Spatial-temporal Changes And Driving Forces Of Vegetation Coverage In Hengduan Mountains In Past 20 Years

As an important ecological barrier for rivers in the southwest of China,Hengduan Mountains plays a vital role in the green and sustainable devel opment of the ecological environment in southwest China and its surrounding areas,But global warming has brought huge challenges to the terrestrial ecosystems on which humans depend.As the most important part of the terrestrial ecosystem,vegetation play s an important role in regulating climate,maintaining soil and water,and windbreak and sand fixation.With the increase of population and the acceleration of economic construction,regional climate change and human activities have significantly affected vegetation.Exploring the spatial and temporal distribution characteristics and change rules of vegetation coverage scientifically,and analysis of the relationship between vegetation with climate and non-climate is of great significance for the sustainable development of regional ecological environment and the protection of ecological environment.However,the current research differs greatly in terms of research scale,research content,and research methods.Therefore,based on the Normalized Difference Vegetation Index(NDVI),Enhanced Vegetation Index(EVI),Landsat remote sensing images of typical regions,climatic factors,vegetation types,terrain data,land use and other auxiliary data,using non-parameters Sen trend analysis and Mankendall test,partial correlation analysis,grey correlation analysis,residual trend analysis and other methods,this paper studied the spatial-temporal characteristics of vegetation coverage in Hengduan Mountains from 2000 to 2018,and analyzed the climate driving forces of vegetation coverage changes from multiple time scales(month,season,growing season)and multiple spatial scales(pixel,county,ecological area).Based on Landsat images,the non-climate driving forces affecting vegetation coverage were discussed,an d the differences between NDVI and EVI at spatial-temporal changes and driving forces of Vegetation Coverage were compared.The main conclusions of this study are as follows:(1)The annual average NDVI and EVI of the study area were 0.47 and 0.25,respectively,which showed obvious spatial differences due to the difference of topography,latitude,and vegetation types.The vegetation coverage of the study area was gradually improving from 2000 to 2018.The largest growth rate was the broad-leaved forest ecological area,followed by the meadow ecological area,coniferous forest ecological area and rain forest ecological area.Vegetation improvement and degradation were more pronounced at low altitudes and smaller slopes.The difference between mean values of NDVI and EVI was that the difference was smaller in low coverage area and the difference was larger in high coverage area;NDVI was more sensitive to the vegetation change trend in low coverage area,and EVI was more sensitive to the vegetation change tren d in high coverage area.(2)The correlation between vegetation and climate in study area was more complex.Vegetaion coverage was generally positively correlated with temperature,weakly negatively correlated with precipitation,and most strongly negative ly correlated with relative humidity.Most of vegetation in high latitudes was positively correlated with air temperature especially in spring and negatively correlated in low latitudes especially in summer;the effects of precipitation and relative humidity on vegetation varide greatly with the season.Terrain changed the relationship between vegetation and climate.The vegetation in the valley area was negatively correlated with temperature and positively correlated with precipitation and relative humidit y,especially in summer.EVI had a stronger positive correlation with climate,while NDVI had a stronger negative correlation with climate,but it varied depending on the type of climate and season.(3)The dominant climate factor affecting vegetation in s tudy area was temperature.Latitude and topography would change the relationship between vegetation and climate.In valley areas,precipitation and relative humidity were the dominant climatic factors that affected vegetation especially in summer and autum n.As altitude increasing,temperature gradually becomed the dominant climatic factor that affected vegetation growth.Vegetation in high latitudes was mainly driven by temperature and precipitation.At low latitudes,temperature and relative humidity were the main driving factors.NDVI in the low-cover vegetation area was more affected by climate than EVI,and EVI in the high-cover vegetation area was more affected by climate,but the climatic factors,vegetation types and season would change this relationship.(4)Vegetation and climate existed a lag period of 0-4 months or longer.Vegetation had the fastest response to temperature in high latitudes,followed by precipitation and relative humidity.Vegetation had the fastest response to precipitation and had the slowest response to temperature in low latitudes.The response of vegetation to climate in river valley areas was generally faster than in high altitude areas.In general,the types of vegetation that respond to climate from fast to slow were marsh> meadow> shrub> forest > cultivated vegetation,but there were differences due to different vegetation indices and climatic factors.The lag of NDVI in high latitudes was stronger than that of EVI in terms of temperature,while the lag of precipitation and relative humidity was weaker in NDVI.In low latitudes,the lag of climate to NDVI was generally stronger than EVI.(5)Non-climate factors were important cause of vegetation change in the study area.The largest area was the significant improvement vegetation coverage with dominating by climate in the study,followed by the non-climate improvement area,the climate and non-climate improvement area,and the climate and non-climate area with significant degradation area was smallest.The overall positive effect on the vegetation was far greater than the negative effect,and the impact on vegetation in low-altitude areas was stronger than that in high-altitude areas.The impact of human activities on vegetation was two-sided,but the positive effects of human activities in improving vegetation coverage were far greater than the negative effects.In addition to human activities,geological disasters and fires were also one of the reasons for the significant degradation of vegetation by non-climate factors.