| Climate warming leads to frequent droughts on the Mongolian Plateau,and causes a series of ecological problems.Due to the increase in intensity and frequency of recent drought events,vegetation phenology is undergoing profound changes.However,little is known about the mechanisms of drought affects the vegetation restoration to start of growing season and end of growing season,especially in arid and semi-arid regions.In this study,based on MODIS NDVI data from 2000 to 2021 and GIMMS NDVI data from 1982 to 2015,an extended NDVI long-time series data set(1982-2021)was obtained by linear regression analysis and cross-validation,and the vegetation phenological parameters were extracted by the dynamic threshold method of Savitzky-Golay filter,and the temporal and spatial variation characteristics of the dataset in recent 40 years were analyzed.On this basis,the standardized precipitation evapotranspiration index(SPEI)was calculated as a drought monitoring index based on ERA5-Land temperature and precipitation data,and the frequency and intensity characteristics of drought in different seasons on the Mongolian Plateau were further analyzed,and then the sensitivity characteristics and spatial and temporal differences of pre-season drought on phenological parameters were quantitatively studied.It is very important to understand the relationship between pre-season drought and vegetation phenology in order to simulate and predicting the evolution of drought,vegetation phenology and ecosystem processes under sustained climate change.The main conclusions obtained from the study are as follows: follows.(1)From the spatial and temporal trends of seasonal drought,the SPEI of the Mongolian Plateau has been decreasing significantly in all seasons in all seasons in the past 40 a,and the aridification of the Mongolian Plateau is becoming more and more serious;the drought intensification is the most serious in summer,and the rate of drought intensification in other seasons is spring,autumn,and winter in descending order;spatial distribution,except for the drought in the northwestern part of the study area in winter,the SPEI of different seasons in other areas are In terms of spatial distribution,except for the northwestern part of the study area in winter,the SPEI in all other areas showed a decreasing trend and drought intensified.The analysis of drought frequency and drought intensity in different seasons in the Mongolian plateau region in the last 40 a revealed that the highest frequency of drought occurred in winter,followed by spring,autumn and summer;the intensity of drought occurred in summer and winter was higher than that in spring and autumn;in addition,the frequency of drought and drought intensity in Mongolia were higher than those in Inner Mongolia,indicating that the degree of drought in Mongolia was more severe than that in Inner Mongolia.(2)The 1982-2021 multi-year mean SOS for vegetation on the Mongolian Plateau was mainly concentrated in early April to mid-May.In terms of interannual trends,the overall return of vegetation to the Mongolian Plateau shows a significant forward trend.The areas with an early vegetation SOS trend accounted for 70.95% of the total area of the study area.The SOS of different vegetation types on the Mongolian Plateau differed significantly,with forests returning to green earlier and desert grasslands returning to green later The multi-year mean of vegetation death on the Mongolian Plateau is mainly concentrated in October to early November.In terms of interannual trends,the vegetation dieback period on the Mongolian Plateau as a whole showed a significant delayed trend;spatially,76.45% of the study area showed a delayed trend in EOS.There are significant differences in EOS between different vegetation types on the Mongolian Plateau,with alpine grassland having the earliest withering period and desert grassland having the latest dying period.(3)From the spatial distribution of SOS,EOS and SPEI correlations on the Mongolian plateau,the January-March drought trend led to shift of SOS,and the January drought trend with a pre-season length 4 had a greater influence on SOS.the June-September drought trend shifted the EOS,and the June drought trend with pre-season length 5 had a greater influence on the EOS.(4)SOS of each vegetation type was mainly negatively correlated with pre-season drought of different lengths,indicating that the pre-season drought trends led to SOS shift.Among them,SPEI in April had a greater influence on SOS of each vegetation type.The relationship between EOS of each vegetation type and SPEI from June to October was predominantly negatively correlated,indicating that pre-season drought trends resulted in delayed EOS. |
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