Spatio-Temporal Characteristics And Influence Factors Of Vegetation Coverage In The Southwest China

Fractional vegetation cover(FVC)is an important indicator to measure the surface vegetation condition,the important essential data to describe ecosystem structure and function and the significant mark of regional ecosystem change.The changing process of FVC is extremely complicated due to the influence of natural factors and human activities.How to understand the process deeply and analyze the response mechanism to climate,topography and human activities in regional scale has been become hot spot of academic community.Southwest China is an important ecological barrier with abundant biodiversity and complex and diverse ecosystems;meanwhile,the ecology is sensitive and fragile extremely.The ecological and environmental problems in this region are be concerned by more researchers.Studying on the spatial-temporal variation characteristics of FVC in Southwest China is helpful to explore the driving factors of vegetation dynamics and to provide scientific reference for regional ecological environment protection.FVC is an index which evolved from moderate resolution imaging spectrometer(MODIS)normalized difference vegetation index(NDVI)data.The spatio-temporal patterns of vegetation coverage in Southwest China from 1982 to 2016 were analyzed using FVC data.Maximum value composites,linear regression methods,and partial correlation analyses were used to investigate the monthly and annual FVC variations,analyze the spatial distribution and changes in the FVC,and examine the relationships between the FVC and climatic factors,topograpic factors and human acitivitis.The main results are as follows:(1)The higher FVC occurred from June to September in Southwest China.The minimum value of FVC occurred in February(0.3497)while the maximum value occurred(0.5766)in August.Annual FVC increased significantly from 1982 to 2016(P<0.05),and the slope of the trend line was 0.0016.The seasonal FVC and the growing season FVC also showed increases.Spring had the fastest FVC growth rate(0.0024).(2)The average FVC value for Southwest China was 0.46.The higher FVC values(0.5 to 0.6)were mainly distributed in the eastern part of Henduan Mountains,including the Sichuan basin,southwest of Yunnan Province,southeast of Tibet and the mountain border of Guangxi and Guizhou Provinxce.In contrast,the western part of Henduan Mountains had lower FVC(less than 0.2),including the northwestern Tibet,Qinghai Province,western Sichuan Province and the central areas of other provinces(cities)and counties.The vegetation improvement area was significantly greater than the degradation area,and the area of increasing trend accounted for 74.0%of the total vegetation coverage.The area of increased vegetation coverage was mainly concentrated in the eastern part of Henduan Mountains,and the decreased vegetation area was mainly concentrated in western part of Henduan Mountains and central regions.(3)Considering the stability and sustainability of FVC,the moderately stable area is largest which accunted for 34.1%of the total area.The highest stabale area has least distribution area that accounted for 0.45%of the study area.For the lower,higher and unstable areas were accunted for 31.1%,18.6%and 15.8%of the total area,respectivily.So,the dominant areas were the moderately and lower stable areas.In the future change trend of FVC in Southwest China,the area of continued&improved accounted for 32.2%;the unchanged area accounted for 22.95%;the continued°radation area accounted for 27.22%and the uncertainty area accounted for 17.6%.Both of the areas of anti-sustainability°radation and anti-sustainability&improvement can be neglected.(4)There was a clear difference in the relationships between FVC and sunshine duration,temperature and precipitation among different months,and there was a time lag in the response of FVC to precipitation.Precipitation had a crucial effect on the FVC variation in spring,summer,autumn,and winter.The influence of different topographic factors on FVC is mainly reflected in that the elevation of>5000m,on the overcast slope had the lower FVC value.(5)The time series of FVC in Southwest China is influnced by climate and human activities,and the contribution rates are 51.2%and 48.8%,respectively.The increase area of FVC caused by climate change is larger than the decrease area,and the increase area of FVC caused by human activities is slightly larger than the decrease area.In the past 35 years,the impact of human activities on vegetation in southwest China has been continuously strengthened.The construction of artificial afforestation and ecological civilization city in each province(city)is conducive to the increase of FVC,while the urban population growth and urbanization expansion mainly lead to the decrease of FVC.