Forest floor conditions causing regional differences in the habitat of forb species in Japanese beech forest, eastern Japan

Abstract

In cool temperate forest zones in eastern Japan, certain forb species like Cimicifuga simplex in light^-snow regions grow on slopes of beech forests, while in heavy^-snow regions its habitat is rather limited to valleys adjacent to beech forests. This research study reveals reasons for this difference in forb species habitats. As key factors, the study focuses on differences in the lighting environment during the leafing season as well as the state of tree leaf litter sedimentation on the forest floors in both light^-snow and heavy^-snow environments. Regardless of the amount of snow, the volume of tree leaf litter sedimentation on the forest floor was large on slopes of beech forest and small in valleys. In heavy^-snow beech forests, the number of layers of litter sedimentation as well as the density of litter compaction within a certain thickness of sedimentation were larger compared to light^-snow regions. In terms of the lighting environment during the summer season, no difference was observed on slopes of beech forest between light^-snow and heavy^-snow environments. However, over the spring season in light^-snow regions, the lighting environment of the forest floors was good during the leafing seasons, because foliation begins with the herbaceous layer and leaf expansion of canopy trees comes later. On the other hand, in heavy^-snow regions, the lighting environment of the herbaceous layer was poor during the leafing season, since foliation starts from canopy trees as snow covering the floor remains longer. In valleys, foliation of herbaceous layer preceded that of shrubs even in heavy^-snow regions. This phenomenon was attributed to the fact that lingering snow in the valley floor covers the shrubs alongside the valley, which in turn delays foliation of the shrubs. Two factors are considered reasons restraining the growth of forb species in beech forests in heavy^-snow regions: 1） the large number of layers and the high density of litter sedimentation at the forest floor constrict the establishment and germination of forb species, and 2） the poor lighting environment from spring to early summer （which are supposed to be productive seasons） is due to lingering snow that makes the crown canopy of beech forests foliate before forb species. However, the study found that forb species are able to grow in valleys in heavy^-snow regions, because the volume of litter sedimentation is small and herbaceous layer begins and completes foliation before shrubs do.