Phytosociological characteristics of Betula davurica–Quercus crispula forests in Japan based on a comparison with summergreen broad-leaved forests in Northeast Asia

Betula davurica and deciduous Quercus forests are among the principal forest types in Northeast Asia. They are found throughout continental Northeast Asia and in some mountainous regions of Central Honshu and eastern Hokkaido in the Japanese Archipelago. B. davurica–Quercus crispula forests in Japan are significant for understanding the phytogeography of Japan and Northeast Asia. However, their relationships with B. davurica–Quercus mongolica forests in continental Northeast Asia remain poorly understood. We compared the species composition of B. davurica–Q. crispula forests of Japan, with those of Q. crispula dominant forests and B. davurica–Q. mongolica forests using phytosociological methods. The results suggested that B. davurica–Q. crispula forests of Japan had a significant number of species common to the order Lespedezo bicoloris–Quercetalia mongolicae and the class Querco mongolicae–Betuletea davuricae in continental Northeast Asia than in other associations in Japan. Furthermore, our results showed that associations between B. davurica and Q. crispula forests in Central Honshu and Hokkaido are related to different orders in continental Northeast Asia. The relations of the associations between Japan and the continent are considered to have been caused by the environmental similarities, the histories of taxa, and the changes in vegetation distribution.