中国东北多年冻土区植被生长季NDVI时空变化及其对气候变化的响应

多年冻土对气候变化十分敏感,尤其是多年冻土上的植被,易受气候变化影响.东北多年冻土区位于北半球中、高纬度地区,是我国第二大多年冻土区,同时也是欧亚大陆多年冻土带的南缘.本文基于1981—2014年LTDR和MODIS 两种数据集对东北多年冻土区植被生长季归一化植被指数(NDVI)时空变化特征进行分析,同时结合气象数据,分析植被对气候变化的响应.结果表明: 研究期间,东北多年冻土区植被生长季平均NDVI呈显著增加趋势,年增加0.0036.空间逐像元NDVI变化趋势具有明显的空间异质性. 研究区80.6%区域的植被NDVI具有显著增加趋势(P<0.05),7.7%的区域呈显著减少趋势(P<0.05).不同类型多年冻土区的植被NDVI增加强度不同,依次为连续多年冻土区>不连续多年冻土区>稀疏岛状多年冻土区>季节冻土区,NDVI增加趋势最大值(>0.004)所占的面积比例依次为连续多年冻土区>不连续多年冻土区>稀疏岛状多年冻土区>季节冻土区.多年冻土全区尺度下,植被生长季NDVI与平均气温呈显著正相关关系(r=0.79,P<0.01),与降水呈较弱的负相关,表明气温是东北多年冻土区植被生长的主控因子.研究区的多年冻土退化对植被生长起到积极的促进作用,尤其是在连续多年冻土区和不连续多年冻土区,植被NDVI增加强度更为剧烈.尽管增加的地表温度可以加快植被生长、增加植被覆盖,但长期来看,多年冻土退化甚至消失会阻碍植被生长.

Spatiotemporal variations of growing-season NDVI and response to climate change in permafrost zone of Northeast China

Permafrost is extremely sensitive to climate change. In particular, aboveground vegetation on permafrost has great sensitivity to the change. The permafrost zone of Northeast China, within middle and high latitudes of the Northern Hemisphere, is the second-largest region of permafrost in China. It is at the southern edge of the Eurasian cryolitho zone. This study analyzed growing-season spatiotemporal variation of the normalization difference vegetation index (NDVI) in this permafrost zone and the correlation between NDVI and climate variables during 1981-2014 based on LTDR, MODIS data set and meteorological data. Mean growing-season NDVI significantly increased by 0.0036 a^-1 over the entire permafrost zone. The spatial dynamics of vegetation cover in the zone had strong heterogeneity on the pixel scale. NDVI showing a significant (P<0.05) increasing trend accounted for 80.6% of the permafrost area, and NDVI showing a significant (P<0.05) decreasing trend accounted for 7.7% of the entire study area. The trend of NDVI was variable in different kinds of permafrost zones which increased in order of continuous permafrost zone>discontinuous permafrost zone>sparse island permafrost zone>seasonal frozen region. The proportion of area for maximum increasing trends of NDVI (>0.004) in different permafrost regions was in order of continuous permafrost zone>discontinuous permafrost zone>sparse island permafrost zone>seasonal frozen region. Additionally, at the entire permafrost zone scale, the mean growing-season NDVI was positively significantly correlated with growing-season mean temperature (r=0.79, P<0.01), but weakly negatively related with growing-season precipitation, which indicated that temperature was the dominant influence on vegetation growth during the growing season in the study area. Permafrost degradation in the study area played a positive role in plant growth, especially in continuous permafrost zone and discontinuous permafrost zone. Although increasing ground surface temperature accelerated vegetation cover and growth in the short time, from the long term point of view, permafrost degradation or disappear might hinder vegetation growth.