Climatic and historical factors controlling horizontal and vertical distribution patterns of two sympatric beech species, Fagus crenata Blume and Fagus japonica Maxim., in eastern Japan

The Japanese Archipelago is unique when viewed in terms of beech flora, since two native species, Fagus crenata Blume and Fagus japonica Maxim., occur sympatrically there. In order to examine the most important environmental or historical factors restricting the geographical ranges of these beech species in eastern Japan (34–44°N and 137–143°E), horizontal and vertical distributions were comparatively examined in detail. The study used two kinds of data sets: (1) DS1, constructed by assembling data from literature, herbarium specimens, etc., and (2) DS2, based on a mesh vegetation database. The upper range limit was expected to be in equilibrium with the current climatic conditions for both species (correlated with temperature factors for F. crenata and with snow depth for F. japonica). The lower range limit was also expected to be in equilibrium with the present climatic condition for both species, but the importance of competition with evergreen trees was also suggested. Dispersal limitation and a topo-geological barrier (for F. japonica) were expected to strongly restrict the northern range limit. Under contrasting climatic conditions in winter (between the Pacific and the Sea of Japan sides in eastern Japan) the geographic ranges of the two beech species are differentiated because of a difference in tolerance to heavy snowfall. A range shift model that assumes a migration lag along a horizontal direction because of dispersal limitation can explain the observed distribution patterns of these beech species in relation to climatic change in the Quaternary.