放牧对藏北紫花针茅高寒草原植物群落特征的影响

Effect of grazing on community characteristics and species diversity of Stipa purpurea alpine grassland in Northern Tibet

基于放牧试验,研究了不同放牧强度下藏北地区紫花针茅(Stipa purpurea)高寒草原植物群落特征的变化规律。结果表明,随着放牧强度的增强,植物群落盖度、地上生物量均呈现显著降低的趋势;紫花针茅等禾草类植物的重要值逐渐降低,莎草类中青藏苔草(Carex moorcroftii)、牲畜不喜食的杂类草及有毒有害植物均有增加的趋势;如果持续过度放牧,植物群落表现出由紫花针茅等禾草为建群种的草地型向青藏苔草(Carex moorcroftii)、杂类草(Herbarum variarum)草地型过度的趋势;在中度放牧强度下,紫花针茅高寒草原α物种多样性达到最高水平,而继续增强放牧强度,则造成各项指标的迅速降低。

Alpine grassland is widely distributed in Northern Tibet. Stipa purpurea alpine grassland is one of the most important and representative meadow types in this area. In recent years,due to global warming and human activities such as overgrazing or excessive reclamation,the alpine grassland in Northern Tibet has undergone an accelerated and large-scale degradation. The grassland＇s productivity and biological diversity have decreased significantly and locally,this has also become a great obstacle to sustainable social,economic and ecological development. To investigate community characteristics and α diversity of Stipa purpurea alpine grassland under different grazing intensity in Northern Tibet is critical not only to the understanding of local degradation mechanisms,but also to the establishment of rational grassland management procedures,as required for continued healthy and sustainable development of alpine grassland. In our study,based on a grazing experiment in Northern Tibet grassland ecosystem,the community characteristics of Stipa purpurea alpine grassland under different grazing intensity were investigated. The research was conducted at the grazing experimental site of Anduo County,Naqu,Tibet Autonomous Region,at an average elevation over 4,500 m above sea level. The experiment included four different stocking rates：no grazing （0 head/hm^2,CK）,light grazing （2.395 head/hm^2,LG）,moderate grazing （3.593 head/hm^2,MG） and heavy grazing （5.988 head/hm^2,HG）. Based on the method of community investigation,community structure characteristics,such as species height,coverage and density were recorded. Aboveground biomass within areas of 0.5 m×0.5 m quadrat was also measured during the grass vigorous growth period. The species important value and summed dominance ratio were calculated by combining relative coverage,relative frequency and relative height. The species important value and summed dominance ratio were selected as comprehensive evaluation index of the importance of the species. The species diversity was measured using the Shannon-Wiener index and Pielou evenness index. The results showed that,with increasing grazing intensity,the above-ground biomass and the total coverage of plant communities followed a decreasing trend. Compared with the no grazing and light grazing treatments,the above-ground biomass and total coverage in the heavy grazing treatment were significantly lower. The important value of Stipa purpurea and other species was reduced gradually with increasing grazing intensity. Sedges such as Carex moorcroftii and other forbs,which the local livestock was rather avoiding,and also poisonous grasses were observed increased. The Shannon-Wiener index and Pielou evenness index changed according to the same pattern：MG﹥LG﹥HG﹥CK. The species diversity index of alpine grassland in this area reached the highest level under moderate grazing intensity. However,when the grazing intensity continued to increase beyond that level,the indices would decline rapidly. Comprehensive analysis indicated that the grassland had not only the highest diversity index under moderate grazing intensity,but could also resist a certain carrying capacity and prevent grassland degradation. Moderate grazing should be the most reasonable management strategy in Stipa purpurea alpine grassland. However,if grazing intensity continues to increase,the Stipa purpurea grassland type in this area would be transformed into a Carex moorcroftii,Herbarum variorum grassland community,which was clearly found to be of lesser economic and ecological value under the observed conditions.