Dynamic characteristics of sandy vegetation landscape pattern based on dimidiate pixel model

Abstract: The relationship between landscape pattern and process is a hot research topic in southern Gurbantunggut Desert. Desertification in fact is a process of landscape changing. Taken Landsat MSS/TM/ETM+ image as remote sensing data sources in selections of typical meteorological year (1977, 1990, 2001 and 2010) from 1977 to 2010, characteristics of the sandy vegetation landscape pattern were analyzed by using theory of landscape ecology, RS and GIS. Firstly, using remote sensing images based on pre-treatment, vegetation coverage classification map was gained by the dimidiate pixel model; then landscape pattern of desert vegetation coverage was analyzed using Fragstats 4.0 software to compute landscape pattern index or landscape analysis. The desert vegetation area changes and its mutual transition relationships with other types were analyzed with area ratio and transition matrix methods by ENVI software. The migration path of vegetation patch gravity center was analyzed with computing patches gravity center of different coverage of vegetation by ArcGIS software. The results showed that during the periods of 1977-1990 and 2001-2010, medium coverage desert vegetation area and its proportion were increasing, and vary low coverage desert vegetation area and its proportion were decreasing. While during the period of 1990-2001, vary low coverage desert vegetation area and its proportions were increasing, and low, medium and high coverage desert vegetation area and its proportion were decreasing. During the periods of 1977-2010, desert vegetation showed a changing trend of "restoration - degradation - restoration" cycle in southern Gurbantunggut Desert, and the general trend was restorative. In recent 40 years, landscape fragmentation showed a growing trend, and landscape heterogeneity was enhanced at landscape level. Fragmentation of different desert vegetation coverage changed with fluctuating in different trend at class level, from which relatively high coverage of vegetation fragmentation was increased and relatively low degree of coverage of vegetation fragmentation was decreased. During the period of 1977-2010, gravity center of vegetation patch fluctuated alternately with from SW to NE, and the track of gravity center shaped like a Z. Overall, gravity center of vegetation patch moved to NE with precipitation increasing. On the whole, vegetation patches were expanded from southwest margin of the desert to northeast of the desert heartland. Evolutionary direction of desert vegetation coverage, the degree of landscape fragmentation, percentage of different desert vegetation coverage, fragmentation of different desert vegetation coverage and migration direction of vegetation patches had close relationship with precipitation fluctuating. Therefore, evolution of desert vegetation landscape pattern had a strong relevancy with precipitation frequency. Along with the increasing annual precipitation, dominant role changed from bare land to vegetation with higher coverage. The bare land with a 91 mm annual precipitation had very low vegetation coverage vegetation. The area with 129.5 mm precipitation had the low vegetation coverage whereas the medium vegetation coverage area had 184.4 mm, and high vegetation area had a 221.4 mm annual. In last 40 years, both precipitation as a primary factor and topography induced water redistribution on the landscape as a secondary factor affected vegetation distribution pattern evolution on a landscape in southern Gurbantunggut Desert. Landscape pattern of the desert vegetation had an obvious response to change of annual precipitation, indicating that desert vegetation would continue to be in a restoring trend. Landscape fragmentation would continue to be in an increasing trend. Dominant effect and connectivity of medium and high coverage vegetation would have an enhancing trend, and vegetation patches would expand towards to northeast of the desert heartland in the future.