Spatiotemporal Variation Of Vegetation Phenology And Its Response To Snow Change In The Qinghai-Tibet Plateau

Vegetation phenology is one of the key parameters for monitoring vegetation ecosystem changes at regional and global scales,and studying its change characteristics and its response to snow cover changes in the context of global warming can help to understand the response mechanism of high-altitude ecosystems to global warming.Based on MODIS-NDVI data and snow cover data,this paper obtains vegetation phenology and snow phenology on the Qinghai-Tibet Plateau,discusses their spatiotemporal differentiation characteristics,and analyzes the response law of vegetation phenology to snow phenology,and the main conclusions are as follows:(1)From 2001 to 2020,from northwest to southeast on the Qinghai-Tibet Plateau,the start of the vegetation growing season gradually advanced,the end gradually delayed,and the length of the growing season gradually extended,and there were obvious laws of latitude and topographic differentiation.During the study period,the start of the growing season,the end of the growing season and the length of the growing season showed significant advance,slight delay and significant increase trend.The spatial heterogeneity of inter-annual variation of vegetation phenology parameters is obvious.In most regions,the variation range of vegetation phenology indexes is small and most of them are not significant.Although the area with large variation range of each index is small,it basically passes the significance test(P < 0.1).(2)The vegetation phenology of the plateau as a whole and most ecological divisions were altitude sensitive,and with the increase of altitude,the length of the growing season,the start of the growing season and the end of the growing season showed a trend of shortening,postponing and advancing respectively.In terms of the overall interannual variation of the region,the extension trend of growing season length decreased and then increased with the altitude gradient,and the extension trend showed a strong significance level on all the altitude gradients.The advance amplitude of the initial stage weakened with the increase of altitude as a whole,and the significance was stronger in the lower altitude area.The terminal phase manifests as greater delays at low and high altitudes.The interannual variation of growing season length in most ecological zones is mostly an increasing trend,and the end stage is mostly a delayed trend.(3)During the 2000-2019 snow cover year,the spatial distribution of snow phenology was uneven.On the whole,high-altitude areas have more snow cover days,early start of snow cover days and later end of snow cover days,while low-lying hinterland is the opposite.The climate and topography are obviously different.The overall interannual variation in average snow cover days,the start of snow cover days,and the end of snow cover days were weakly increase,slightly delay and slightly ahead of the trend,respectively.(all of which failed the significance test of P<0.1).The interannual variation of phenological parameters of each snow cover had obvious spatial heterogeneity.(4)The phenological parameters of snow cover in the Qinghai-Tibet Plateau as a whole and in each sub-district also had altitude correlation.As the altitude increases,the snow cover days,the start of snow cover days,and the end of snow cover days were increase,advance,and delay,respectively.In addition,for the Qinghai-Tibet Plateau as a whole,the interannual variation of snow phenological parameters in high altitude areas was greater than that in medium and low altitude areas,and the parts of each parameter that passed the significance test were distributed in high altitude areas.(5)Changes in snow phenology affect changes in surface vegetation phenology.Among the phenological factors of snow cover,the end of snow cover days had the greatest influence on the phenological parameters of vegetation throughout the day,while the start of snow cover days was the smallest.The correlation between vegetation phenology and snow phenological factors on the Qinghai-Tibet Plateau was obviously spatially different,and the phenological factors of each snow cover were mostly negatively correlated with the phenological factors of each vegetation.Areas whose correlation passes the significance test have absolute values greater than 0.3 or 0.5.At the same time,3500 m altitude is an important dividing line for the correlation coefficient.