三江源区近30年植被生长动态变化特征分析

Analysis of vegetation dynamics using GIMMS NDVI3g in the Three-Rivers Headwater Region from 1982 to 2012

三江源区为长江、黄河、澜沧江提供了丰富的水资源,其地表植被状况与我国水安全和生态安全密切相关。由于所使用的数据时间序列长度有限,以往研究缺乏近30年整体视角的连续时间序列研究,也很少分析多时段变化趋势的持续性。为此,本文利用最新发布的1982-2012年的GIMMS NDVI3g数据集,采用多个嵌套时间序列的分析方法,在多个空间尺度上,研究了近30年来三江源区各季节植被活动的变化趋势和过程。研究表明,1)三江源区生长季和各季节NDVI在近30年均呈增强趋势,春季植被变化具有明显的阶段性,1998年前后NDVI分别呈显著增加、减少趋势。2)生长季、春季和夏季NDVI的增加随时段延长而趋缓,尤其是春季减缓迅速甚至停滞,而秋季植被活动持续增强。3)尽管区域尺度生长季、春季和夏季NDVI变化趋缓,但在像元尺度显著变化的区域范围却多呈增加趋势。4)不同植被类型显著变化的季节、时段存在差异。针叶林和灌丛NDVI主要在春季前几个时段增加显著,荒漠、草甸的增加主要集中在春季、夏季前几个时段。5)各季节NDVI在14个时段增加趋势持续性的差异,引起各季节对生长季植被活动变化贡献的变化。总的趋势是,前几个时段,春季NDVI变化较为剧烈,夏季NDVI的贡献最大,而随时段延长,逐渐过渡到秋季NDVI变化幅度、变化量最大,夏季次之。

The Three-Rivers Headwater Region(TRHR)refers to the source area of the Yangtze River,Yellow River and Lancang River,an important water resource.Vegetation activities in this region are closely related to water and ecological security in China.As a critical component of terrestrial ecosystems,vegetation has become one of the important issues in global climate change.Numerous case studies have been conducted on vegetation dynamic trends over relatively short study periods however,few long-term studies from the 1980 sto recent years have been completed because the normalized difference vegetation index(NDVI)datasets limited.Additionally there are few quantitative analyses of the consistency of vegetation trends over time.In this study,the latest GIMMS NDVI3 g datasets from 1982 to 2012were used to identify any spatiotemporal patterns in vegetation changes over the growing season and seasonal NDVI at regional,biome and pixel scales in the TRHR.To explore the change process linear regression over progressively longer periods from the initial year of the study was applied to detect vegetation growth changes in TRHR.At the regional scale,although a positive trend of growing season and seasonal NDVI were found during past three decades,there were two distinct periods with opposite trends in spring NDVI,a significant increase and a significant decrease before and after 1998,respectively.As the study period increases,the rates of NDVI increase in the spring and summer growing season markedly reduced,but in autumn the NDVI increased.However,significant NDVI increases were found in most seasons.The season and period with significant trend in NDVI differed among vegetation types.Significant increases in NDVI were observed in spring during the first few periods for coniferous forest and shrub and in spring and summer during the first few periods for desert and meadow areas.The difference in persistence of NDVI trends caused variation in the contribution of seasonal vegetation activity to plant growth.During the first few periods,the largest NDVI increase and the largest contribution were generally found in spring and in summer,respectively.During the last few periods,both the largest NDVI increase and contribution were found in autumn.It can be inferred that increases in NDVI during the growing season was mainly due to both the earlier start of vegetation growth and accelerated vegetation activity during previous periods and from both the accelerated vegetation growth and a delay in the end of the growing season during latter periods.