;在中度和重度沙化区,高寒草甸的建群种高山嵩草已被家畜不喜食或更具抗性的植物种所取代,而在极重度沙化的流动沙丘上无植被生长;从过牧的退化草甸到半流动、流动沙丘,植物种多样性呈显著的降低趋势。轻度沙化草甸物种数、个体密度和丰富度指数最多;中度沙化草甸的Shannon-Wiener指数和均匀度指数最大,而优势度指数最小;在沙化过程中,高寒草甸的植被盖度显著下降,地上生物量也在下降,虽然轻度、中度和重度沙化草地的地上生物量显著高于极重度沙化区,但前者之间却无显著差异。地下根系生物量也呈显著下降的趋势。过牧是造成高山嵩草草甸沙化的主要原因。

For determining the impacts of desertification on vegetation composition, structure and species diversity of alpine Kobresia steppe meadow and understanding the causes of desertification, a margin area of severe desertification region in Anduo County, Tibet Autonomous Region, was selected as study site. According to the different levels of desertification： slight desertification （SLD）--moderate desertification （MD）--severe desertification （SD） very severe desertification （VSD）, the floristic features of total 20 samples were investigated and analyzed. The results drawn from this case study are listed as follows：①The composition and structure of alpine Kobresia steppe meadow in SLD rangeland differed significantly from MD, SD, and VSD, the species with more drought resistance and inedible by livestock were the dominant species of SD rangelands. No any plant was found in the VSD rangelands. ②There is a significant negative correlation between different stages of desertification and species diversity from overgrazing pastures to semi-shifting and shifting dunes. The SLD rangeland had largest mean number of species and individuals, and Richness index values, but the MD grassland had largest Shannon-Wiener index values, Evenness index values, and smallest Simpson index values. ③The vegetation cover declined significantly from 91.8% （overgrazing pastures） to34.8% （semi-shifting dunes） and to 0% （shifting dunes）. ④The SLD, MD, and SD grasslands had significant higher aboveground biomass than the VSD, but no significant difference existed between the SLD, MD, and SD grasslands （P〈0.05）. There were significant difference in root biomass and dry matter among different desertification levels （SLD, MD, SD, and VSD）. The SLD had the highest root biomass and dry matter, followed by MD, SD and VSD. ⑤Overgrazing is the primary cause of desertification of alpine Kobresia steppe meadow and it accelerates the expanding of desertification in the region.