Variations In Frozen Soil Environment And Its Effect On Alpine Vegetation Over The Source Region Of Yellow River

The alpine ecosystem of the source region of the Yellow River(SRYR)was fragile and sensitive.In recent years,because of the influence of global climate change and human activity disturbance,there were a series of ecological environment deterioration phenomenon in SRYR,and degradation in frozen soil environment was the main factor in accelerating the deterioration of the alpine ecosystem.However,current research about variations in frozen soil environment mainly had been carried out around the construction project,and the field sites were sparse and concentrated,which cannot meet the requirement of long-term monitoring the frozen soil environment.In addition,the study on relationship between frozen soil environment and vegetation was still in the qualitative stage,and the quantitative analysis was lacking.In this study,for the relationship between frozen soil environment and vegetation under the background of climate change in SRYR,we conducted systematic study in SRYR on climate change characteristic during 1982-2014,soil freeze-thaw cycle,frozen soil environment change,spatial-temporal variations of vegetation NDVI and phenology,and the quantitative relationship between vegetation and frozen soil environment.Results showed that air temperature,ground surface temperature and precipitation in SRYR increased from 1982-2014,and precipitation decreased before 2007 and then increased after that.The climate condition changed from "warm-dry" to "warm-wet".During 2002-2011,the dates of the onset of thawing in spring and complete thawing advanced by 3.1±1:1.4 days and 2.46± 1.4 days;The dates of the onset of freezing in autumn and complete freezing were delayed by 0.9 ±1.4 days ? 1.6± 1.1 days;The complete thawing duration increased by 5.76±1.1 days and the frozen duration decreased 4.16±2.08 days The annual average surface soil moisture decreased in 2003-3010 at the rate of 0.0012cm3/(cm3·a),and areas with higher initial soil moisture values showed decreasing trends,while the areas with lower initial soil moisture concentrations showed increasing trends over the same time period.The freezing indices in SRYR decreased during 1980-2014,and the thawing indices and thawing-freezing index ratios increased.The change point of the FI and TI time series occurred in 2000/2001.The average soil frozen depth in SRYR was 1.52m,and decreased at the rate of 0.0093m/a in 1980-2014.The average vegetation NDVI higher in southeast and lower in the northwest of the study area,increasing at the rate of from 1982-2014.The start of vegetation growing season and the end of vegetation growing season were advanced,and the length of growing season shorten.The changes in vegetation and frozen soil environment had the spatial-temporal dependency,and the distribution of vegetation types had dependency with the hydro-thermal properties of frozen soil environment.The beginning and duration of different freeze-thaw cycle stages affected the vegetation phenology.The vegetation NDVI had positive relationship with land surface temperature and surface soil moisture,and had negative relationship with phase transition of water in freeze-thaw cycle.In particular,land surface temperature in low temperature period,surface soil moisture in March and July,and phase transition of water in freeze-thaw cycle in spring had significant influence on vegetation NDVI.Vegetation NDVI had significant negative relationship with the freezing indices and soil frozen depth and positive relationship with thawing indices.That was,with weakening in soil freezing ability and lower of soil frozen depth,the vegetation NDVI increased.In addition,the duration of complete freezing affected the vegetation NDVI significantly.Above all,the climate in SRYR changed from "warm-dry" to "warm-wet" in 2007.The frozen soil environment showed a trend of degradation,and the average vegetation NDVI showed an increasing trend.The temperature rise was the main factor affecting the changes in frozen soil environment and alpine ecosystem over SRYR,and the influence of precipitation was more complicated.