Gradients controlling vegetation in Alnus japonica forests in Kushiro mire, Hokkaido, Japan

This study examined factors controlling the vegetation in Kushiro mire, northern Japan, especially in the alder and reed communities. The alder community was classified into four types that were arranged by canonical correspondence analysis (CCA) as follows: Carex lasiocarpa type, Carex augustinowiczii type, Persicaria type, and Spiraea type. The four types exhibited differences in size (tree height) and in the ratio of sprouting stems. Although plants growing on the forest floor primarily determined the species composition of the community, shading by alder crowns had no marked effect on the community gradient. Therefore, hydrochemical variables could control the growth and occurrence of both the floor plants and alder tree size. The differences between reed community and the alder community sites were characterized as heavily eutrophic by the Kruskal–Wallis test. Overall, the chemical variables pH and P₂O₅ and the hydrochemical variables maximum and range of water level were positively correlated with the CCA axis, and effectively explained the community gradient in relation to alder growth. These variables could be controlled by the inflow of neutral and turbid water from river floods or run-off, which would increase pH by replacing the acidic mire water, ash content, and P₂O₅ by conveying suspended inorganic particles that adsorb phosphorus.