Vegetation Change Trend And Its Response To Key Ecological Factors In Junggar Basin

Junggar Basin,as the main geographical unit in northern of Xinjiang,its environmental changes will affect the ecological environment changes in the northern of Xinjiang.Vegetation index,as an important index to measure regional ecological environment,directly reflects the ecological status of Junggar Basin.In this paper,remote sensing data,ENVI and GIS software are used to analyze the temporal and spatial variation of normalized difference vegetation index(NDVI)in Junggar Basin,to understand the characteristics of NDVI changes in the basin,and to describe the key ecological factors affecting vegetation changes in the basin.In this study,the remote sensing image data of MOD09A1,the surface temperature data of MOD11B3,the land classification data of MCD12Q1 and the precipitation data of TRMM in the growing season from 2002 to 2019 were used.The trend changes of NDVI,soil salinity,soil moisture,surface temperature,precipitation and human factors in growing season and the correlation between NDVI and the above key ecological factors were analyzed by numerical synthesis,Mann—Kendall trend analysis,Hurst trend analysis and correlation analysis.Finally,the contribution rate of each factor to the change of NDVI in the basin was calculated by geographic detector.Through the above research,the following results can be obtained:(1)The NDVI of Junggar Basin showed an obvious increasing trend from 2002 to 2019,with the increasing regional area accounting for 63.50%—90.93% in each season,with an average of 77.98%,and local areas showed a decreasing trend.The increase of precipitation and temperature promoted the growth and development of ephemeral plants in spring and autumn,so the NDVI growth in spring and autumn was obviously better than that in other seasons.In addition,from 2002 to 2019,the future change direction of the region with significant NDVI change is the same as the current change direction.(2)The trend change direction of accumulated precipitation and soil moisture in Junggar Basin is the same as that of NDVI,and NDVI is positively correlated with accumulated precipitation and soil moisture.During 2002—2019,the accumulated precipitation in Junggar Basin showed an overall increasing trend,and the area with an increasing trend in each season accounted for 58.72%—94.08%,with an average of71.31%.The soil moisture in the basin is increasing on the whole and decreasing locally,and the area with increasing soil moisture in each season accounts for 51.31%—71.57%,with an average of 59.35%.According to the correlation analysis between NDVI and accumulated precipitation,NDVI and soil moisture,the area with positive correlation between NDVI and accumulated precipitation accounts for23.26% of the basin area,mainly concentrated in natural areas,there is a positive correlation between NDVI and soil moisture in the basin,accounting for 23.23% of the whole basin area,especially in Gurbantunggut Desert.(3)NDVI in Junggar Basin has a negative correlation with soil salinity and surface temperature.In the growing season of the basin,the area where the surface temperature is decreasing and increasing is almost equal,among which the decreasing area accounts for 51.95% of the basin area.The overall surface temperature of the basin is increasing in spring and decreasing in autumn.In the basin,the soil salinity showed a decreasing trend as a whole,with the decreasing area accounting for 46.50%—86.78% in each season,with an average of 70.68%.In the low-lying areas,the degree of soil salinization increased and the vegetation decreased.The results of correlation analysis between NDVI and surface temperature and between NDVI and soil salinity show that the area where NDVI is negatively correlated with surface temperature accounts for 22.98% of the basin area,mainly concentrated in oasis area.The negative correlation between NDVI and soil salinity accounts for 29.64% of the basin area,mainly concentrated in deserts and desert areas.(4)The study quantifies the influence of human activities on vegetation change in Junggar Basin through the difference between theoretical NDVI predicted by precipitation and observed NDVI.The results showed that human activities played a decisive role in increasing vegetation in oasis area.The quantitative results are consistent with the development intensity of cultivated land in different areas of oasis in recent years through trend analysis.(5)Precipitation and soil moisture are the dominant factors affecting NDVI changes in desert and desert areas of the basin,while human activities are the dominant factors affecting NDVI changes in oasis areas.The contribution rates of key ecological factors in Junggar Basin and its regions are as follows.Junggar basin: soil moisture(0.30)> precipitation(0.23)> human activities(0.17)> soil salinity(0.16)>surface temperature(0.14);Gurbantunggut Desert: soil moisture(0.42)> precipitation(0.24)> soil salinity(0.16)> surface temperature(0.12)> human activities(0.07);Desert area of Junggar Basin: soil moisture(0.32)> precipitation(0.28)> soil salinity(0.17)> surface temperature(0.16)> human activities(0.07);Oasis of the basin: human activities(0.34)> soil moisture(0.21)> precipitation(0.17)> surface temperature(0.141)> soil salinity(0.139).