Factors affecting the risk of Sasa grassland decline due to Abies homolepis invasion
Landscape Ecology and Management
factors affecting tree invasion were analyzed at Ocitiai Pass in the area of Mt. isurugi Park in Shikoku, Japan. At the area, Abies homolepis has been invading into Sasa hayatae grassland. Pattern of A. homolepis expansion in the grassland was confirmed by comparing the distribution maps of 1954, 1962, 1975, 1990 and 2000, which were made by using aerial photographs taken in the years. Area of A. homolepis expanded with the year. Invaded area is decreased with the distance from the area where
... homolepis has existed in 1954. A model to estimate distribution of aboveground volume of S. hayatae was developed, in order to evaluate effect of Sasa covering the study area to A. homolepis invasion. Aboveground volume of S. hayatae could be categorized into 4 types in relation to relative photon fiux density at ground level; SviI>185%, 10.6%<SviII ≦ 18.5%, 5.5%< SviIII ≦10.6%, and SviIV ≦ 5.5%. Tree based model allowed to estimate the distribution of 4 Sasa-types for entire study area by using topographical index and index of wind velocity from southward as explanatory variables, which topographical factors were calculated from Digital Elevation Model (DEM). By overlaying the distribution maps of A. homolepis and Sasa-types, it was clarified that the rate of A. homolepis invasion was remarkably decreased in the Sasa-type Svi IV because of the darkest light condition. In the S, I, invasion rate was slightly decreased comparing to the S, H & III, probably due to its stressful site condition; the most wind-swept and the driest in the area.