Long-term spatial distribution of vegetation and sand movement following the commencement of landscape conservation activities to curb grassland encroachment at the Tottori Sand Dunes natural monument

摘要

San'in Kaigan Geopark is recognized as a United Nations Educational Scientific and Cultural Organization (UNESCO) Global Geopark, and the Tottori Sand Dunes in the park are designated as a Natural Monument of Japan. In 1900, the area that constitutes the Tottori Sand Dunes was part of a vast coastal sand dune system. However, the encroachment of grasslands (grasslandisation) became a significant threat to the area and by 1991 about 42% of the surfaces of the sand dunes were covered with vegetation. This prompted the initiation of landscape conservation activities, primarily weeding, in 1994. We analysed the spatial distribution of sand movement based on aerial land survey data captured intermittently since 1964, and propose a method for evaluation of the vegetation cover ratio per 30 x 30-m cell (pixel polygons; PPs), based on unmanned aerial vehicle (UAV) photographs and satellite data. Our analysis showed that the inland slope of the second row of sand dunes has shifted landward by around 30 m due to extremely high deposition rates. The third row of sand dunes has also moved inland. Thus, the Oigo-suribachi Basin has gradually been buried along the coast due to significant sand sedimentation along the inland slope of the third row of sand dunes. We analysed the spatial distribution of the vegetation cover ratio on 9 May 1990, when the area of the encroaching grasslands was largest. A vegetated area with a vegetation cover ratio exceeding 0.40 extended from the first row of sand dunes to the west of the windbreaks located in the sand dune area. In the area in close proximity to the first row of sand dunes, the vegetation cover ratio was 0.51 in 1990 and reached 0.56 in 1995. However, it decreased rapidly to 0.14 by 1996, and since 2000 it has fluctuated within the range of 0.01-0.13.