| Vegetation is the natural link between soil,atmosphere,water and other ecological elements,and is a comprehensive indicator of global change research.The Hexi area is located in the western part of Gansu Province.The mountain basins in the area are inter-distributed,the geological structure is complex and diverse,and the topography is undulating,forming a “mountain-oasis-desert” which is a typical representative landscape pattern in the arid semi-arid area.In recent years,with the continuous changes in the global climate and the increasing interference of human activities,the Hexi region is facing a serious ecological crisis.At present,the Hexi region is in the critical period of creating the golden section of the Silk Road Economic Belt,and it is urgent to strengthen the ecology.Protection and construction.This paper takes MODIS NDVI remote sensing image products as the main data source,and uses surface vegetation as the monitoring object.With RS and GIS technology,it reveals the spatial and temporal changes of vegetation in Hexi area from 2000 to 2017,and reflects the special “mountain-oasis--The heterogeneity of the “desert” system,in order to provide scientific support for the subsequent restoration of ecosystems and the sustainable development of resources,and provide a basis for long-term monitoring of vegetation.The main conclusions are as follows:(1)The vegetation in Hexi area has a periodic change pattern of about 5 years,and the overall level of NDVI is low,showing a spatial distribution pattern of "high mountain and oasis areas,low desert areas,and decreasing from southeast to northwest".Qilian Mountains are dominated by high and medium-low values,high values are mainly concentrated in Heihe River and Shiyang River basins,middle and low values are mainly distributed in Shule River basin;corridor oasis area is dominated by high and medium-high values,Heihe River and Shiyang River basin account for a larger proportion;Gobi desert area is dominated by low values,Shule River Basin accounts for a larger proportion.The vegetation grows best at the altitude of 3100 m,the slope of 27 degrees and the northward direction.The vegetation cover area is larger in the altitude range of 1300-4200 m,the slope range of 0-6 degrees and the northward direction.(2)The interannual variation of vegetation in Hexi area shows a fluctuating trend,with 2009 as the dividing year.The vegetation coverage was relatively poor in 2000-2009,NDVI fluctuated sharply and grew slowly;the vegetation coverage was relatively good in 2009-2017,and NDVI fluctuated slowly and grew rapidly.The area transfer shows that the degree of vegetation restoration in the later period(2009-2017)is greater than that in the earlier period(2000-2011),especially in the transition zone between Qilian Mountains and plain areas and in the desert areas of Minqin County.The area change of "mountain-oasis-desert" zoning shows that the area of NDVI in the Qilian Mountains tends to increase,with Shule River basin contributing the most in the 0.2-0.4 area,Heihe River and Shiyang River basin contributing the most in the 0.5-1 area,and the area of low grade tends to decrease,with Shule River basin contributing more significantly.The area of NDVI in corridor oasis area tends to decrease,with Shule River and Heihe River basin contributing the most,medium-level area maintaining stability,high-level area showing an increasing trend,and Shiyang River and Heihe River basin contributing the most.The area of NDVI low-grade in Gobi desert area tends to decrease,while the area of NDVI low-grade tends to increase.Heihe and Shule River basins contribute the most,and the vegetation of middle and high-grade tends to be stable.(3)The vegetation changes in Hexi area are relatively stable overall,and the high coefficient of variation coefficient is mainly concentrated in the corridor oasis area,and the Heihe River Basin accounts for a large proportion.These vegetations are mainly distributed in the periphery of the main oasis and around the immigration area.The median area is mainly concentrated in the Qilian Mountains,and the Shule River Basin accounts for a large proportion.The vegetation types in the area are mostly grassland,desert and alpine meadows,and the intensity of change is severe.The low-value area is mainly concentrated in the Gobi desert area,with the largest proportion of the Shule River basin.The area is mostly desert and sparse vegetation,followed by the corridor oasis area and the Qilian mountain area.The former is mainly concentrated in the main oasis,and the cultivated vegetation in the area is flourishing.The latter is mainly concentrated in the Heihe River and Shiyang River Basin,which is rich in forest resources.(4)The research on vegetation change pattern in Hexi area shows that 32.63% of the area is in an improved state,and the degraded area only accounts for 1.26%.The vegetation as a whole is in a stable and well-developed state.The Qilian Mountains are mainly stable and invariant,followed by the obvious improvement.The Shule River Basin has the largest proportion and is widely distributed.The Heihe River Basin is the second and mainly distributed in the northern part of the mountain.The Shiyang River Basin has the smallest proportion and is mostly distributed in the mountainous areas.Midwest.The corridor oasis area is mainly improved,and the Heihe River Basin accounts for the largest proportion,mainly distributed in the periphery of the main oasis and the periphery of the immigrant area;the second is stable and constant,mainly distributed in the main oasis.The Gobi desert area is mainly stable and invariant,mainly concentrated in the Shule River Basin,followed by the obvious improvement type,mainly concentrated in the Heihe River Basin and the Shiyang River Basin.The degraded type mainly occurs in the transition zone between urban construction land and desert and oasis.It is affected by urbanization,crop planting structure and natural conditions,and the vegetation coverage is poor.(5)The study on the sustainability of vegetation change in Hexi area shows that the vegetation changes in the area are mostly weak and persistent,and the strong persistent areas are mainly concentrated in the corridor oasis area.28.51% of the region continued to develop in the direction of improvement,1.11% continued to develop in the direction of degradation,and 4.36% turned to the opposite direction.These vegetations were mostly slightly improved,and vegetation still had degradation risks. |
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