Relationship Study Between Vegetation Cover Change And Extreme Temperature Based On MODIS EVI Data In Hunan Province

As an important part of terrestrial ecosystem,vegetation connects the material circulation and energy flow of each circle and is considered as an important way of water circulation and biogeochemical circulation.On the condition of the global warming,extreme temperature events occur frequently.The study on the change of vegetation cover and its response to extreme temperature in Hunan province is of great significance to national geographical condition monitoring and the consecutive development of the economic society sustainable.As an important content of "the 12 th five-year plan of surveying and mapping",national geographical condition monitoring is now in the stage of experimenting.At present,the relation of vegetation cover change and the average temperature is studied a lot.However,the understanding on the relation between the change of vegetation and extreme temperature and the response of vegetation of different types to extreme temperature,is not comprehensive enough and the current researches for central regions,especially in Hunan province are very deficient.Therefore,based on MODIS EVI data of 2000-2016 and temperature data during this period in Hunan province,the paper analyzes the temporal and spatial variation characteristics of EVI in Hunan province,its response relation between condition and extreme temperatures with different spatial and temporal scales and different types of vegetation by the maximum synthesis method,multiple linear regression method,trend analysis,standard deviation analysis,correlation analysis method.The main conclusions are as follows:(1)In the past 17 years,the extreme temperature index of Hunan province has shown an increasing trend,among which the extreme low temperature has the largest increasing trend while the extreme high temperature has the smallest.Additional,the average temperature is between the two,and the annual distribution of the three is relatively consistent,showing a single-peak distribution.Based on the mean value and trend distribution of extreme temperature index in Hunan province,the spatial distribution of mean value and trend has the consistency: they are high in the south,east and low in the north,west.The temperature in Hunan province shows the trend that the climate in the warm region becomes warmer and warmer and that in the cold region gets colder and colder.(2)The average spatial distribution of the vegetation EVI in Hunan province shows an apparent characteristic of latitude and longitude,and the vegetation cover in the west of Hunan province is relatively good,while the vegetation cover in the middle and the north of Hunan province is relatively poor,where the mean value of the coniferous forest,the broad-leaved forest,the bush and the grass is larger.In most parts of Hunan province,the vegetation EVI is in an increasing trend,and the area of the increasing trend of all types of vegetation EVI is much larger than that of the decreasing trend.There are obvious regional differences in the stability of the vegetation cover in Hunan province,and the middle fluctuation is more and the high fluctuation is less.From the perspective of time change,in the past 17 years,the mean annual EVI of vegetation in Hunan province and the mean monthly EVI of vegetation in growth season showed an overall growth trend,and EVI values of vegetation of all types showed an upward trend,indicating that the vegetation growth situation in Hunan province showed an overall improvement trend.(3)In Hunan province,vegetation EVI is mainly positively correlated with temperature,and extreme low temperature is the main factor affecting vegetation growth.The correlation coefficient between vegetation EVI and monthly temperature change in the within-growing-season was higher than that in the annual scale and there is no obvious lag between vegetation coverage and temperature.In terms of different types of vegetation,coniferous forest,cultivated vegetation,broad-leaved forest,shrub and grass are greatly affected by extreme low temperature,while meadow and marsh are greatly affected by extreme high temperature.The time-delay effect of vegetation EVI and temperature in the inter-growing-season is similar to that in the within-growing-season,but there are some differences in the time-delay effect of vegetation EVI and temperature in different vegetations.Furthermore,the responses of meadow and marsh to mean temperature and extreme temperature showed three months hysteresis in September.