基于像元二分法的沙地植被景观格局特征变化分析

景观格局与过程的关系研究是目前景观生态学的主要目标和研究热点,而沙漠化是一种典型的景观演化过程。基于景观生态学原理,运用遥感与地理信息系统技术,以古尔班通古特沙漠南缘为研究区,选择近40 a(1977-2010年)间不同年降水量梯度的代表年份(1977年、1990年、2001年和2010年),利用Landsat MSS/TM/ETM+遥感数据对沙地植被景观格局变化进行了分析。结果表明:1977-2010年间,古尔班通古特沙漠南缘沙地植被呈现恢复—退化—恢复交替变化趋势,总体处于恢复趋势中;景观水平上沙地植被破碎化程度处于增加趋势,景观异质性增强,斑块形状趋于复杂;类型水平上不同盖度沙地植被破碎化程度呈不同趋势波动,总体呈盖度相对高的植被破碎化程度增加,而盖度相对低的植被破碎化程度降低的趋势。1977-2010年间,沙地植被斑块重心在西南-东北方向交替呈现,重心轨迹形成"Z"形。总体来看,植被斑块由沙漠西南缘向沙漠腹地东北方向扩张。年降水量波动与沙地植被盖度演化方向、景观破碎化程度、不同盖度沙地植被比例、不同盖度沙地植被破碎化程度、不同盖度沙地植被斑块重心迁移方向均具有密切关系,即干旱区沙地植被景观格局演化特征与年降水波动具有很强的关联性。

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.