陕北长城沿线风沙区植被指数变化及其与气候的关系

Vegetation change and its relationship with climate in the region along the Great Wall in northern Shaanxi

陕北长城沿线风沙区位于毛乌素沙漠东南部边沿,属毛乌素沙地向东南移动的最活跃地段,生态环境十分脆弱。使用1981～2003年23a长时间序列的NOAA/AHRR NDVI数据、气候资料,分析了陕北长城沿线风沙区植被覆盖的历史演变及其与气候因子的关系。结果表明:(1)陕北长城沿线风沙区植被覆盖状况23a来尽管有波动起伏,但是整体在持续转好,年平均NDVI增加了10.62%。低覆盖率植被面积在减少,高覆盖率植被面积在增加。夏季的NDVI值最高、波动起伏最大,其次是秋季;春、夏、秋三季的NDVI具有明显的上升趋势,季平均NDVI年增长率夏季最大,秋季次之;夏、秋季NDVI与年NDVI具有很高的相关性,这两个季节的植被状况基本决定了全年的植被分布状况。NDVI年变化曲线为单峰型,春季NDVI缓慢增加,秋季NDVI降低速度比较快。(2)年平均NDVI与温度的年际变化相关不明显,各季节NDVI与温度相关也不明显。近年来长城沿线风沙区的年降水量没有明显增加,而年平均NDVI线性增加趋势显著,降水量是引起NDVI年际波动的主要因子,非气候因素是年平均NDVI线性增加的主要原因。降水量与NDVI存在着明显的年相关和隔季相关。年降水量与年NDVI的相关,冬季降水量与春季NDVI的相关,春季降水量与夏季NDVI的相关,夏季降水量与秋季NDVI的相关性都非常高。(3)非气候因素中生态保护和环境建设等人为措施,如植树造林、草原围栏封育等是导致植被显著增加的重要原因。

The wind-born sandy region along the Great Wall in northern Shaanxi Province of China is located in the southeast edge of the Maowusu desert. The ecosystem is highly vulnerable and it is active in moving southeastward over Maowusu. Interannual variation of vegetation coverage and its relationship with climate in the region between 1981 and 2003 were examined by using the monthly GIMMS （Global Inventory Modeling and Mapping Studies ）NDVI （Normalized Difference Vegetation Index ） data and local meteorological records. Vegetation coverage of the region was generally improved in the past 23 years but with substantial variations. Area of lower vegetation coverage decreased in contrast to the increase of higher vegetation coverage area, Summer NDVI was the highest in a year followed by its autumn value. Increasing trends of NDVI were found in spring, summer and autumn. The vegetation conditions in summer or autumn determine the annual coverage. Correlation between NDVI in the two seasons was found to be highly correlated with its annual values. The NDVI increased slowly in spring and decreased rapidly in autumn. The annual mean NDVI showed positive correlation with precipitation, but not with temperature. There were significant positive correlations between winter precipitation and spring NDVI, spring precipitation and summer NDVI, and summer precipitation and autumn NDVI. The correlations between summer and autumn precipitations and annual NDVI were both significantly positive. The results indicate that rainfall is a key factor in determining the distribution of vegetation over the region. Annual precipitation did not increase significantly during the study period, but annual NDVI continuously increased. This impli the main driver for NDVI interannual variations and anthropogenic factors such as reforestation and es that precipitation was grassland recovery werethe cause for the increasing NDVI trend.