TEMPORAL AND SPATIAL EVOLUTION OF NDVI IN SEASONAL FREEZE-THAWING AREA OF CHINA

Vegetation is the most important composition part of land ecological system and is sensitive to the change of global climate. The characteristic of nearly 37a temporal and spatial evolution of NDVI in China’s seasonal freezing-thawing area was studied focusing on the target of China’s seasonal freeze-thawing area, utilizing methods of GIS spatial analysis and mathematical statistics and based on the dataset of AVHRR GIMMS NDVI and MODIS NDVI during 1982 to 2018. It showed that nearly 37a NDVI in China’s seasonal freezing-thawing area fluctuated with an increasing trend in the range of 5.292~6.635. Besides, the coverage degree of vegetation increased dramatically. Sandy land developed from the direction of desertification to oasisization. This work provides scientific evidence for the sandy land ecological evaluation of China’s seasonal freezing-thawing area and regional sustainable development. Introduction Vegetation which plays an important role in the aspects of energy exchanging, water cycle and climate adjustment at the land surface is a sensitive indicator for climate change (Twumasi et al. 2019). Normalized difference vegetation index (NVDI) is an important index for large scale vegetation coverage and productivity, whose value could characterize the activity strength of vegetation and further demonstrate the characteristics of structure and function of ecological system (Wang et al. 2019). Large scale and long sequence vegetation change research utilizing remote sensing technique is an important method to illustrate the change characteristic of ecological system (Vrbicanova et al. 2020). It has been reported that the vegetation activity at the global scope demonstrated an increasing trend (Nan et al. 2010). Vegetation coverage of China and its eastern area all demonstrated a change to some extent (Tucker et al. 1985). Vegetation coverage change which was influenced jointly by climate change and human activities was the direct result of regional ecological environmental change (Zhen et al. 2019). Besides, the illustration of the relationship between the temporal and spatial evolution of vegetation and climate change and human activities could provide theoretical evidence for coping with the climate change and enhancing the adaptability of ecological system. Related researches have been done. Mohammat et al. (2013) studied the change of NDVI in different seasons during 1982 to 2009 in Inner Asia, illustrating that the changing to green trend at regional scale stopped in the 1990s with the main reason of cold spring and dry summer. Piao and Fang (2003) studied the change of China’s NDVI from 1982 to 1999 illustrating that the spring was the season whose average NDVI of China’s vegetation has the most dramatic increasing trend and the fastest increasing rate. Besides, the moving up of the growth season of China’s vegetation was the main approach responding to global changing. Freezing-thawing erosive area was the region with the characteristic of strong freezing-thawing as well as the corresponding freezing-thawing erosive landscape. China’s seasonal freezing and thawing area is situated at the eastern border of giant sandy land at the middle latitude in Eurasian Continent which is also the eastern border of the *Author for correspondence: <duhs@163.com>.