In 1952, T. SUZUKI drew attention to the importance that the direction of prevailing wind in winter and the range of mountains determine the composition and the distribution of Japaneese forest vegetations.<br> An understanding of the subject in detail can best be obtained by local investigations of vegetation surveyed in several lines running across the Japan proper.<br> Considering of above mentioned, the writer surveyed the forest vegetation of Oku-Nikko in the Hokuetsu-North Kanto section where the boundary of the Japansea-type and the Pacific-type climate is found.<br> 1. Below are the listed the associations in Oku-Nikko.<br> 1, Vaccinieto-Pinetum betuletosum Usui<br> 2, Alnus Maximowiczii Forest (Rank: association)<br> 3, Tsugetum diversifoliae Kurita<br> 4, Tsugetum thujopsidetosum Usui Subass. nov.<br> 5, Rhodoreto-Pinetum Maeda et Usui Ass. nov.<br> 6, Abies homolepis Forest (Rank: association)<br> 7, Sasamorpheto-Fagetum sasosum nipponicae Usui Facies nov.<br> 8, Ulmetum Davidianae Suz-Tok.<br> 9, Toisusu Urbaniana Forest (Rank: association)<br> 10, Larix leptolepis-Betula platyphylla var. japonica Forest (Rank: subassociation)<br> 2, Alnus Maximowiczii Forest is a secondary forest developed in denuded area of Alpine zone, and the vegetation type of its Calamagrostis-Carex type forest floor stands in parallel relation to the secondary forest floor of Alnus firma Forest in montane zone.<br> 3, The development of Abies homolepis Forest lying between Tsuga and the beech forest climax is restricted in the mountain side of the volcano of Mt. Nantai and can not be found in the tertiary rocks of the area.<br> As to the cause of the restriction, the writer considers as follows.<br> The steeper the slopes the lower descend the Tsuga forest in the mountain side of tertiary rocks in immediate contact with the beech forest. While in the slopes of volcano, Sasa nipponica, the dominant species of beech forest floor enlarge its territory up to the subalpine zone, and prevent the natural regeneration of the Tsuga forest. So that we can easily recognize the appearance of transitional zone dominated with Abies homolepis being left behind the upper and lower climax forest.<br> 4, In the beech forest climax in Pacific Coast of Japan proper, Sasamorpha purpurascens in the dominant species in the forest floor, but in Oku-Nikko, the inner montane region of the Pacific Coast, it is extensively replaced by Sasa nipponica.<br> This is due to the following reason.<br> Owing to the Pacific type climate characterized by less amount of snowfall in winter, the distribution of Sasa purpurascens is restricted to the foot part of the mountain where snowfall is enough to support the survival of the species. On the contrary, Sasa nipponica extend its territory supported by the hemicryptophytic character, that is, for lack of the buds on the upper nodes of the culm, the life form of it represents a change from phanerophyte to hemicryptophytic character.