森林立地54(1)，7〜17 2012
Jpn J For Environ
54 (1), 7–17, 2012
Patch mosaic distribution of overstory vegetation in a
riparian
buffer strip along a meandering V-shaped valley of
Oyabu creek,
central Kyushu, southwestern Japan
Satoshi Ito1, Tomomi Marutani2,
Osamu Shimizu1
1Faculty of Agriculture, University of Miyazaki, Japan
2Faculty of Agriculture, Hokkaido University, Japan
We investigated the relationship of vegetation to geomorphology in a V-shaped valley bottom of an incised meandering mountainous stream in a transition zone from fir-hemlock forests to cool-temperate deciduous broad-leaved forests in Kyushu, southwestern Japan. The bank of a channel reach (410 m in length) of a forth-order valley of Oyabu creek was divided into 32 units, and overstory vegetation, slope characteristics, flood frequency and other geomorphic features for 15 m width from the vegetation edge were surveyed. The bank vegetations of 32 units were classified into three types: Tsuga sieboldii forests (ridge type), Quercus crispula forests (secondary broad-leaved type) and Betula grossa forests (valley-bottom type). These types were related to slope forms, soil depth and slope inclination, but not to flooding frequency. These results suggested that the bank vegetation type classified by the overstory species composition in the V-shaped valley was influenced by hillslope processes rather than by the fluvial process of the main creek. The slope forms were related to the incised meander and geomorphic features such as the back slopes and the escarpments. These results suggested that river meander and geomorphic features may also have significant impacts on patch mosaic formation in a riparian strip in a V-shaped valley through the promotion of different bank slope characteristics through long-term lateral erosion processes.
Key words : flooding frequency, geomorphic process, slope form, soil depth, species composition
伊藤 哲，丸谷知己，清水 收：九州山地の穿入蛇行河川のV字谷渓岸における森林のモザイク分布．森林立地54:7-17,2012
九州山地のモミ・ツガ林―夏緑林樹林の移行帯において，穿入蛇行河川のV字谷渓岸植生のタイプと斜面形状，土壌深度および洪水頻度との関係を解析した。一ツ瀬川支流大藪川上流域の4次谷区間（410m）の右岸を地形および相観に基づいて長さ5-17mの32個のユニットに分割し，渓岸から15mの範囲の高木層（林冠層）および亜高木層（樹高＞8m）の個体の樹種および胸高直径を調査した。また，各ユニットの斜面形状，受け盤・流れ盤の別および土壌深度を調査するとともに，過去の降水データおよび推移データを用いて，洪水に伴う各ユニットの冠水頻度を算出した。32ユニットの植生はツガが優占する尾根型植生，ミズナラが優占する二次林型植生およびミズメ等が優占する水辺型植生の3タイプに区分された。これらの植生タイプは斜面形状，斜面傾斜および土壌深度の違いと対応していたが，洪水による冠水頻度とは対応しなかった。これらの結果は，調査対象域の植生が河川の洪水攪乱よりも斜面の地形プロセスに影響を強く受けていることを示していた。また，斜面形状の分布は，受け盤・流れ盤および河川の蛇行と関係していた。以上の結果から，穿入蛇行河川のV字谷の渓岸植生のモザイク分布は，長期的な側方浸食の影響を通した河川の蛇行と受け盤・流れ盤の影響で形成されていると考えられた。
キーワード：種組成，斜面形状，冠水頻度，土壌深度，地形プロセス