Variation Of Vegetation EVI And Its Response To Climate Change In The Western Sichuan Plateau From 2000 To 2017

Vegetation is the basis for the existence and development of ecosystem,and plays a leading role in hydrological regulation,environmental evolution and energy cycle.Dynamic changes of vegetation are sensitive indicators in the ecological environment,with obvious characteristics of interannual and seasonal changes,and show significant spatial differentiation characteristics in vertical structure affected by topographic factors.Climate is the dominant factor affecting the growth and distribution of vegetation.At the same time,the changes of surface vegetation will have corresponding feedback effects on regional climate and global climate system.The dynamic monitoring of vegetation can reflect the trend of climate change to some extent.The Western Sichuan Plateau is an important ecological security barrier and a vital ecological functional area in China.It has complex hydrothermal conditions and environmental gradients.And the study of its ecological distribution pattern and vertical variation characteristics has a positive effect on understanding the spatial differentiation law of the plateau ecosystem.Based on the MODIS EVI data and elevation data from 2000 to 2017 in the Western Sichuan Plateau,this thesis analyzes the temporal and spatial distribution characteristics,variation trends and vertical differentiation of vegetation changes in the Western Sichuan Plateau over the past 18 years,and combines the temperature and precipitation data from 39 meteorological sites of the Western Sichuan Plateau to study the response and driving mechanism of vegetation EVI to regional climate in western Sichuan Plateau.The main conclusions are as follows:(1)The mean annual EVI of vegetation in the Western Sichuan Plateau over the past 18 years shows a volatility growth trend with a growth rate of 0.60%/(10a)and a fluctuation range of 0.23-0.26.The interannual variation trend and rate of vegetation vary in different seasons,and increase with the volatility in spring,summer and growing season,but decrease in autumn.The interannual variation trend of vegetation EVI in different altitude gradients is close to the whole year and each season,but the difference of variation rate and fluctuation range is significant.(2)From 2000 to 2017,the annual average EVI of vegetation in the Western Sichuan Plateau is 0.25.The overall EVI of vegetation increases gradually from west to east,with significant regional differences.The high-value areas are concentrated in the Zoige Grassland,the Aba-Hongyuan County,the Yalong River Basin and the Dadu River Basin,and the eastern part of the Lancang River Basin;the low-value areas are mainly distributed in high-altitude areas such as Jacques Mountain,Gnie Mountain,Gongga Mountain and Queer Mountain in Dege County.From the seasonal scale,the spatial distribution characteristics of EVI values in spring and autumn vegetation are relatively close,and they are characterized by low west and high east.The distribution of vegetation in summer and growing season is good,and they all have the characteristics of low distribution in the north and high in the south.In addition,with the continuous increase of altitude,EVI of vegetation in the Western Sichuan Plateau in the whole year and in each season increases gradually at first and then decreases continuously.(3)The EVI annual change rate of vegetation in the Western Sichuan Plateau is between-0.02 and 0.12.The overall state of vegetation in the plateau is mainly stable,with more improved area than degraded area.The improvement areas are mostly distributed in the eastern margin of Aba Prefecture,and the degraded areas are most obvious in Shiqu County and Yajiang County.Vegetation EVI shows improvement trend,basically unchanged,and degraded trend in all seasons,of which has the most area proportion,are distributed in the growing season,spring and summer,respectively.With the change of altitude gradient,the trend of vegetation EVI varies with seasons.(4)The EVI response to climatic factors in the Western Sichuan Plateau has a significant hysteresis effect.In spring,the correlation between the vegetation EVI and the average temperature during the same period is the highest,and the EVI response to the precipitation is delayed by two months.The response between the summer vegetation EVI and temperature shows a one-month lag,while the response to the precipitation shows a two-month lag.The response lag time for temperature and precipitation in the autumn vegetation EVI is one month.The growth season vegetation EVI has the closest relationship with the temperature in the same period,while the precipitation is two months behind.(5)The relationship between the average annual vegetation EVI and temperature in the Western Sichuan Plateau is not significantly positively correlated,and the areas are mainly distributed in the Aba County and Zoige County in the northeast and Shiqu County in the northwest of the plateau;and it is negatively correlated with precipitation and distributed in blocks in high altitude areas in the south-central part of the Western Sichuan Plateau.For different seasons,the area proportion that is positively correlated with temperature is from large to small in the order of spring,growing season,summer and autumn.The area proportion positively correlated with precipitation is growing season,summer,autumn and spring from large to small.(6)The vegetation EVI in the Western Sichuan Plateau is generally driven by non-climate factors.At the significance level of 0.05,the area affected by climatic factors accounts for 4.56%.Among them,temperature-driven and precipitationdriven account for 1.32% and 1.07%,respectively.The areas of strong-driven by temperature and precipitation are relatively large,accounting for about 1.86%;while the weak-driven regions are scattered and the distribution characteristics are not obvious.The areas where vegetation EVI is affected by climatic factors in all seasons are spring,growing season,summer and autumn from large to small.