Changes in different land cover areas and NDVI values in northern latitudes from 1982 to 2015

Climate warming leads to vast changes in the land cover types and plant biomass in the northern high-latitude regions.The overall trend is of shrubland and tree lines moving northwards,while changes in different land cover types and vegetation growth in response to climate change are largely unknown.Here,we selected land areas with latitudes higher than 50°N as the study area.We compared the land cover type changes and explored relationships between the normalized difference vegetation index(NDVI)values of different land cover types,air temperature,and precipitation during 1982-2015 based on dynamic grid.The results indicated that forest and shrubland areas increased as a large area of grassland shifted to forest and shrubland.The snow/ice,tundra and grassland largely have decreased from 1982 to 2015.Although approximately 277.3×103 km2 of barren land(6.2%of the total barren land area in 1982)changed to tundra,the tundra area still decreased because some tundra shifted to forest and grassland.The NDVI values of tundra significantly increased,but the shrubland showed a decreasing trend.Temperature in the growing season(June to September)showed the largest positive correlation coefficients with the NDVI values of forest,tundra,grassland,and cropland.However,due to shrubification processes and plant mortality in shrubland areas,the shrubland NDVI showed negative relationship with annual temperature but positively correlated with monthly t.Taken together,although there is large room for improvement of the land cover type data accuracy,our results suggested that the land cover types in high-latitude regions changed significantly,while the NDVI values of the different land cover types showed different responses to climate change.