Tibetan Plateau's Phenological Changes And The Analysis Of Its Driving Factors

Tibetan Plateau is the sensitive region of global climate change.Its hydrothermal conditions are in the limit of biological growth and its ecosystem is extremely fragile.The study of the phenology and its driving factors of the Tibetan Plateau is of great significance to the study of the evolution of the Qinghai-Tibet Plateau ecosystem under climate change.Based on MODIS NDVI data set from 2001 to 2018,Timesat3.1 was used to extract Start of growth season(SOG),End of growth season(EOG),Length of growth season(LOG),accumulation of NDVI,Absolute increase(AIV),and Relative increase Value(RIV).Median filtering and S-G filtering were used to smooth NDVI data in advance.The extraction method of SOG and EOG adopts dynamic threshold method,and the threshold were set as 0.2 and 0.6 respectively.Based on the above calculation,the interannual changes of SOG,EOG and LOG were analyzed by the least square method.Considering the influence of groundwater and vegetation types on vegetation phenology,the SOG and EOG in river and lake buffer zones and different vegetation types were analyzed.Considering the influence of permafrost on vegetation phenology under the condition of climate warming,the SOG and EOG in different frozen areas were specifically analyzed.Pearson correlation coefficient was used to analyze the influence of climate factors on SOG and EOG.At the same time,the relationship between climatic factors and vegetation phenology was studied by proper zoning of climatic factors and altitude.The results are as follows:(1)There are obvious spatial differences in vegetation phenology in the Tibetan Plateau.From the southeast to the northwest,SOG was postponed from mid-to-late April to May,EOG was advanced from early-October to mid-September,and LOG was gradually shortened from 141-181 days to 102-131 days.SOG was advanced by 1.56 days/year from 2001 to 2008 and postponed by 0.31 days/year from 2009 to 2018.(2)As the distance to the river and lake becomes longer,SOG was delayed and EOG was advanced.The 3km is the best river and lake buffer zone.With the SOG later,the EOG earlier,the effect of river and lake recharge on Vegetation phenology increases.For the SOG of Tibetan Plateau,the meadow type <the shrub type <the grassland type;For the EOG,the shrub type < the meadow type < the grassland type.The SOG and EOG of the permafrost degradation zone were between the permafrost zone and seasonal permafrost zone.(3)The SOG of eastern region is controlled by the temperature in April and May and the precipitation in January and February,while the western region is controlled by the precipitation in May.The western region was controlled by the temperature in September,the central and eastern regions by the temperature in October,and the precipitation in August and September control the whole.The phenology of the plateau was altitude dependent,for every 100 m increase in altitude,the SOG was delayed by 0.58 days and the EOG was advanced by 0.43 days.The above results lay a foundation for understanding the plateau phenological change and analyzing the driving mechanism of ecological environment change.