Study on the degrading process and vegetation succession of Medicago sativa grassland in North Loess Plateau,China

The characteristics of community succession of M.sativa artificial grassland with ages from 1 to 30 years was studied using space for time method in Liudaogou catchment,in the north Loess Plateau in Shenmu County of Shaanxi Province.Results show that M.sativa productivity was low in general attributed to drought and poor soil fertility conditions.The highest fresh biomass was 4 000-5 000 kg/hm~2,but the flourishing duration was short no more than 6 years.Relevant managing measures,such as cultivation and fertilization that can ameliorate soil conditions and slow down competitive stress of M.sativa group,were suggested to be applied for the purpose of prolonging the life-span of artificial grasslands to some extent.The planting of M.sativa can significantly accelerate the succession process toward natural vegetations.M.sativa grassland can naturally succeed into the stage of stable prairie community-Stipa bungeana community in 10 years.Floristic species belonging to the three big families of Leguminocae,Compositae,and Gramineae played critical role in the degrading process of M.sativa grassland.The species number belonging to the three families occupied(54.5%)-78.6% of total numbers appeared in respective stage.As the main companion species of Stipa bungeana community in the studied area,Artemisia capillarys,Astraglus melilotoides and Lespedesa davurica reflected the arid climate condition and poor soil fertility.