Key project(02103) supported by National Education Department of China; Key project(02A008) supported by the Education Department of Hunan Province,China; Project(2005090) supported by Central South University of Forestry and Technology; Project(03JJY3007) supported by the Natural Science Foundation of Hunan Province,China; Project supported by the Rewarding Project for Excellent PhD Thesis of Hunan Province,China; Project(07031B) supported by Scientific Research Fund of Central South University of Forestry and Technology;