基金This work is supported by the National Natural Science Foundation of China （No. 41574123）. We greatly appreciate Dr. Dai for providing us with his recently acquired field data from Africa. We also acknowledge the anonymous reviewers and editors for careful review of our manuscript and providing us with their comments and suggestion to improve the quality of the manuscript. The final publication is available at Springer via http ：//dx. doi.org/10.1007/s 12583-017-0749-1.