Vegetation Evolution Characteristics And Toughness Evaluation In Minjiang River Basin

Among the many vegetation indexes,it is the one that can most clearly monitor the vegetation growth and observe the vegetation coverage,Normalized vegetation index(NDVI)is widely used at home and abroad.It is the most effective index to observe the changes of plants and ecosystems in various regions of the world.It plays an important role in restoring the ecological environment and protecting regional vegetation.Due to the large north-south span of Minjiang River Basin and extremely complex terrain,the climate and vegetation in the South and North form typical vertical zone differences.This paper is based on the monthly data and monthly precipitation of NDVI Based on the data and monthly temperature data,the trend,periodicity and persistence of NDVI in interannual,intra annual,four seasons and growing seasons in Minjiang River Basin are analyzed,as well as the year in which NDVI changes;The relationship between annual cumulative precipitation,annual average temperature and NDVI in Minjiang River Basin from 1999 to 2019 is analyzed respectively,and the extreme drought year is determined by using standardized precipitation index(SPI)and the proportion of extreme drought area;finally,the stability coefficient,resistance coefficient and resilience coefficient are determined to evaluate the vegetation toughness in Minjiang River Basin.The main contents of this paper are as follows:(1)The overall vegetation coverage of Minjiang River Basin is good,and the area with NDVI value above 0.6 accounts for 91.92% of the whole Minjiang River Basin.In the division of vegetation areas,dense vegetation areas account for the highest proportion;the change trend of NDVI shows an upward trend in interannual,four seasons and growing seasons;from the perspective of sustainability,except winter season,the future NDVI development trend of other three seasons,interannual and growing seasons is the same as that in the past Similarly,it will continue to rise;In terms of periodicity,there are about 11 a cycles in interannual,spring,summer,autumn and growing season.(2)In terms of spatial change,the area with slight improvement of interannual NDVI change trend accounts for the largest area,accounting for 72.26%,indicating that the overall vegetation coverage in Minjiang River Basin is developing towards the improvement trend;the spatial difference of vegetation coverage change in different seasons is obvious.In Minjiang River Basin,the season with the most improvement of NDVI change trend is spring,followed by summer,and the area with the most degradation of NDVI in winter,accounting for the total area 32.90% of product;The spatial variation trend of NDVI in growing season is roughly the same as that in interannual,and its improved area accounts for 87.19% of the total area of Minjiang River Basin.There are three degraded areas in interannual,four seasons and growing seasons,namely the distribution area of glacial vegetation and frozen soil in the west,the plain area with frequent human activities in the East centered on Chengdu,and the distribution area of alpine vegetation with high altitude in the middle.(3)The mutation of interannual NDVI is most likely in 2012.The variation of NDVI occurred in 2018 in spring,2009 and 2012 in summer,2002 and 2012 in autumn,2011 in winter and 2009 and 2012 in growth season.The variation years occurred before and after the Wenchuan earthquake,i.e.the late 2000 s and the early 210 s.(4)From 1999 to 2019,when the precipitation and temperature at all research scales showed an upward trend,the upward trend was compared.It was found that the rising rate of precipitation and temperature in summer was the fastest,and that in winter was the slowest.The correlation between precipitation,temperature and NDVI was analyzed(correlation and lag),it is found that vegetation can respond to precipitation changes in the current month,while the response to temperature has a lag of one month.Based on the monthly precipitation,the annual SPI value is calculated,combined with the proportion of extreme drought area,and the extreme drought years in Minjiang River Basin from 1999 to 2019 are determined as 2000 and 2003.(5)Using stability index,resistance index and resilience index to analyze the Minjiang River Basin,it is found that the local stability of green vegetation distribution is large,and the local stability of glacial permafrost and alpine vegetation distribution is poor;the resistance and resilience in 2000 and 2003 are different,but the resistance index and resilience index in glacial permafrost area are small.