Spatial And Temporal Variation Of Fractional Vegetation Cover And Its Relationship With Environmental Factors In Qaidam Basin During 2000-2015

As an vital part of the ecosystem,vegetation has an important impact on the energy flow,material circulation and information transmission of surface ecosystems.Vegetation and climate interaction is one of the most important processes in terrestrial ecosystems.Especially in the context of global warming,the relationship between vegetation and climate has become an important topic in geography and ecology research.Fractional vegetation cover can reflect the state and structure of vegetation growth,and is an important indicator for climate change,soil erosion and hydrological processes,which is widely used in global change research.The Qaidam Basin,located on the northern Qinghai-Tibet Plateau,is a typical arid and semi-arid climate with a fragile ecological environment.Changes in fractional vegetation cover can represent regional or even national climate change to some extent.Therefore,clarifying the interaction between vegetation and environmental factors has important theoretical and practical significance for the study of ecological environment in other arid regions.This paper selected MODIS-NDVI data and meteorological data from 2000 to2015,and analyzed the temporal and spatial variation characteristics of fractional vegetation cover in the Qaidam Basin based on pixel dichotomy model and trend analysis.Using the ground measured data obtained by umanned aerial vehicle,the accuracy of model was verified.According to the temporal and spatial distribution of meteorological data such as precipitation,temperature,relative humidity,sunshine hours and evapotranspiration,correlation analysis and partial correlation analysis were used to further study the relationship between fractional vegetation cover,precipitation and sunshine hours and so on from the interannual and seasonal scales.And the response characteristics of vegetation to environmental factors was explored.At the same time,combined with land cover data from different years,the human factors affecting fractional vegetation cover were studied.The results were as follows:There was a good linear relationship between NDVI and fractional vegetation cover,which was more suitable as a parameter of pixel dichotomy model than other vegetation indices;The pixel dichotomy model extracted the histogram inflection point and maximum value as parameter thresholds to invert the fractional vegetation cover with high precision,which was fit for the inversion of fractional vegetation cover in arid regions.The average fractional vegetation cover in the Qaidam Basin had a semi-ring decreasing trend from the east and the southeast to the northwest inland area of the basin.The overall fractional vegetation cover was low,mostly value were concentrated below 20%.The core area of the oasis of the Golmud-Numhon borderline in the central basin anf in the low mountainous area of southeas were covered with more than 40%.Stipa purpurea grassland and meadow vegetation such as Kobresia,Phragmites australis and Tamarix hohenackeri Bunge were developed.Some Kunlun Mountains and northwestern inland areas had lower fractional vegetation cover,which mostly below 10%.Under the background of climate warming,except for some of the southwestern Kunlun Mountains,the low mountainous areas in the southeast and the mountainous areas south of the Qilian Mountains,the fractional vegetation cover in the Qaidam Basin increased significantly from 2000 to 2015.Especially in the desert area of Stipa purpurea steppe-Salsola collina Pall,the piedmont alluvial plain east of Wulan and the central oasis core area increased most obviously.The vegetation cover of saline-alkali wetlands around lakes such as Dabson Lake and Taijinair Lake was decreasing.Among them,the precipitation increased from 2000 to 2007,and the fractional vegetation cover increased mainly in the mountainous areas outside the basin;fractional vegetation cover in this decreased significantly since 2007 due to the increased in construction land such as saltworks.While the increase in agricultural land in the central and western basin increased fractional vegetation cover to a certain extent.The spatial distribution of annual precipitation and relative humidity in 2000-2015 was relatively consistent,showing a decreasing trend from southeast to northwest.The interannual and seasonal changes of precipitation were obvious,and itwas increasing and concentrated in summer in 16 years;Relative humidity had a slight upward trend,with an average annual relative humidity of 35.5%;The overall temperature rised from 2000 to 2015,showing a decreasing distribution from south to north.The region of southern mountainous had a significant warming trend,and two high-value appeared centers around Golmud and Lenghu;Sunshine hours and evapotranspiration decreased from east to west.Among them,the number of sunshine hours in spring had the highest value,but there was no significant difference in the interannual and seasonal seasons;The evapotranspiration showed an overall upward trend,and the seasonal differences were large,with summer and spring evapotranspiration being higher than autumn.Fractional vegetation cover is different from different environmental factors.The fractional vegetation cover was positively correlated with precipitation and relative humidity at the interannual scale,and the extremely significant positive correlation region had good overlap in the east low mountainous area of Dulan;Fractional vegetation cover was mainly negatively correlated with sunshine hours and evapotranspiration,and plant transpiration had much less effect on evapotranspiration than soil evaporation;The relationship between fractional vegetation cover and temperature was not significant,but the contribution of temperature to evapotranspiration was higher than that of precipitation.That was,temperature had little direct impact on fractional vegetation cover,and indirectly affects fractional vegetation cover by factors such as evapotranspiration;The area of the significantly related area between fractional vegetation cover and the main environmental factors was precipitation > relative humidity > evapotranspiration > sunshine hours >temperature,which meant the contribution of moisture factor to fractional vegetation cover was higher than that of heat factor,and precipitation was the main limitation affecting vegetation.There was a large discrepancy in the relationship between fractional vegetation cover and environmental factors in different seasons,but they were all less directly related to temperature.The relationship between fractional vegetation cover and climatic factors in spring was significantly lower than that in summer and autumn inthe Qaidam Basin.Fractional vegetation cover was mainly affected by evapotranspiration in spring.The relatively large evapotranspiration in spring made the fractional vegetation cover negatively correlated with precipitation and relative humidity in some areas;There was still a good agreement between precipitation and relative humidity in summer,which was significantly negatively correlated.And precipitation was the dominant factor affecting summer fractional vegetation cover;The correlation between fractional vegetation cover and other environmental factors had been improved in autumn apart from temperature,and relative humidity and sunshine hours substitute precipitation had become the main limiting factors affecting vegetation cover.Precipitation in autumn was still positively correlated with fractional vegetation cover due to hysteresis effect and lower evapotranspiration.