Spatial-temporal Changes And Influencing Factors Analysis Of Vegetation Coverage In Yulin

Vegetation plays the role of material circulation and energy flow in nature,and maintains the balance of the ecosystem.The value of the vegetation coverage can evaluate the health of the local ecosystem.Yulin City has serious wind erosion,desertification,soil erosion,sparse vegetation coverage and fragile ecological environment,which is the key implementation area of the national project of returning farmland to forest.In this paper,vegetation coverage is used to explore the effect of vegetation restoration in Yulin City after returning farmland to forests.However,the existing research is limited by the inconvenience of data storage and the performance of a single computer to process data.Most of the selected research time series are short and the spatial resolution is low.Based on this,In this paper,with the help of the GEE(Google Earth Engine)platform,the Landsat Surface Reflectance images in Yulin City from 2000 to 2019 are processed online in batches,such as cloud removal,annual NDVI maximum value synthesis,vegetation coverage calculation,and cropping.So that the spatial distribution characteristics and temporal-spatial variation of vegetation coverage in Yulin City during the past 20 years were studied.at the same time,the impact of climatic conditions(temperature,precipitation),human activities(population,land use change,returning farmland to forest,agricultural production)on vegetation cover changes was analyzed using meteorological data,land use type data,and socioeconomic data.The main conclusions reached are as follows:(1)The spatial distribution of vegetation coverage in Yulin City increases from northwest to southeast.The average annual vegetation coverage increases from 41.07% in 2000 to 52.30%in 2019,with an annual growth rate of 1.37%/a(P<0.05).During the past 20 years,62.99%of Yulin City’s vegetation coverage areas were in a state of improvement,of which 32.3% of the areas were significantly improved(P<0.05),mainly distributed in Jiaxian,Wubao County,and the eastern part of Shenmu City.In the past 20 years,the stability of vegetation coverage in Yulin City has mainly fluctuated slightly,and the fluctuation in the northwest region is relatively large;the future trend of vegetation coverage will be dominated by continuous improvement.(2)During the period from 2000 to 2019 in Yulin City,the average annual temperature and annual precipitation showed an overall upward trend.The spatial distribution is characterized by high in the southeast and low in the northwest,and the temperature increased from the flat to the mountain.Vegetation coverage is mainly positively correlated with annual average temperature and annual precipitation.The areas with significant positive correlation with temperature are mainly concentrated in Fugu County and Shenmu City,and the areas with significant positive correlation with precipitation account for 11.48%.It is distributed in Hengshan District and the eastern part of Yuyang District.Precipitation has a promoting effect on vegetation growth.The response of vegetation coverage to precipitation in the growing season has a time lag.(3)The average residual value of vegetation coverage in the past 20 years is 0.035,which indicates that the disturbance of vegetation coverage by human activities in Yulin City is mainly facilitation,and the area where this facilitation effect is gradually enhanced accounts for 66.41%,while Dingjingbian County Human activities in places such as the South have inhibited vegetation recovery.Due to the conversion of farmland to forests,2579 km~2 of cultivated land in Yulin City was converted into forest and grass,and the expansion of urban land occupied cultivated forest and grass,which seriously degraded the vegetation coverage.The vegetation coverage in the study area increased first and then decreased with the increase of population density,and the population density of 195 persons/km~2 was the turning point of the change trend.There was a significant positive correlation between grain yield and vegetation coverage in Yulin City(P<0.05).The increase in the number of cattle and sheep before 2015 made overgrazing serious,resulting in a decrease in vegetation coverage.At the same time,the accumulated afforestation area in Yulin City in 20 a totaled 1.5433 million hectares,which significantly increased the forest land in Yulin City and greatly promoted the restoration of vegetation coverage.