History of landform–vegetation relationships in the upper Azusa River basin, central Japan

The relationship between landforms and vegetation was examined to clarify the long-term effects of geomorphic processes on vegetation structures on mountain slopes. This study was conducted in the Genbunzawa and Zenroku-zawa watersheds in the upper Azusa River basin, central Japan, where landslides and sagging landforms prevail. In the middle and lower parts of the watersheds, landslides characterized the vegetation structures. Larch (Larix kaempferi) forests occupied the scarp slopes of large landslides, while spruce (Picea jezoensis var. hondoensis) forests occurred mainly on landslide deposits. Even-aged alder (Alnus inokumae) forests and mature fir (Abies homolepis) forests dominated the alluvial cones that form in the lowest part of the watersheds. However, spruce and fir-spruce forests were found on a large debris-flow lobe (LDL) in the alluvial cone in the Genbun-zawa watershed. The LDL seems to have controlled the distribution of debris-flow paths on the alluvial cone because both recent debris-flow paths and even-aged alder forests caused by debris-flow disturbance were restricted to the area south of the LDL. Dendrochronology and radiocarbon dating results suggest that a large landslide occurred in the middle of the Genbun-zawa watershed ca. 370–350 years ago and had lasting influences on the vegetation by creating new edaphic conditions on debris-flow lobes and by changing the local disturbance regime in the alluvial cone.