6Daily G.Nature Services:Societal Dependence on Natural Ecosystem[M].Washington D C:Island Press,1997:122-134.
7Costanza R,d'Arge R,de Groot R,et al.The value of the world's ecosystem services and natural capital[J].Nature,1997,387(6630):253-260.
8Millennium Ecosystem Assessment.Ecosystems and human well-being:Biodiversity synthesis[R].Washington D C:World Resources Institute,2005.
9PIAO Shi-long,FANG jing-yun,ZHOU Li-ming,et al.Changes in biomass carbon stocks in China's grasslands between 1982 and 1999[J].Global Biogeochemical Cycles,2007,21,GB2002,doi:10.1029/2005GB002634.
10Jian Ni.Estimating net primary productivity of grasslands from field biomass measurements In temperate northern China[J].Plant Ecology,2004,174:217-234.
