LaboratoryofQuantitativeVegetationEcology;
InstituteofBotany;
ChineseAcademyofSciences;
Beijing100093(China)/E-mail:wangzp5@yahoo.com;
StateKeyLaboratoryofGasGeochemistry;
LanzhouInstituteofGeology.ChineseAcademyofSciences;
Lanzhou730000(China);
CollegeofResourceandEnvironment;
BEijingNormailUniversity;
Beijing100875(China);