1. State Key Laboratory of Vegetation and Environmental Change; Institute of Botany; Chinese Academy of Sciences; Beijing 100093 China
2. Department of Plant Sciences; University of California; Davis California 95616 USA
3. Department of Ecology and Evolutionary Biology; University of California; Irvine California 92697 USA
4. Department of Ecology and Evolutionary Biology; University of Arizona; Tucson Arizona 85721 USA