Abstract
The floristic composition and stand structure of lowland forests were studied in the Ishikari Lowland, central Hokkaido, northern Japan. In 30 plots, diameters at breast height (i.e., DBH) of trees ≧5cm were measured, and phytosociological measurements were carried out on the vascular plants. Phytosociological classification methods were applied to 30 vegetation samples from the plots, and additional 10 samples obtained from the literature. Lowland forests were classified into the following two communities: the Ulmus japonica-Syringa reticulata community, and the Fraxinus mandshurica van japonica-Lysichiton camtschatcense community (includes 4 under units and 2 stand groups). These two communities were comparable to Syringo-Fraxinetum mandshuricae and Dryopterido monticolae-Fraxinus mandshuricae japonicae, respectively. Differences in floristic composition among plots were clearly shown by the first axis of detrended component analysis (DCA). Correlations between scores of the first axis and climatic features clarified that the sites of the F. mandshurica van japonica-L. camtschatcense community were warmer, and were deeper in snow cover than those of the U. japonica-S. reticulata community. Thus, the difference in floristic composition between two communities was probably caused by climatic conditions. On the other hand, equitability of tree dominance (relative basal area) was lower in the F. mandshurica van japonica-L. camtschatcense community than in the U. japonica-S. reticulata community. Since there was a difference in soil type between the two communities, it seems that not only climatic features but edaphic conditions were responsible for the difference in stand structure between two communities.
