**Acknowledgements**

We thank Dr. M. Kitao, Dr. H. Tobita, Dr. H. Utsugi (FFPRI) and Prof. Y. Maruyama (currently Nihon University) for their invaluable comments on this study. Thanks are also given to researchers at the Hokkaido Research Center, FFPRI for their encouragement. We are grateful to the technical staff of Hokkaido Research Center, FFPRI for maintenance of the arboretum. We thank Ms. Saki Fujita (Hokkaido University) for English improvement and Ms. M. Ohbuchi for assistance in the experiments.
