北海道東部幌岩山の針広混交林の植生構造とその形成要因としての土壌特性

抄録

The vegetation structure and soil properties of mixed deciduous broadleaf/conifer forest were studied on Mt. Horoiwa (376 m altitude), eastern Hokkaido, Japan. 154 grids (1 ha : 100 m×100 m) were set out on an aerial photograph of the study area, and their vegetation was classified into three types by ratio of conifer ; broadleaf, mixed broadleaf/conifer and conifer forests. Further, the topography of the grids was categorized into ridge, valley and slope, and the aspect of slopes was determined in 8 directions. Broadleaf or mixed broadleaf/conifer forests and conifer forests were primarily distributed in valleys or on slopes facing north, north-east or east and on ridges or slopes facing south, south-west or west, respectively. In 18 quadrats, DBH of trees (over 2 m in height) were measured, and phytosociological measurements were carried out on the vascular plants on the forest floor (below 2 m in height). Two communities were recognized on the basis of similarity in relative dominance of trees, and on species composition of the forest floor : broadleaf-dominated forest and conifer-dominated forest. The former community was distributed on the slopes facing north, north-east or east ; the latter was on the slopes facing south, south-west or west. Soil types of the broadleaf-dominated and the conifer-dominated forests were identified to B_D type (mesic brownish forest soil) and B_B type (xeric brownish forest soil), respectively. Synthetic indices of habitat fertility were provided using the soil chemical properties of horizon A, which showed a tendency to become gradually reduced in xeric habitats (i.e., ridges or slopes facing south, south-west or west). Soil chemical properties showed clear differences between the two community types. In particular, a close relationship between the relative dominance of conifers (mainly Abies sachalinensis) and habitat fertility was recognized.