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We analyzed effects of seed-hoarding by rodents on the variation in seed mass and seed success for a perennial forest undergrowth plant — Symplocarpus renifolius Schott ex Miquel — in Hokkaido, northern Japan. Although density of rodents differed greatly between seasons, more rodents were always captured in mesic Sasa sp. patches with dense foliage than in wet Lysichiton sp. patches. In the season with fewer rodents, they cached seeds close to the original places irrespective of vegetation, while in the season with abundant rodents, they transported seeds further and cached seeds disproportionately in Lysichiton patches. Seeds missed by rodents were larger than seeds that were eaten or that survived. Sasa patches are more suitable for seedlings to establish and a size advantage was observed there, but even small seeds could establish in Lysichiton patches, although seedling success was lower. We concluded that maternal plants of Symplocarpus renifolius increase their reproductive success by having small to middle-sized seeds transported to suitable sites while offering larger seeds as rewards to the transporters. Since the variation in seed mass was not correlated with the biomass per seed of the maternal plant, the large variation in seed mass is considered to have evolved through the density-dependent hoarding by rodents.

Regional variation in size and shape of Sphagnum hummocks of eight lowland mires throughout Hokkaido Island, northern Japan, was examined in relation to climatic factors. The size and shape of hummocks significantly differed among the regions: low and flat hummocks occurred in the northern region and next to the Sea of Japan; moderately raised and conical hummocks occurred in the western part of the Pacific coast; and extremely raised and cylindrical hummocks occurred in the eastern region. Multiple regression analysis revealed that the mean and maximum heights and height/basal area of hummocks were negatively correlated with evapotranspiration rate (E0) in summer, duration of sunshine, and air temperature. Of these, evapotranspiration rate in summer positively correlated with radiation and thermal factors. Mean height of hummocks within a mire was increased with precipitation during the growing season. Because the regional difference of the hummock form was not related to the difference in nutrient conditions, the effects of evapotranspiration and precipitation prevailed over edaphic conditions. Higher E0 in summer was considered to cause water stress for hummock species in the northern region and next to the Sea of Japan, whereas heavy rainfall relieved it in the western region of the Pacific coast. Consequently, lower and flat hummocks were a result of the higher E0 in summer, moderately raised and conical hummocks of the medium E0 and high precipitation, and extremely raised and cylindrical hummocks of a lower E0 due to prolonged foggy days.Key words: evapotranspiration, duration of sunshine, minerotrophy, ombrotrophy, precipitation.

Foliar phenologies of forest understory plants were categorized, and the distribution pattern of leaf habit was examined among different forest environments. Various patterns of foliar phenology were found, especially in herbaceous plants. In addition to the seasonal light regime controlled by the phenology of canopy trees, differences in the length of period with snow cover led to the divergence. Perennial-leaved plants predominate in intensely shaded habitats while annual-leaved plants are more abundant in less shaded habitats. The shade tolerance of perenniel-leaved plants can be considered a preadaptation to snow tolerance. In contrast with the perennial-leaved plants, biennial-leaved plants with leaves overwintering 1 year appear to be favored in euphotic habitats with high insulation both in spring and in autumn. These species are effective competitors in spring because of rapid emergence of current leaves, probably through retranslocation of resources accumulated in the previous year. Another adaptive trait is found in heteroptic plants simultaneously having summer-green leaves and overwintering leaves; these types of leaves seem to function in predictable and quite different environments in a year. Key words: foliar phenology, growth form, light resource, overwintering leaf, snow cover.

In the summer of 1988, a vegetation survey of the Siberian taiga was conducted in a discontinuous permafrost zone of northern China where a destructive fire had occurred in the spring of 1987. The dominant forest cover of this area was larch, with spruce forests occurring in wet valleys and pine forests on steep, dry slopes. The vertical structure of forests showed that the spruce forests were preceded by larch forests; however, most of the larch forests were self-renewable probably because their habitats were too dry for spruce trees to compete. In larch forests, the 1987 fire seriously damaged perennial herbs; in spruce forests, however, most of the herbaceous plants were protected by wet conditions of their habitats, except feather mosses, which could not survive in the habitat opened by fire. Many plants of the burnt habitats recovered vegetatively; in particular, the burnt stems of birch trees frequently sprouted many shoots and consequently appeared to be contributing to the rapid reestablishment of larch, which is generally preceded by birch trees in ecological succession. Pine forests were not self-renewable; however, they seemed independent of the normal course of succession by dominating dry habitats where their competitors were not successful.