Influence Factors Of NDVI Change And Evaluation Of Grassland Ecological Service Function

Global climate change has a huge impact on terrestrial ecosystems.Vegetation is the main body of the terrestrial ecosystem and one of the most active factors in the global material cycle and energy conversion,and acts as an"indicator"in climate change research.Vegetation change is affected by a variety of factors,climatic factors affect the trend and fluctuation degree of vegetation changes,and topographic factors affect the basic distribution of vegetation.In high-altitude areas,temperature is the dominant factor affecting vegetation growth.In arid regions,precipitation is the main controlling factor for vegetation growth,and the response of vegetation growth to precipitation has a certain time lag.The Qilian Mountains are one of the important mountain ranges in the arid region of northwest China,with complex topography,large elevation differences,diverse vegetation types,and are sensitive to climate and human activities.Under the background of global climate change and the implementation of ecological engineering,monitoring and evaluating the vegetation in the Qilian Mountains,and exploring the main controlling factors affecting the vegetation change are of great significance to the ecological security and vegetation restoration in the Qilian Mountains.This study uses"3S"technology（ie GIS,RS,GPS）,combined with SPOT-VGT-NDVI data,ecological region range,vegetation type data,meteorological data,ground survey data,DEM and other data to analyze the different ecological regions of Qilian Mountains,Different vegetation types,different stages of ecological engineering implementation,vegetation cover change patterns and their influencing factors,and the value of grassland ecosystem services in Qilian Mountains were evaluated.The results of the study are as follows:（1）The NDVI of vegetation in the Qilian Mountains is generally low,and is higher in the southeast than in the northwest.From 1998 to 2018,vegetation NDVI showed a fluctuating upward trend,with a growth rate of 0.32%/a.The forest NDVI value and rate of change are the largest,and the coefficient of variation is the smallest,while the NDVI value and rate of change of the desert are the smallest,and the coefficient of variation is the largest.The change rate of NDVI in the Qaidam Basin desert ecoregion and the Pamir-Kunlun Mountains-Aljin Mountains alpine desert steppe ecoregion was relatively small,only 0.14%/a and 0.27%/a,while the steppe-like desert ecoregions and rivers in the central Inner Mongolia Plateau The change rate of the source area-Gannan alpine meadow steppe ecological area was larger,which were 0.54%/a and 0.57%/a,respectively.The area of the low vegetation coverage area in the northwest gradually shrank,the middle vegetation coverage area gradually increased in the middle,and the high vegetation coverage area in the southeast increased significantly.The proportion of NDVI in 0-0.2 decreased from 43.18%in 1998 to28.97%in 2018,the proportion of NDVI in 0.2-0.8 remained basically unchanged,and the proportion of NDVI in 0.8-1 increased from 0.12%to 12.31%.The variation of vegetation NDVI in Qilian Mountains showed a trend of overall improvement and partial degradation.The areas with benign and malignant development trends in the future accounted for 42.82%and 26.40%of the total area,respectively,of which the area with continuous degradation accounted for 25.56%.The growth rate of NDVI in different stages of ecological project implementation is between 0.38-1.85%/a,and the growth rate is in the following order:after environmental protection inspection>mid-term protection period>continuous protection period>initial stage of protection>before project implementation.The proportion of low vegetation coverage areas decreased from 39.89%before the implementation of the project to 31.50%after environmental inspections.The percentage of coverage areas increased from 0.15%to 10.04%.（2）The variation range of the average temperature in the Qilian Mountains from 1998 to2018 was between-17.45 and 7.96°C,and the annual average temperature in the whole region was-3.72°C.The spatial distribution of air temperature does not change significantly with latitude and longitude,but changes significantly with altitude gradient.The average precipitation in the Qilian Mountains from 1998 to 2018 showed the characteristics of low in the west and high in the east.The average annual precipitation in the whole region was 317.54mm,and the precipitation in the growing season was 299.03 mm.The precipitation in the growing season accounted for 94.17%of the annual precipitation.The annual maximum NDVI value has the greatest correlation with precipitation in June,followed by July,August,and April.The regions with positive correlations account for 74.33%,71.55%,67.05%and65.33%of the total area,respectively,indicating that vegetation has a significant effect on precipitation.The lag response effect,the annual maximum NDVI is dominated by precipitation in June.Different vegetation types have different responses to precipitation.The effective precipitation of shrubs,meadows,and alpine vegetation occurs after June,and the precipitation of temperate grasslands and deserts occurs after May.,desert and alpine vegetation maximum NDVI values were most strongly correlated with cumulative precipitation in July-August,June-July,April-August,April-July and March-June.The annual maximum NDVI value had the highest positive correlation with August,with a positive correlation of 93.41%of the area.The negative correlation with April is the highest,with63.79%of the regions showing a positive correlation.There is a negative correlation with the temperature in April and June,and a low positive correlation with the air temperature in May,July and August,and the correlation coefficient is not significant.（3）With the increase of slope and altitude,vegetation NDVI value and change trend showed a trend of first increase and then decrease,and the change law of variation coefficient was opposite to the former two.The vegetation NDVI on the east slope（67.5-112.5°）and southeast slope（112.5-157.5°）was larger,and the vegetation NDVI on the south slope（157.5-202.5°）and southwest slope（292.5-337.5°）was smaller.The change trend of vegetation coverage was different in different slopes.The southeast slope had the highest growth rate of 0.34%/10a,and the southwest slope had the lowest growth rate of 0.29%/10a.The coefficient of variation of vegetation cover is the largest in the north slope,which is0.1536,and the smallest in the southeast slope,which is 0.1328.（4）When there are sufficient data indicators for the evaluation of ecosystem service value,the evaluation area is small,and the accuracy of the results is required to be high,the method based on the price of the unit ecosystem service product is selected;the ecosystem service function of different regions needs to be compared,the evaluation area is large,and data acquisition is difficult,select the method based on the value-equivalent factor per unit area.The total value of natural grassland ecological service functions in Qilian Mountains is3449.42×10⁸yuan.Among them,the ecological service value per unit area of mountain meadow is the highest,which is 5819.82 yuan·hm^-2;the ecological service value per unit area of temperate desert is the lowest,which is 10 120.21 yuan·hm^-2.The total value of ecological services of alpine meadows is the highest,which is 2430.61×10⁸ yuan,and the contribution rate is 70.46%;the total value of ecological services of lowland meadows is the lowest,which is 46.31×10⁸ yuan,and the contribution rate is 1.34%.The order of contribution rate of ecological service value of each grassland type is alpine meadow>temperate grassland>alpine grassland>mountain meadow>alpine desert>temperate desert>lowland meadow.This paper analyzes the vegetation coverage characteristics,change trend,stability,future trend and vegetation response to climate and terrain in Qilian Mountains from the perspective of ecology,botany and human activities,and explores the main control factors affecting vegetation growth.The research results can improve the awareness of decision makers and the public on the ecological security of Qilian Mountains,and provide scientific basis and policy recommendations for the protection,utilization and restoration of Qilian Mountains.