Controlling factors of the geographic difference in Siebold's beech (Fagus crenata Blume) forest vegetation between the Pacific Ocean side and the Sea of Japan side region

Abstract

We reviewed our recent studies on the cause of geographic difference of Siebold's beech (Fagus crenata) forest vegetation in Japan with special reference to the effect of snow-pack on tree regeneration. 　(1) A direct gradient analysis of forest vegetation along snowfall gradient showed that the species diversity index declined clearly along the snow depth gradient. One third of the tree species was not distributed in the snow-rich region ( >2.0-m in snow depth). Other species were distributed from snow-poor region (< 2.0-m in snow depth) to snow-rich region. However, they lost their relative importance in snow-rich conditions exceptF. crenata, which was the only species that gained very high dominance in snow-rich region. (2) Regarding the snow pressure tolerance hypothesis, traits of beech forest species; i.e. trunk bend, trunk height decrease, trunk damage and basal sprouting rate were compared along the snow pressure gradient. General trends recognized are 1) trunk bend and trunk height decrease; 2) trunk injury increase; and 3) maximum size decrease. However,F. crenata was the only species whose trunk form and maximum size were hardly influenced by snow pressure.(3) The causes and timing of seed death in early regeneration of F. crenata was studied at 15 sites along a snowfall regime in countrywide scale. Seed production did not significantly differ between snow-rich regions and snow-poor regions. Pre-dispersal seed mortalities by insects were higher in snow-poor regions than those in snow-rich regions, but this only seemed to be a minor factor influencing the population. A large proportion of the viable seeds were killed in winter. Mortality in winter was much higher in snow-poor regions than that in snow-rich regions, and this factor was strongly correlated with the geographic variation of seedling regeneration probability. Germination experiments made clear the reason of high mortality of beech nuts in winter, and suggested that effects of desiccation, rodents, fungi and bacteria were very strong. (4) These results showed the close relationships between the snowfall regime and F. crenata regeneration, and are consistent with its distributional and environmental development since Last Glacial time.