The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (Grant No. 21607176), the Natural Science Foundation of Hunan Province, China (Grant No. 2017JJ3516), the Research Foundation of Education Bureau of Hunan Province, China (Grant No. 16B274), and The Open fund for key discipline of Forestry of Central South University of Forestry and Technology (Grant No. 2016ZD11). The authors also thank the reviewers for their constructive comments and suggestions.