河西山地绿洲荒漠植物群落多样性研究 Ⅱ放牧扰动下草地多样性的变化特征

Studies on the population diversity of plant community in Hexi mountain-oasis-desert area, Gansu, China Ⅱ Impacts of grazing pressure on species diversity in steppe

对张掖山地绿洲荒漠各草地群落放牧扰动下物种丰富度及物种替代速率 (β多样性 )进行了测度 ,结果表明 :在放牧压力梯度 (轻度、中度、重度和过度 )上 ,山地荒漠草原、山地典型草原、高山草甸草原和盐渍化草甸的物种丰富度呈单峰曲线 ;荒漠草原物种丰富度在过牧压力下最高。在放牧压力下各草地群落 βw和 βt值 (物种替代速率 )由大到小的序列是 :高山草甸草原 >盐渍化草甸 >山地典型草原 >山地荒漠草原 >荒漠草原 ;βc和 βr的测度结果兼有反映草地群落放牧压力梯度上物种替代速率和植物群落物种丰富度的内涵。二者的测度结果为 :山地典型草原>高山草甸草原 >盐渍化草甸 >山地荒漠草原 >荒漠草原。

The study was carried out within the scope of Zhangye city, Gansu province, northwest of China. The sampling area consisted of two geographic areas, i.e. midsection of the northern slope of Qilian mountain forest grassland area and Zhangye desert oasis area with the altitude ranging from 1 450 m to 3 700 m. There occurred five steppes along the altitudinal gradient, i.e., desert steppe(1 470 m^1 800 m), saline meadow(1 460 m～1 500 m), montane desert steppe(1 800 m～2 350 m), typical steppe(2 350 m～2 950 m)and alpine steppe(2 950 m～3 700 m).Main disturbance imposed on these steppes was grazing. In order to uncover the responses of species diversity of steppe to grazing pressure, a measurement on species richness and β diversity in each of these steppes was conducted. The result showed that (1)the species diversity represented by number of species of all steppes excluding desert steppe were highest under moderate grazing pressure and declined with increased grazing pressure and reached minimum under over grazing pressure;(2) The pattern of species diversity in desert steppe under grazing pressure, i.e., it was highest under over grazing pressure and quite stable under the other three grazing pressure stages ( light, moderate and heavy grazing ), was associated with the specific environment in which desert grassland involved;(3) The trend of β diversity of βc and βr among the five steppes was alpine steppe > saline meadow >typical steppe >montane desert steppe>desert steppe, while the trend of β diversity of βw and βt among the five grasslands was typical steppe >alpine steppe > saline meadow >montane desert steppe>desert steppe;(4)Over grazing along with the unreasonable grazing strategy might be responsible for the degradation of grassland in the study area.