1. Forestry College, Northwest A&F Univ., Taicheng Rd., No. 3, Yangling 712100, China; and Forest and Landscape, Faculty of Life Sciences, Copenhagen Univ., Rolighedsvej 23, 1958 Frederiksberg C, Copenhagen, Denmark.
2. Professor, Forest and Landscape, Faculty of Life Sciences, Copenhagen Univ., Rolighedsvej 23, 1958 Frederiksberg C, Copenhagen, Denmark.
3. Professor, Forestry College, Northwest A&F Univ., Taicheng Rd., No. 3, Yangling 712100, China (corresponding author).