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Based on a database of more than 40 years of second production process and energy flow records for Maduo,Qumalai and Yushu counties,a dynamic model of the stress within grassland ecosystems was established using a nonlinear regression method for this source regions of the Yangtze and Yellow Rivers.The results show that dynamic curves of stress within grassland ecosystems in the three counties were in the shape of an inverted "U" during the period 1965-2007.It also revealed that the variation in actual amount of livestock inventories reflected the general trends of the stress within the grassland ecosystems in the source regions,although there were many other factors for the increase or reduction in grassland ecosystem stress.