Analysis On The Spatio-temporal Characteristics Of Phenology And Its Driving Factors In The Qinghai-Tibet Plateau

In recent years,remote sensing of phenology has become a useful tool to reveal the response and feedback of vegetation dynamics to global climate change due to its multitemporal phase,wide-coverage,continuous space coverage,and long-time series.Based on the Moderate Resolution Imaging Spectroradiometer data product MOD09A1,enhanced vegetation index(EVI)was calculated,and the time series EVI of the growing season in the Qinghai-Tibet Plateau(QTP)from 2001 to 2016 was reconstructed in light of the double logistic function filter.The vegetation phenology was extracted using dynamic threshold method(SOG,Start of growth season;EOG,End of growth season;LOG,Long of growth season),the QTP vegetation phenology and EVI spatio-temporal variation characteristics at the beginning and end of the vegetation growing season were analyzed and discussed the response relationship of the QTP vegetation phenology with the climate change.The results show that:(1)From 2001 to 2016,the vegetation SOG in the QTP shows an advance trend with an advance rate of 1.68d/10a;the vegetation EOG shows a slight trend of advance,with the advancement rate of 0.53d/10 a,which was smaller than that of SOG;the LOG shows a slight extension trend,with an extension rate of 0.14d/10 a.The SOG in QTP varied between day of year(DOY)110 and DOY 170 from 2001 to 2016,with the deterioration of hydrothermal conditions and uplift of the terrain,the vegetation SOG is gradually delayed from the southeast to the northwest in the plateau.The vegetation EOG spatial distribution gradually advanced from southeast to northwest,mainly occurrs between DOYs 240 and 300 days.Qinghai Province was later than Tibet Province as a whole.As the elevation increases,the vegetation LOG is gradually extended from southeast to northwest,primarily occurrs between DOYs 90 and 170 days.The vegetation LOG in Qinghai province is generally longer than that of Tibet province.(2)Vegetation phenology in the QTP is highly dependent on elevation.In regions with an elevation of 2,500 to 5,000 m,the elevation increased by 1,000 m each time,the vegetation SOG is delayed by 9.8 days,EOG is advanced by 4.6 days and LOG is extended by 14 days.In the past 16 years,the phenological change rate has also changed from negative to positive with the increase of elevation.With the increase of elevation,the advance trend of SOG has changed from fast to slow.After 4,500 m,the delay trend has changed from slow to fast.EOG has changed from advance to delay at the elevation of 3,400 m.The LOG at 3,000 m has changed from shortened to gradually extended,with gradually larger extension extent.(3)The interannual EVI change(1.3%/10a)at the initial of the vegetation growing season shows an increasing trend of fluctuation and is higher than EVI(0.3%/10a)at the end of the growing season in the QPT from 2001 to 2016.The EVI gradually decreases from southeast to northwest,and EVI of Qinghai province is higher than that of Tibet province on the whole,and the vegetation coverage gradually decreases with the increase of elevation.The EVI variation spatial distribution in the QPT at the initial and end of the vegetation growing season is significantly different,among which the degraded areas are small,and the areas that are basically unchanged and slightly improved account for a large proportion.(4)EVI at the initial of the vegetation growing season in the QTP has the strongest correlation with the air temperature during the period of DOYs 110 to 170 and the precipitation during the period of DOYs 94 to 154.The significant influence of air temperature almost covers the entire study area,and the precipitation mainly affects the southwest of Tibet and the southwest of Qinghai.EVI at the end of the vegetation growing season in the QTP has the strongest correlation with the air temperature and precipitation during the period of DOYs 240 to 300.The influence of air temperature is widely distributed,while the influence of precipitation is relatively concentrated in the central and southwestern part of Tibet and the eastern part of Qinghai province.The response of vegetation SOG in the QTP to air temperature is relatively rapid,with a lag of about 0 to 15 days,while the response to precipitation lags 15 to 30 days.With the co-influence of air temperature and precipitation,vegetation EOG responds quickly,with a lag time of about 0 to 15 days.(5)The SOG in the QTP affected by climate factors accounts for 20.47% of the total area,among which driven by air temperature and precipitation strongly is 0.4%,driven by air temperature mainly is 5.9%,driven by precipitation mainly is 10.4%,driven by air temperature and precipitation weakly are 3.77%.The regional distribution of vegetation EOG driven by climate factors is similar to that of vegetation SOG,accounting for 21.88% of the total area,among which driven by air temperature and precipitation strongly is 0.36%,driven by air temperature mainly is 7.18%,driven by precipitation mainly is 10.62%,driven by air temperature and precipitation weakly are 3.72%.Overall,the vegetation SOG coverage in over 79% areas is not driven by air temperature and precipitation,while vegetation EOG in over 78% is not driven by air temperature and precipitation.