Spatio-Temporal Variation Of Vegetation EVI In Central Yunnan Under The Background Of Climate Aridity

As the global climate warms,the intensity and frequency of regional droughts are increasing,which has a major impact on ecosystems.The response mechanism of vegetation to meteorological drought is complex.Its growth dynamics,structure and function may be affected by drought,and it also has a feedback effect on drought.Central Yunnan is a region with high frequency of drought.The study on the response characteristics of vegetation to meteorological drought in Central Yunnan can reveal the relationship between regional drought and vegetation,which is of great practical significance for promoting the development of regional social economy and the restoration and protection of ecological environment.In this paper,based on the surface meteorological observation data and vegetation remote sensing data,Sen's slope estimation method,correlation analysis method and multiple regression residual analysis methods are used.Based on the Standardized Precipitation Evapotranspiration Index(SPEI),the long-term change characteristics of climate aridity in Central Yunnan were analyzed.Furthermore,we analyzed the spatiotemporal variation characteristics and trends of Vegetation in central Yunnan by using EVI(Enhanced Vegetation Index),and revealed the response rule of EVI to drought.The main research conclusions are as follows:(1)In recent 60 years,there was a significant decrease in SPEI in central Yunnan,especially after 2002,when there was a significant abrupt decrease in SPEI and drought,and the decrease trend was the most significant in autumn(P < 0.01).The whole region of central Yunnan is prone to drought,especially the southern part of Yuxi,central and northern part of Kunming,western and northern part of Chuxiong and central part of Qujing.In addition to Yuanmou County in the northern part of the Jinsha River Basin,there was a slight upward trend(wetness),while the rest of the region showed a consistent downward trend(climate aridity).The average frequency of drought in four seasons was31.6% ? 34.5%,with the highest frequency in winter and spring,but the maximum increase in summer.The affected area of drought showed an expanding trend,from partial regional drought to regional and regional drought,and the affected area expanded most obviously in summer.The spring drought intensity has been increasing significantly in recent 20 years.(2)EVI in central Yunnan showed a significant increasing trend from 2000 to 2019(P < 0.01),and the increase was most obvious in grassland ecosystem,followed by forest ecosystem and farmland ecosystem.From the perspective of four seasons,EVI increased most obviously in autumn,followed by winter and spring,and EVI remained basically unchanged in summer.The spatial distribution of EVI was higher in the southwest and southeast,while the low EVI was mainly distributed in the northeast of Kunming and the middle and north of Qujing.EVI showed a significant increase in the area of 29.4%,mainly distributed in Qujing and other areas in the northeast,and a significant decrease in the area of 4%,scattered in central Kunming,northeast Yuxi and northwest Chuxiong.The future change trend of EVI was mainly anti-persistent,and the area proportion was as follows: from increase to decrease(57.1%)> continued to increase(29.2%)> changed from decrease to increase(7.1%)> continued to decrease(6.4%)> random change(0.1%)= not significant(0.1%),and the future change trend of EVI values in most areas changed from increase to decrease.(3)There was a certain correlation and lag between EVI and drought in the vegetation in central Yunnan.In most years,the rising trend of EVI was interrupted or decreased when there was a mild drought or more drought.The farmland ecosystem was more sensitive to drought,followed by grassland ecosystem.Forest ecosystems are the least sensitive to drought.EVI was most sensitive to drought in winter.In the growing season,EVI in May and June had a strong correlation with SPEI of the previous month,and the lag response to drought was significant.The lag of EVI to drought was not obvious in July.There was a 2-month lag in EVI response to drought in August and September.(4)With the increase of drought intensity,the EVI value of vegetation decreased,and the degree of deviation from normal value was greater.The EVI of grassland ecosystem showed the highest sensitivity to light drought,medium drought and severe drought.In the case of extreme drought,the EVI value of farmland ecosystem decreased the fastest.Forest ecosystems are not sensitive to moderate drought intensity and below drought intensity.On the whole,EVI was more sensitive to medium-and long-term drought than to short-term drought.Precipitation is an important factor affecting EVI response of vegetation in the study area.In the regions with significant effects of drought on vegetation,human activities mainly promoted the EVI value in the growing season.When human activities had a certain positive effect on the growth of EVI in the significantly related regions,the effect of drought on vegetation could be weakened.However,human activities have a negative effect on EVI of vegetation in urbanized areas,which makes the vegetation in some areas where drought is not serious show a significant downward trend.