Dynamics Of Vegetation Autumn Phenology And Their Response To Extremely Climate Change In The Qinghai-tibet Plateau

Under the background of global warming,the extreme climate events has drawn wide attention around the world.Due to the strong sensitivity to the climatechange andhaving large amplitudefor vegetations in Qinghai-Tibet Plateau,extreme climate change is one of the important drivers ofchange in plateau ecology and environment.The scientific assessment of extreme climate events andtheireffects on vegetation will be of guiding significance tovegetation ecological restoration,formulate effective disaster prevention and mitigation measures,and also help to build stable plateau ecological barrier.This thesis focused on the response of vegetation phenology to extreme climate events on the Qinghai-Tibet Plateau.Based on the daily meteorological data,general international 23 extreme climate index was selected to analyze the spatial and temporal variations of extreme temperature and extreme precipitation events;GIMMS3g,MODIS,SPOT remote sensing data were used toanalyze the spatial and temporal distribution of vegetationautumn phenology on Qinghai-Tibet Plateau;the focus has revealed the plateau vegetation autumn phenology changes in response to extreme weather events and a series of work.The main conclusions are listed as follows:(1)From 1982 to 2012,the cold-related indexes in extreme temperature events showed significant downtrend,and the warm-related index and extreme value index showed a significant upward trend,the warming rate atnight washigher than the increasing rate of day index.For extreme precipitation events,the total precipitation amount,simple daily intensity index,weak precipitation days,maximum rainfallin a day,maximum 5-day precipitation andmaximum consecutive dry days,all their change showed slightly increasing trend,and thesimple daily intensity index and maximum 5-day precipitation showed a significant increasing trend.However,changes of theextremely heavy precipitation,heavy precipitation,rainfall days and the maximum consecutive wet days showed a downward trend,the maximum consecutive wet days decline significantly.In general,the extreme temperature indexes showed significant warming trends during 1982-2012,and indices derived from daily minimum temperature warming faster than indices from daily maximum temperature,which indicates that the index at night was more sensitive to climate change than index in the day.Though the total rainfall experienced increasing trend over half of the study area,the maximum consecutive dry days further increased,and the maximum consecutive dry days reduced significantly.These results indicated that the rainfall displays an uneven features in Qinghai-Tibet Plateau in both temporal and spatial scales,and they could imply that Qinghai-Tibet Plateau will be more warming and drier in future.(2)The end date of vegetation grow season(EGS)occurred during period between 300-300 Julian days,with the average ofSPOT data is about 270 days,and the average of MODIS and GIMMS3 g data were both around 275 days.The EGS exhibited a westward trend in arrival,ranging from the end of September at the eastward edge,to the early October at the western edge,and late as the middle of October in the northwest parts of Qinghai-Tibet Plateau.The interannual variation EGS in Qinghai-Tibet Plateau showed the tendency of advance based on the three datasets,however,these results are all not statistically significant.The EGS in the whole study area showed obvious elevation dependency,it occurred in advancewith the altitude increase.In addition to the eastern forest,EGS of alpine meadow ecological regions shows a delaytrend withthe altitude increase,EGSin other biomesshows advanced along with the elevation increase.In general,there have been no significant changes in the EGS for the vegetation in Qinghai-Tibet Plateau,but it shows obvious regional characteristic(being advanced in central area and delayed in the southwest area)and elevation dependency(being advanced with the rise of the elevation).(3)The temperature plays animportant role in promoting the delay trend of vegetation autumn phenology;however,the positive relationship has been weakened with the preseason period extend,particularly in the positive relationship between different preseason average temperature and EGS for alpine meadow area of three rivers biome and farming ecological biome.Compared with temperature,the rainfall shows less effect on the change of EGS.The accumulative rainfall in August and September can promote the delay trend of EGS inmost partof the study area,however,the relationship between the EGS and other preseason periods is slight.For the different ecological zones,there is a negative relationship between cumulative precipitation ofpreseason periods and EGS in forest ecological regions;however,it shows a positive relationship between EGS and cumulative precipitation in preseason periods in meadow and steppe ecological region.In general,EGS in most part of study area was controlled by temperature,the control of temperature on EGS peaked in September,and the cumulative precipitation for 7-9 month control the highest percentage of area of EGS.(4)It shows a positive relationship between the warm index of extreme temperature events with EGS,while there is a negative correlationbetween the EGS and cold-relatedindex,including cold nights,cold days,frost days and icing days.Different from extreme temperature events,both of the proportion ofpositive and negative relationship betweenannual extreme precipitation index and EGSwerefairly close.During different preseason periods,the warm index in extreme temperatures events for1 to 3 months before the EGSplayan obvious role in promoting the delay trend of EGS,and the cold index shows obvious inhibitory effect on the EGS.Most of the precipitation events of 1 to 2 months before the EGSplay a significant role in promoting the extension of the EGS,however,it presents a negative relationship betweenthe maximum length of dryspell with EGS.In general,the raise of warm-related index can promote the extension of the EGS.On the contrary,the cold-related index will inhibit the growth of vegetation,and result in an early end date of the grow season.The responses of EGS to the extreme rainfall is more complex,extreme rainfall promote the extension of the grow season in meadow and grass biomes;however,it will inhibit the vegetation growth in forest biomes.