Study On The Change Of Permafrost In Mongolian Plateau And Its Influence On Spring Phenology

Permafrost,as a product of cold climate change,undergoes significant degradation in the context of global warming.Permafrost degradation plays a key role in regulating vegetation growth of dry and cold grassland in permafrost regions.It affects the hydrothermal environment of vegetation growth through the change of phase change energy,and then affects the spring phenology of vegetation.Especially on the Mongolian plateau where permafrost is widely distributed,its ecosystem is sensitive and fragile,and is highly vulnerable to the influence of climate and permafrost directly or indirectly.Therefore,in this study,the satellite results of MOD11A1 and MYD11A1(the sixth version)and MODIS NDVI data are used to extract the start of growing season(SOS)and permafrost area of the Mongolian Plateau from 2003 to 2019 respectively,and combined with a variety of data,the temporal and spatial variation characteristics of permafrost and SOS from 2003 to 2019 and their relationship with meteorological and non-meteorological factors are analyzed.On this basis,the multiple linear regression equation between SOS and main influencing factors is established,and the variable sensitivity is calculated.The main research conclusions are as follows:(1)Compared with the permafrost boreholes,the simulation results obtained from the surface frost number driven by remote sensing data have higher accuracy,except for the isolated permafrost,but all other types are higher than 75%.The multi-year permafrost area on the Mongolian Plateau is about 57.07×10~4km~2.From 2003 to 2019,the area of permafrost on the Mongolian Plateau showed no significant reduction trend,with a reduction rate of 0.91×10~4 km~2·a^-1.There are significant regional differences in permafrost changes.The degradation range and degree of permafrost in the east of 105°E are higher than that in the west.The altitude-dependent results of permafrost indicated that the interannual amplitude of permafrost is obvious at the altitude of 500-2500 m.(2)The start of thaw(SOT)of the multi-year permafrost active layer on the Mongolian Plateau is concentrated in the 105th-135th day(mid-April to mid-May).The interannual variation of SOT of the permafrost active layer indicated a slight advance trend(-0.18d·a^-1).The SOT of permafrost active layer in permafrost area and permafrost change area of Mongolian Plateau becomes later with increasing altitude.The correlation showed that the significant decrease in multi-year permafrost area is related to the significant decrease of snow cover and snow depth,as well as the significant increase in NDVI.The SOT of permafrost active layer advanced with the increase of air temperature and solar radiation in winter,and correlations with snow cover,snow depth,NDVI and precipitation were weak.(3)The multi-year mean value of vegetation start of growing season(SOS)in permafrost area of the Mongolian Plateau was about 132d(early May),with an overall advance of-0.45 d·a^-1.The vegetation SOS was the latest for cropland and the earliest for forest under different land covers.Vegetation SOS was latest in areas with high permafrost probability(80-100%).Areas with earlier vegetation SOS showed an earlier trend of its change,and areas with later vegetation SOS showed a delayed trend of SOS change.The advancement rate of vegetation SOS was larger for forest and cropland types.The vegetation SOS under different permafrost change rates showed an early trend.The vegetation SOS in permafrost area,permafrost change area and permafrost unchanged area of the Mongolian Plateau became later with increasing altitude.(4)About 63.68%of the permafrost active layer SOT in the study area was earlier than the vegetation SOS,with an average of 5.26d earlier.The difference in the average date of cropland was the largest,and the difference in forest areas was the smallest.The sensitivity of vegetation SOS to permafrost active layer SOT showed significant positive correlation across land cover types(P<0.01),with forest land being the highest and cropland being the lowest.Partial correlation analysis indicated that there were regional differences in the effects of permafrost active layer SOT on vegetation SOS:4.85%of regional vegetation SOS was earlier than permafrost SOT,and under the influence of vegetation SOS advance and temperature factors,permafrost began to thaw;3.43%of regional permafrost SOT was earlier than vegetation SOS,and permafrost began to thaw,and the thawed water entered the active layer,providing the necessary temperature and moisture conditions for vegetation growth in the region,and jointly promoting vegetation SOS with factors such as temperature.(5)Vegetation SOS was significantly negatively correlated(P<0.05)with permafrost active layer SOT,air temperature and precipitation in both permafrost and permafrost change zones,and the vegetation SOS had the highest sensitivity to air temperature in both zones.The comparison of vegetation SOS sensitivity to permafrost active layer SOT in the three zones showed that permafrost change zone vegetation SOS had the highest sensitivity to permafrost active layer SOT(-0.28).The sensitivity of vegetation SOS to air temperature was ranked the highest in the three zones,and the sensitivity of precipitation was the highest in the three zones in the permafrost change zone(-0.44).