近30年中国陆地生态系统NDVI时空变化特征

气候变化已明显影响到陆地植被的活动,但在不同生态系统间存在差异,研究不同陆地生态系统归一化植被指数(NDVI)的时空变化特征,不仅可揭示各生态系统植被活动对气候变化的响应规律,而且可为我国不同生态区制定应对气候变化的策略和生态文明建设提供科学依据。基于1982—2012年GIMMS NDVI3g和中国陆地生态系统类型数据,利用一元线性回归、集合经验模态分解和相关分析等方法,研究了近30年中国各陆地生态系统NDVI的时空变化特征,分析了其与气候事件的关系。结果表明,近30年中国植被活动显著上升,年平均归一化植被指数(ANDVI)的上升幅度为0.0029/10a(P0.05),年最大归一化植被指数(MNDVI)的上升幅度为0.0076/10a(P0.01);植被活动显著增强的区域主要是分布在东部季风区的农田和森林生态系统,显著下降的区域主要是分布于西北的荒漠生态系统和东北的森林生态系统;尽管ANDVI和MNDVI线性趋势的显著性有所差异,但农田、森林、草地和水体与湿地生态系统的NDVI总体呈非稳定的上升趋势,上升过程中伴随着较大波动,荒漠生态系统的NDVI呈下降趋势,植被退化显著;与线性趋势不同,各生态系统植被活动的残差趋势包含"上升—下降"两个阶段,并相继于20世纪90年代到21世纪初发生转折;上述5类生态系统的植被活动存在不同尺度的周期特征,年际周期波动特征(1.9—7.6a)比较显著,而年代际周期(10.7a和22.2a)的显著性相对较差;各生态系统的空间异质性在趋强过程中存在2.1—7.1a的年际周期节律;海洋与大气环流的短周期脉动与各生态系统植被活动的周期性节律有着明显关联,ENSO事件和太阳活动是推动植被活动周期性振荡的重要因素。

Spatiotemporal variations of NDVI in terrestrial ecosystems in China from 1982-2012

Vegetation, an important component of terrestrial ecosystems, tells the story of the ecological response to climate change. In the context of climate change, vegetation changes have directly affected substance exchanges and energy balance of regional land-atmosphere interaction processes. Additionally, in China, the vegetation activity of varied terrestrial ecosystems has responded to global climate change in significantly different ways. As a vital remote sensing indicator, the normalized difference vegetation index (NDVI) reflects vegetation growth and coverage, and time series analysis can describe the temporal evolution and spatial variability of vegetation very well. Thus, studying the spatiotemporal variation of NDVI in terrestrial ecosystems not only reveals the response of vegetation in those ecosystems to climate change, but can provide a scientific basis for action on climate change and ecological considerations in an increasingly urbanized country like China. Based on the Global Inventory Modeling and Mapping Studies (GIMMS) NDVI3g dataset for 1982-2012, and the macro structure data of the Chinese terrestrial ecosystem, this study utilized one-dimensional linear regression, ensemble empirical mode decomposition (EEMD), and correlation analysis to examine spatiotemporal variation in the NDVI of each terrestrial ecosystem in China. It also analyzed the relationship between NDVI and climate events over a recent 30-year period. The results show that the average normalized difference vegetation index (ANDVI) and maximum normalized difference vegetation index (MNDVI) increased (0.0029/10 a, P < 0.05 and 0.0076/10 a, P < 0.01, respectively) over the same period. These phenomena indicate the activity of vegetation strengthened in China. In addition, owing to its sensitivity to water-thermal factors, MNDVI can be a useful supplement for monitoring vegetation change. Furthermore, the areas with significant increases in vegetation activities were mainly the farmland and forest ecosystems in the eastern monsoon region, and the areas with significant decreases in vegetation activities were mainly the forests in Northeast China and the desert ecosystem in Northwest China. With ANDVI and MNDVI showing linear trends at different significance levels, the NDVIs of the farmland, forest, grassland, and water and wetland ecosystems showed a general upward trend with large fluctuations, while the NDVI of the desert ecosystem decreased, with obvious vegetation degradation. However, EEMD detected an inflection point among the trends in the five ecosystems that was distinct from the linear trend. This occurred in the 1990s and early 21^st century when the NDVI trend shifted from increasing to decreasing. The decomposed results indicated that the NDVI of the ecosystems had oscillation characteristics on an inter-annual (1.9-7.6 years) and an inter-decadal (10.7-22.2 years) scales, with the former being statistically more prominent. The increasing spatial heterogeneity of the NDVI of different terrestrial ecosystems also showed inter-annual periods of 2.1-2.7 years, with increasing regional differences in vegetation activity. Furthermore, the short period-pulsation of the air-sea system from the El Niño-Southern Oscillation and solar activity influenced vegetation activity via climatic factors, which was evidently related to the oscillation characteristics in all ecosystems.