We thank Ji-Shen Wang for processing photographs. We are grateful to Wen Zhong for comments on the early draft of the manuscript. We also thank the two anonymous reviewers for their helpful comments and valuable suggestions on the revision of the manuscript. The research was financially supported by the National Natural Science Foundation of China (Grant no. 31672341).
Received: 20 January 2017
Revised and accepted: 3 May 2017
Published online: 23 October 2017
Editor: Herman de Jong