Article Effects of Urban Forest Types and Traits on Soil Organic Carbon Stock in Beijing Xinhui Xu 1,2, Zhenkai Sun 1,2,*, Zezhou Hao 1,2, Qi Bian 1,2, Kaiyue Wei 1,2 and Cheng Wang 1,2 1 Research Institute of Forestry, Chinese Academy of Forestry, Beijing 100091, China; xinhui...