Yang Xinbing<sup>1,2</sup>,Yu Xinxiao<sup>1</sup>,Wu Jun<sup>3</sup>,Xiao Yang<sup>1</sup>,Chen Huixin<sup>1</sup>,Sun Qingyan<sup>1</sup>
1.College of Soil and Water Conservation,Beijing Forestry University,Key Lab.of Soil & Water Conservation and Desertification Combating,Ministry of Education,100083,Beijing;2.Agriculture University of Hebei,071000,Baoding,Hebei;3.Beijing Municipal Bureau of Parks and Afforestation,100029,Beijing:China